Some selected nuclear techniques in research and development

FROM IDEA TO APPLICATION

Some selected nuclear techniques in research

and developmentP R O C E E D IN G S O F A N A D V IS O R Y G R O U P M E E T IN G S A N JOSÉ, C O ST A R IC A , 9 -1 3 M A Y 1977

i m I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y , V I E N N A , 1 9 7 8

F R O M I D E A T O A P P L I C A T I O N

S o m è s e l e c t e d n u c l e a r t e c h n i q u e s

i n r e s e a r c h a n d d e v e l o p m e n t

The following States are Members of the International Atomic Energy Agency:

AFGHANISTANALBANIAALGERIAARGENTINAAU STRA LIAAUSTRIABANGLADESHBELGIUMBO LIVIABRA ZILBULGARIABURMABYELO RU SSIA N SO V IET

SOCIALIST REPUBLIC CANADA CHILE COLOMBIA COSTA RICA CUBA CYPRUSCZECHOSLOVAKIA DEM OCRATIC KAMPUCHEA DEM OCRATIC PEOPLE’S

REPUBLIC OF KOREA DENMARKDOMINICAN REPUBLICECUADOREGYPTEL SALVADORETHIOPIAFINLANDFRANCEGABONGERMAN DEMOCRATIC REPUBLICGERM ANY, FED ER A L REPUBLIC OFGHANAG REECEGUATEMALAHAITI

HOLY SEEHUNGARYICELANDINDIAINDONESIAIRANIRAQIRELANDISR A ELITA LYIV O R Y COASTJAMAICAJAPANJORDANKENYAKOREA, REPUBLIC OFKUWAITLEBANONLIBER IALIBYAN ARAB JAM AH IRIYALIECHTENSTEINLUXEM BOURGMADAGASCARMALAYSIAMALIM AURITIUSMEXICOMONACOMONGOLIAMOROCCONETHERLANDSNEW ZEALANDNICARAGUANIGERNIGERIANORWAYPAKISTANPANAMAPARAGUAYPERU

PHILIPPINESPOLANDPORTUGALQATARROMANIASAUDI ARABIASENEGALSIE R R A LEONESINGAPORESOUTH AFRICASPAINSRI LANKASUDANSWEDENSW ITZERLANDSYRIAN ARAB REPUBLICTHAILANDTUNISIATU R K EYUGANDAUKRAINIAN SO V IET SOCIALIST

REPUBLIC UNION O F SO V IET SOCIALIST

REPUBLICS UNITED ARA B EM IRA TES UNITED KINGDOM OF G REA T

BRITAIN AND NORTHERN IRELAND

UNITED REPUBLIC OF CAMEROON

UNITED REPUBLIC OF TANZANIA

UNITED STA TES O F AMERICA URUGUAY VENEZUELA V IET NAM YUGOSLAVIA ZAIRE ZAMBIA

The Agency’s Statute was approved on 23 October 1956 by the Conference on the Statute o f the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective is “to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world’’.

Printed by the IAEA in Austria February 1978

P A N E L P R O C E E D I N G S S E R I E S

F R O M I D E A T O A P P L I C A T I O N

S o m e s e le c t e d n u c le a r t e c h n iq u e s

in r e s e a r c h a n d d e v e lo p m e n t

P R O C E E D IN G S O F A N A D V I S O R Y G R O U P M E E T IN G O N

A P P L IE D N U C L E A R P H Y S IC S

O R G A N IZ E D B Y T H E

IN T E R N A T I O N A L A T O M IC E N E R G Y A G E N C Y

A N D H E L D IN S A N J O S É , C O S T A R IC A , 9 - 1 3 M A Y 1 9 7 7

IN T E R N A T I O N A L A T O M IC E N E R G Y A G E N C Y

V IE N N A , 1 9 7 8

F R O M ID E A T O A P P L IC A T IO N : S O M E S E L E C T E D

N U C L E A R T E C H N I Q U E S IN R E S E A R C H A N D D E V E L O P M E N T

IA E A , V IE N N A , 1 9 7 8

S T I/ P U B / 4 7 6

IS B N 9 2 —0 — 1 3 1 0 7 8 — 1

F O R E W O R D

In M ay 1 9 7 7 , th e I n te r n a t io n a l A to m ic E n e rg y A g e n c y o rg a n iz e d an

A d v iso ry G ro u p m e e tin g o n ap p lied n u c le a r p h y s ic s . A t th e k in d in v ita t io n o f th e

G o v e r n m e n t o f C o s ta R ic a th e m e e tin g t o o k p la c e a t th e U n iv e rs ity o f C o s ta

R ic a , in S a n J o s é . T h e a im o f th e A d v iso ry G ro u p w a s 'to s tu d y so m e s e le c te d

n u c le a r te c h n iq u e s th a t h av e e m e rg ed fro m fu n d a m e n ta l re s e a rc h in a to m ic and

n u c le a r p h y s ics an d hav e fo u n d th e ir w ay in to ap p lied re s e a rc h and te c h n o lo g ic a l

p ro c e ss e s .

A n im p o r ta n t a s p e c t o f th e G r o u p ’s w o rk w as to e x a m in e th e s e te c h n iq u e s

f o r th e ir s u ita b il ity fo r a p p lic a t io n in m o d e s t la b o r a to r ie s , e s p e c ia lly in d ev e lo p in g

c o u n tr ie s . T h e c o n tr ib u t io n s p re p a re d b y th e p a r t ic ip a n ts f o r th e m e e tin g c o n

s isted o f rev iew p a p e rs e n d e a v o u rin g to t r a c e th e e v o lu tio n o f th e d e scrib e d

te c h n iq u e s fro m th e t im e w h en th e y w ere used e x c lu s iv e ly f o r fu n d a m e n ta l

s tu d ie s to th e p re se n t w h e n m a n y o f th e m a re f ir m ly e s ta b lish e d as v a lu a b le to o ls

in ap p lied re s e a rc h an d te c h n o lo g ic a l d e v e lo p m e n t. T h e p a p ers p a y p a r t ic u la r

a t te n t io n to w o rk and a c t iv it ie s w h e re th e s e te c h n iq u e s are seen to hav e b e e n

s u c c e s s fu l o r ev en u n iq u e ly e f fe c t iv e . I t is h o p e d th a t th is a p p r o a c h w ill a ro u se

th e r e a d e r ’s in te r e s t and p ro v e p a r t ic u la r ly v a lu a b le to th o s e s c ie n tis ts in te n d in g

to in tr o d u c e a c e r ta in n u c le a r te c h n iq u e o r lo o k in g fo r a su ita b le m e th o d to

so lv e a p a r t ic u la r p ro b le m th e y m a y b e fa c e d w ith .

T h e lis t o f te c h n iq u e s in c lu d e d in th e s e p ro c e e d in g s is o f c o u rs e n o t c o m

p le te . T h e s u b je c ts o f b e a m -fo il s p e c tr o s c o p y and s y n c h r o tr o n r a d ia tio n s w ere

d iscu ssed a t th e m e e tin g b u t are n o t in c lu d e d h e re , s in c e th e y re q u ire e x te n s iv e

e q u ip m e n t o r m u lt im ill io n d o lla r a c c e le r a to r s and a re th u s n o t o f ‘p r im a ry ’

in te r e s t to m o d e s t la b o r a to r ie s . M ö s s b a u e r s p e c tr o s c o p y is c e r ta in ly o n e o f th e

m o s t p o p u la r in e x p e n s iv e te c h n iq u e s ; th e s u b je c t p la y e d an im p o r ta n t p a rt in th e

m e e tin g , b u t it w as fe l t th a t a rev iew p a p e r w as n o t n e e d e d a t p re s e n t . A m o n g s t

o th e r u s e fu l b o o k s a v a ila b le , th e re a d e r m a y b e re fe rr e d to ‘M ö ss b a u e r S p e c t r o

s c o p y and its A p p lic a tio n s ’ (S T I/ P U B / 3 0 4 ) , p u b lish e d in 1 9 7 2 b y th e IA E A in its

P a n e l P ro c e e d in g s S e r ie s .

T h e w h o le fa m ily o f te c h n iq u e s b a sed o n n u c le a r o r e le c t r o n re s o n a n c e s , su ch

as N M R (n u c le a r m a g n e t ic r e s o n a n c e ) , E N D O R ( e le c t r o n n u c le a r d o u b le

r e s o n a n c e ) , E P R ( e le c t r o n p a ra m a g n e tic re s o n a n c e ) an d N Q R (n u c le a r q u a d ru -

p o le r e s o n a n c e ) , w as n o t re p re s e n te d a t th e m e e tin g . T h e y w ere th e s u b je c t o f an

IA E A P a n e l in 1 9 7 3 , and a se r ie s o f rev iew p a p e rs , in a fo rm m u c h s im ila r to th e

p re se n t s e t , w as p u b lish e d in A to m ic E n e rg y R e v ie w , 1 9 7 4 , V o l. 1 2 , N o .4 .

A m o n g th e te c h n iq u e s re p re s e n te d h e re is ‘n u c le a r t r a c k f o r m a t io n ’, d e sp ite

th e fa c t th a t a b o o k w as r e c e n t ly p u b lish e d o n th is to p ic . I t is f e l t th a t th is c h e a p

and w id e ly a p p lic a b le te c h n iq u e s till d o e s n o t re c e iv e th e a t te n t io n it d eserv es .

N e u tr o n , s c a t te r in g is re p re s e n te d b e c a u s e th e o b s e r v a tio n h a s b e e n m a d e th a t

s c ie n tis ts a t m a n y re s e a rc h r e a c to r s w h e re n e u tr o n d if f r a c to m e te r s an d s p e c tr o

m e te r s are in o p e r a t io n w o u ld w e lc o m e a d v ice o n h o w to u til iz e th e ir in s tr u

m e n ta t io n fo r ap p lied s tu d ie s .

C O N T E N T S

C h a rg e d -p a r tic le a c t iv a tio n a n a l y s i s ............................................................................................ 1

E . A . S c h w e i k e r t

S p a tia lly se n sitiv e a n a ly t ic a l te c h n iq u e s ................................................................................ 3 3

T .B . P i e r c e

D e te c t io n o f c h a r a c te r is t ic X -ra y s : m e th o d s and a p p l ic a t io n s ............................... 4 9

V. V a l k o v i c

N e u tr o n a b s o r p tio n p h y s ic s in th e d e v e lo p m e n t an d p r a c tic e o f

a c t iv a tio n a n a ly s is ................................................................................................................................ 81

S .S . N a r g o l w a l l a

R e c e n t a n a ly t ic a l a p p lic a t io n s o f n e u tr o n -c a p tu r e g a m m a -ra y

s p e c t r o s c o p y ........................................................................................................................................... 1 2 5

J . A . L u b k o w i t z , M . H e u r t e b i s e , H . B u e n a f a m a

A p p lic a tio n s o f p o s itr o n a n n ih ila t io n ...................................................................................... 151

P . H a u t o j ä r v i , A . V e h a n e n

A p p lic a tio n s o f th e r m a l n e u tr o n s c a t te r in g .......................................................................... 1 8 3

G . K o s t o r z

D e v e lo p m e n ts in io n im p la n ta t io n .............................................................................................. 2 1 5

D . W . P a l m e r

T r a c k fo r m a t io n : p r in c ip le s and a p p lic a t io n s .................................................................... 2 6 1

M. MonninL is t o f p a r t ic ip a n ts .................................................................................................................................. 2 9 9

C H A R G E D - P A R T I C L E A C T I V A T I O N A N A L Y S I S/

E .A . S C H W E IK E R T

C e n te r fo r T r a c e C h a r a c te r iz a t io n ,

T e x a s A & M U n iv e rs ity ,

C o lle g e S ta t io n , T e x a s ,

U n ite d S ta te s o f A m e rica

Abstract

CH A RGED-PA RTICLE ACTIVATION A N A LYSIS.The paper discusses the methodology and application o f nuclear activation with ion beams

(1 < Z < 5) o f energies in the range o f 2 to 10 MeV/amu as a too l for chemical characterization. With these excitation parameters, radioactive products are obtained from virtually all stable elements.

Light element analysis. Oxygen, carbon and nitrogen can be determined at levels as low as a few pplO 9 via 160 ( 3H e,p)18F, 12C(3H e,a)n C and 14N (p ,a )n C respectively. Recently, triton activation has been shown to be inherently still superior to 3He activation for the determination o f oxygen [I60 ( 3H ,n )18F]. Lithium, boron, carbon and sulphur can be detected rapidly, nondestructively and with high sensitivity 0 .25 ppm for Li and B ) via “quasi-prompt” activation based on the detection o f short-lived, high-energy beta em itters (1 0 ms < T i < 1 s).A new development is heavy ion activation, where hydrogen is detected via IH(7Li,n)7Be for example.

Nondestructive multielement analysis. Proton actiyation has the inherent potential for meeting requirements o f broad elemental coverage, sensitivity (ppm and sub-ppm range).and selectivity. Up to 3 0 elements have been determined in Al, Co, Ag, Nb, Rh, Ta and biological samples, using 12-MeV proton activation followed by gamma-ray spectrom etry. These capabilities are further enhanced with the counting o f X-ray em itters, 28 elements (2 6 < Z < 83) have been determined in different m atrices by nondispersive X-ray counting.

Heavy element analysis. Elem ental abundances o f T l, Pb and Bi can be measured with high sensitivity (~ 1 pplO 9) and accuracy using proton activation. 204Pb/206Pb ratios can also be determined with a relative precision o f a few per cent. Although charged-particle activation analysis is a well-established trace analysis technique, broad potential capabilities remain to be explored, e.g. those arising from ultrashort-lived nuclides, heavy ion interactions and the com bination o f delayed and prompt methods.

1 . 0 INTRODUCTION

A s e a r l y a s 1 9 3 8 , S e a b o r g e t a l . [ 1 ] d e s c r i b e d a p r o c e d u r e f o r d e t e r m i n i n g t r a c e a m o u n t s o f g a l l i u m i n h i g h p u r i t y i r o n b a s e d o n d e u t e r o n a c t i v a t i o n . D e s p i t e t h i s e a r l y e x a m p l e o f c h a r g e d p a r t i c l e a c t i v a t i o n a n a l y s i s ( C P A A ) , t h i s c h e m i c a l a n a l y s i s t e c h n i q u e d i d n o t r e c e i v e m u c h a t t e n t i o n u n t i l t h e e a r l y s i x t i e s . T h e r e n e w a l i n i n t e r e s t w a s l a r g e l y d u e t o t h e p i o n e e r i n g w o r k o f S u e [ 2 ] , A l b e r t [ 3 ] . , G i l l [ 4 ] a n d S a i t o [ 5 ] , s h o w i n g t h e u n i q u e p o t e n t i a l o f

1

2 S C H W E IK E R T

t h i s a c t i v a t i o n m o d e f o r d e t e r m i n i n g l i g h t e l e m e n t s ( e . g . b o r o n , c a r b o n , o x y g e n ) a t t h e s u b - p p m l e v e l . T h e s e p r o s p e c t s w e r e c o n s i d e r a b l y e n h a n c e d w i t h t h e i n t r o d u c t i o n o f ^He a c t i v a t i o n b y M a r k o w i t z i n 1 9 6 2 [ 6 ] . An a d d i t i o n a l f a c t o r e s s e n t i a l t o t h e d e v e l o p m e n t o f t h i s f i e l d h a s b e e n t h e i n c r e a s i n g a v a i l a b i l i t y o f s u i t a b l e s o u r c e s o f c h a r g e d p a r t i c l e s i n r e c e n t y e a r s ( c y c l o t r o n s a n d V a n d e G r a a f f a c c e l e r a t o r s ) . T h u s , t h e s c o p e o f CPAA h a s b e e n b r o a d e n i n g r a p i d l y d u r i n g t h e p a s t d e c a d e e n c o m p a s s i n g n ow e l e m e n t a l a n a l y s i s p r o c e d u r e s w i t h p r o t o n s , d e u t e r o n s , t r i t o n s , 3H e , a a n d m o s t r e c e n t l y e v e n L i a n d В i o n s . W i t h t h i s v a r i e t y o f b o m b a r d i n g p a r t i c l e s a n d g i v e n t h e e n e r g y r a n g e o f i n t e r e s t h e r e ( 2 - 1 0 M e V / a m u ) , a g r e a t n u m b e r o f n u c l e a r r e a c t i o n s c a n b e i n d u c e d , y i e l d i n g r a d i o i s o t o p e s w i t h s u i t a b l e d e c a y c h a r a c t e r i s t i c s . I n d e e d f o r v i r t u a l l y a l l s t a b l e e l e m e n t s a c h a r g e d p a r t i c l e a c t i v a t i o n p r o c e d u r e c a n b e d e v i s e d w h i c h w i l l f e a t u r e h i g h s e n s i t i v i t y (p p m t o p p b l e v e l ) a n d s p e c i f i c i t y .

T h e p r e s e n t p a p e r d i s c u s s e s t h e m e t h o d o l o g y a n d a p p l i c a t i o n s o f t h e c h a r g e d p a r t i c l e t e c h n i q u e b a s e d o n d e l a y e d c o u n t i n g . R e l a t e d a n d c o m p l e m e n t a r y p r o m p t t e c h n i q u e s , e l a s t i c a n d i n e l a s t i c s c a t t e r i n g a n d p a r t i c l e i n d u c e d x - r a y e m i s s i o n a r e t r e a t e d e l s e w h e r e [ 7 , 8 ] . T h e f o l l o w i n g t o p i c s a r e e x a m i n e d b e l o w : s e l e c t i o n o f a c t i v a t i o n r e a c t i o n s ;e x p e r i m e n t a l c o n d i t i o n s a n d q u a n t i t a t i v e a s p e c t s ; a p p l i c a t i o n s a s a s i n g l e a n d a m u l t i e l e m e n t t e c h n i q u e f o r l o w , m e d i u m a n d h i g h Z e l e m e n t s ; c u r r e n t s t a t u s o f CPAA a n d a n t i c i p a t e d d e v e l o p m e n t s .

2 . 0 METHODOLOGY

2 : 1 S e l e c t i o n o f A c t i v a t i o n R e a c t i o n s

T h e a c t i v a t i o n r e a c t i o n s m u s t m e e t t h e a n a l y t i c a l r e q u i r e m e n t s o f s e n s i t i v i t y , a c c u r a c y a n d e x p e r i m e n t a l f e a s i b i l i t y .

T h e c o n s i d e r a t i o n s p e r t i n e n t t o t h e a n a l y t i c a l s e n s i t i v i t y a r e b a s e d o n t h e r a t e e q u a t i o n s g o v e r n i n g n u c l e a r t r a n s f o r m a t i o n s a n d t h e d e c a y o f a c t i v a t i o n p r o d u c t s .F o r c h a r g e d p a r t i c l e i n d u c e d r e a c t i o n s , i t c a n b e sh o w nt h a t t h e d i s i n t e g r a t i o n r a t e o f a r a d i o n u c l i d e i n t h et a r g e t a t t h e e n d o f i r r a d i a t i o n i s :

m № f f X l b a d x ( l - e ~ X t ) ( 1 )( > = - S “ J о x x

w h e r e :D : d i s i n t e g r a t i o n s p e r s e c o n dm: w e i g h t o f t r a c e e l e m e n t

№ : A v o g a d r o ' s n u m b e rf : i s o t o p i c a b u n d a n c e o f t h e t a r g e t n u c l i d e

x j : s a m p l e t h i c k n e s s w h e r e t h e i n c i d e n t p a r t i c l ee n e r g y r e a c h e s a v a l u e e q u a l t o t h e t h r e s h o l do f t h e n u c l e a r r e a c t i o n

b : i n t e n s i t y o f p a r t i c l e b e a m a t d e p t h x i n t h et a r g e t

C H A R G E D -P A R T IC L E A C T IV A T IO N A N A L Y S IS

a : r e a c t i o n c r o s s s e c t i o n a t x i n t h e t a r g e t f o rx a n i n f i n i t e l y . t h i n t a r g e t , - d xA : d e c a y c o n s t a n t o f t h e n u c i i d et : i r r a d i a t i o n t i m eA : a t o m i c w e i g h t o f t h e t r a c e e l e m e n t

C h a r g e d p a r t i c l e b e a m i n t e n s i t i e s a r e u s u a l l y e x p r e s s e d i n t e r m s o f t h e b e a m c u r r e n t , I , i n m i c r o a m p e r e s . F o r s i n g l y c h a r g e d p a r t i c l e s , t h e r e l a t i o n s h i p b e t w e e n b a n d I i s a s f o l l o w s :

b = k l = 6 . 2 x 1 0 1 2 1 ( 2 )

C o n s i d e r i n g e q . 1 , i t f o l l o w s t h a t f o r a n a c t i v a t i o n r e a c t i o n t o b e o f i n t e r e s t , i t m u s t f e a t u r e a h i g h y i e l d , w h i c h d e p e n d s u p o n t h e m a g n i t u d e o f t h e c r o s s s e c t i o n a n d t h e i s o t o p i c a b u n d a n c e o f t h e t a r g e t e l e m e n t . T h e e x c i t a t i o n f u n c t i o n , i . e . t h e d é p e n d a n c e o f t h e c r o s s s e c t i o n o n t h e e n e r g y o f t h e i n c i d e n t p a r t i c l e m u s t b e t a k e n i n t o a c c o u n t . I n p r i n c i p l e , t h e b o m b a r d i n g e n e r g y s h o u l d b e a s h i g h a s t h e a c c e l e r a t o r c a n d e l i v e r t o o b t a i n a s l a r g e a n i n t e g r a t e d c r o s s s e c t i o n a s p o s s i b l e , w i t h l i m i t a t i o n s s e t b y t h e o c c u r r e n c e o f i n t e r f e r i n g r e a c t i o n s ( s e e b e l o w ) . E n e r g i e s w i d e l y u s e d i n CPAA r a n g e f r o m 5 - 2 0 MeV f o r p r o t o n s , d e u t e r o n s a n d H e - 3 i o n s a n d f r o m 2 0 - 4 0 MeV f o r H e - 4 i o n s . E x t e n s i v e i n f o r m a t i o n o n e x c i t a t i o n f u n c t i o n s , r e a c t i o n t h r e s h o l d s a n d Q - v a l u e s i s a v a i l a b l e i n t h e l i t e r a t u r e , a u s e f u l r e c e n t c o m p i l a t i o n i s t h a t o f L a n g e [ 9 ] .

S e v e r a l f a c t o r s a f f e c t t h e a c c u r a c y . T h e r a d i o a c t i v e p r o d u c t n u c l i d e s h o u l d h a v e d i s t i n c t i v e d e c a y c h a r a c t e r i s t i c s ( t y p e a n d e n e r g y o f e m i s s i o n s , h a l f l i f e ) . T h i s i s a n e c e s s i t y i n t h e c a s e o f p u r e l y i n s t r u m e n t a l a n a l y s i s . U s e f u l c o m p i l a t i o n s o f t h e d a t a n e c e s s a r y f o r n u c l i d e i d e n t i f i c a t i o n a r e t h o s e o f L e d e r e r [ 1 0 ] , S e e l m a n n - E g g e b e r t [ 1 1 ] , E r d t m a n n [ 1 2 ] , t h e N u c l e a r D a t a G r o u p o f O a k R i d g e [ 1 3 ] , A j z e n b e r g - S e l o v e [ 1 4 ] a n d E n d t [ 1 5 ] . A r e v i e w o n t h e n u c l e a r d a t a r e q u i r e m e n t s f o r a c t i v a t i o n a n a l y s i s a n d t h e s t a t e o f p e r t i n e n t c o m p i l a t i o n s h a s r e c e n t l y b e e n m a d e b y K r i v a n [ 1 6 ] . T h e a b o v e r e q u i r e m e n t i s m u c h l e s s s t r i n g e n t i f t h e r a d i o n u c l i d e o f i n t e r e s t i s s e p a r a t e d c h e m i c a l l y a f t e r i r r a d i a t i o n . T h e n t h e o n l y i n h e r e n t l i m i t a t i o n i s t h a t i m p o s e d b y t h e h a l f l i f e w h i c h m u s t b e s u f f i c i e n t t o c a r r y o u t t h e s e p a r a t i o n .

E q u a l l y i m p o r t a n t f r o m t h e s t a n d p o i n t o f a c c u r a c y i s t h e r e q u i r e m e n t t h a t t h e r e a c t i o n o f i n t e r e s t h a v e n o o r a m in im u m o f p r i m a r y a n d s e c o n d a r y i n t e r f e r i n g r e a c t i o n s [ 1 6 ] . R e a c t i o n t h r e s h o l d s o r Q - v a l u e s a n d C o u lo m b b a r r i e r s m u s t b e c o n s i d e r e d i n a s u r v e y o f p o s s i b l e i n t e r f e r e n c e s .

F u r t h e r , t h e e x p e r i m e n t a l f e a s i b i l i t y c a n b e a f f e c t e d b y t h e o v e r a l l l e v e l o f m a t r i x a c t i v i t y i n d u c e d b y a n a c t i v a t i o n p r o c e d u r e . I n e x a m i n i n g Q - v a l u e s , i t b e c o m e s a p p a r e n t t h a t p r o t o n a n d H e - 3 a c t i v a t i o n w i l l b e p r e f e r r e d m o d e s o f a c t i v a t i o n . I n d e e d d i s c r i m i n a t i o n s o f i m p u r i t y v s . m a t r i x a c t i v a t i o n i s p o s s i b l e i n m a n y c a s e s , b a s e d o n t h e d i f f e r e n c e s i n Q - v a l u e s f o r p r o t o n - i n d u c e d r e a c t i o n s . I n H e - 3 a c t i v a t i o n t h e t h r e s h o l d s f o r ( 3H e , n ) , ( 3H e , p ) , ( 3H e , a ) a r e u n i f o r m l y l o w ; h o w e v e r , a d v a n t a g e c a n b e t a k e n o f t h e

4 SC H W E IK E R T

C o u lo m b b a r r i e r f o r d e t e r m i n i n g l o w Z e l e m e n t s i n h i g h Z m a t r i c e s [ 6 ] . No d i s c r i m i n a t i o n o f i m p u r i t y v s . m a t r i x a c t i v a t i o n i s p o s s i b l e w i t h d e u t e r o n i n d u c e d r e a c t i o n s w h i c h h a v e l o w t h r e s h o l d s f o r t o o m a n y e l e m e n t s , a n d w i t h H e - 4 i o n s w h e r e a l l r e a c t i o n s h a v e h i g h t h r e s h o l d s . A c t i v a t i o n - r e a c t i o n s m ay a l s o b e r e j e c t e d b e c a u s e o f t h e i m p r a c t i c a l h a l f i i f e o f t h e p r o d u c t n u c l i d e . O t h e r l i m i t a t i o n s common t o a l l CPAA p r o c e d u r e s a r e d i s c u s s e d b e l o w .

2 . 2 E x p e r i m e n t a l C o n s i d e r a t i o n s

Beam Monitoring. T h e i t e m s w h i c h r e q u i r e a t t e n t i o n a r e t h e b e a m e n e r g y , i n t e n s i t y a n d h o m o g e n e i t y . E n e r g i e s q u o t e d f o r CPAA d o n e w i t h c y c l o t r o n s s h o u l d b e c o n s i d e r e d " n o m i n a l " o n l y . A c t u a l e n e r g i e s a n d e n e r g y r e s o l u t i o n f r o m t h e s a m e a c c e l e r a t o r m a y v a r y d e p e n d i n g o n t h e l o c a t i o n i n t h e b e a m t r a n s p o r t s y s t e m . A m u c h m o r e c r i t i c a l p a r a m e t e r , h o w e v e r , i s t h e b e a m i n t e n s i t y a s i t t r a n s l a t e s i n t o h e a t d i s s i p a t i o n i n t h e s a m p l e . D u e t o t h e h e a t g e n e r a t e d b y c h a r g e d . p a r t i c l e i r r a d i a t i o n , t h e a p p l i c a t i o n o f CPAA i s s e v e r l y l i m i t e d o r i m p o s s i b l e f o r s p e c i m e n s w i t h l o w m e l t i n g p o i n t s , p o o r t h e r m a l c o n d u c t i v i t y o r h i g h v o l a t i l i t y . S t e p s f o r a v o i d i n g s a m p l e d e s t r u c t i o n d u e t o e x c e s s i v e t e m p e r a t u r e s i n c l u d e : c o n v e r t i n g s a m p l e s i n t o t a r g e t s o f g r e a t e r t h e r m a l s t a b i l i t y ( e . g . a s h i n g o f b i o l o g i c a l s p e c i m e n s , p r e p a r a t i o n o f s a m p l e s a s t h i n t a r g e t s ) , c o o l i n g a n d / o r r o t a t i o n o f s a m p l e s d u r i n g i r r a d i a t i o n a n d , f o r s a m p l e s w i t h l a r g e s u r f a c e a r e a s , s w e e p i n g o r d e f o c u s i n g o f b e a m s . T h e h o m o g e n e i t y o f t h e b e a m m u s t a l s o b e c o n s i d e r e d a l o n g w i t h i t s i n t e n s i t y . G r e a t c a r e m u s t b e t a k e n t o o b t a i n a b e a m o f u n i f o r m b e a m d e n s i t y . G i v e n a t y p i c a l 1 cm2 s a m p l e a r e a t o b e i r r a d i a t e d , n o r m a l f o c u s i n g e q u i p m e n t s h o u l d b e s u f f i c i e n t t o p r o d u c e a u n i f o r m 1 cm 2 b e a m s p o t . A s i m p l e w a y o f t e s t i n g t h e b e a m d e n s i t y c o n s i s t s o f s u b j e c t i n g a t h i n p l a s t i c s h e e t ( e . g . m y l a r ) t o a v e r y s h o r t i r r a d i a t i o n , b e a m i n h o m o g e n e i t i e s t h a t c o u l d n o t b e t o l e r a t e d a r e r e v e a l e d b y d a m a g e i n p l a s t i c f o i l .O t h e r a p p r o a c h e s f o r a c h i e v i n g h o m o g e n e o u s b e a m s r e l y o n t h e u s e o f d i f f u s e r f o i l s o r m a g n e t i c s w e e p i n g o f a t i g h t l y f o c u s e d b e a m . W i t h a b e a m o f u n i f o r m d e n s i t y a n d p r o p e r s a m p l e c o o l i n g , b e a m i n t e n s i t i e s o f s e v e r a l p A / c m 2 h a v e b e e n a p p l i e d . U p p e r l i m i t s f o r b e a m i n t e n s i t i e s v a r y d e p e n d i n g o n t h e n a t u r e a n d e n e r g y o f t h e p r o j e c t i l e a n d t h e t h e r m a l c o n d u c t i v i t y o f t h e t a r g e t [ 1 7 ] . B e a m i n t e n s i t i e s c a n b e e s t i m a t e d b a s e d o n t h e a c t i v i t i e s a c c u m u l a t e d i n t h i n f o i l s p l a c e d o n t o p o f t h e s a m p l e . T h e s e m i g h t s i m p l y s e r v e a s f l u x m o n i t o r s o r m i g h t b e t h e s t a n d a r d s u s e d f o r q u a n t i t a t i v e c a l c u l a t i o n s ( s e e b e l o w ) . I n s t r u m e n t a t i o n f o r r e a l t i m e m o n i t o r i n g o f b e a m i n t e n s i t y a n d h o m o g e n e i t y s h o u l d i n c l u d e a F a r a d a y c u p a n d a n a d d i t i o n a l d e v i c e c a p a b l e o f s e n s i n g a t l e a s t g r o s s b e a m i n h o m o g e n e i t i e s t h a t c a n d e v e l o p d u r i n g b o m b a r d m e n t . A c o l l i m a t o r m a d e o f f o u r e l e c t r i c a l l y i s o l a t e d s e g m e n t s w a s f o u n d a d e q u a t e i n o u r w o r k [ 1 8 ] .

Samples and Their Handling. T h e p h y s i c a l c h a r a c t e r i s t i c s o f s a m p l e s , t h e i r c l e a n i n g a n d t h e p r e s e r v a t i o n o f t h e i r i n t e g r i t y n e e d t o b e c o n s i d e r e d . I n t h e c a s e o f s o l i d s , t h e a r e a t o b e i r r a d i a t e d s h o u l d b e f l a t a n d p o l i s h e d . A s a l


r e a d y i n d i c a t e d , a t y p i c a l d i m e n s i o n f o r t h e b e a m s p o t i s 1 cm . F o r p o w d e r s , p e l l e t i z i n g i s r e c o m m e n d e d ; g r a p h i t e a n d b o r i c a c i d a r e e x c e l l e n t b i n d i n g a g e n t s [ 1 9 ] . L i q u i d s c a n b e e n c a p s u l a t e d i n t h i n - w i n d o w c e l l s a n d c a n , w i t h p r o p e r p r e c a u t i o n , b e i r r a d i a t e d i n v a c u u m o r i n a i r [20, 2 1 ] . G a s e s h a v e a l s o b e e n a c t i v a t e d u s i n g f l o w - t h r o u g h i r r a d i a t i o n c e l l s [ 2 2 ] . I n t h e c a s e o f s o l i d s , o n e o f t h e s i g n i f i c a n t a d v a n t a g e s o f a c t i v a t i o n a n a l y s i s i s t h e p o s s i b i l i t y o f r e m o v i n g s u r f a c e c o n t a m i n a t i o n w i t h a p o s t - i r r a d i a t i o n m e c h a n i c a l o r c h e m i c a l e t c h d e e p e n o u g h t o r e m o v e t h e a c t i v a t e d s u r f a c e l a y e r i n c l u d i n g r e c o i l a c t i v i t y . T h e e t c h i n g p r o c e d u r e m u s t b e c a r e f u l l y d e s i g n e d i n t h e c a s e o f s u b - p p m d e t e r m i n a t i o n s p a r t i c u l a r l y o f s u c h e l e m e n t s a s o x y g e n o r c a r b o n t o a v o i d e r r o r s d u e t o " g r i n d i n g i n " o f c o n t a m i n a n t s b y m e c h a n i c a l a b r a s i o n o r r e a d s o r p t i o n o f a c t i v i t y o n t o t h e s a m p l e s u r f a c e i n t h e c h e m i c a l e t c h [ 2 3 ] . S e v e r a l a u t h o r s h a v e f o u n d t h a t r a d i a t i o n d a m a g e c a n r e s u l t i n d i f f e r e n c e s i n t h e e t c h i n g r a t e . E v e n i f o n l y a v e r y s m a l l f r a c t i o n o f t h e s a m p l e s u r f a c e r e m a i n s i n c o m p l e t e l y c l e a n e d , t h i s c a n s i g n i f i c a n t l y a f f e c t t h e r e s u l t s . T h e a c c u r a c y o f t h e a n a l y s i s i s c r i t i c a l l y d e p e n d e n t o n t h e p r e s e r v a t i o n o f t h e s a m p l e i n t e g r i t y d u r i n g p a r t i c l e b o m b a r d m e n t . P a r t i a l o r t o t a l l o s s e s o f t h e s p e c i e s o f i n t e r e s t c a n o c c u r d u e t o s a m p l e h e a t i n g , d i f f u s i o n , r e c o i l . C o n v e r s e l y , a d d i t i o n a l i m p u r i t i e s a r e d r i v e n i n t o a s a m p l e b y r e c o i l d u r i n g b o m b a r d m e n t . " A n o m a l o u s " d i f f u s i o n o f s o m e t r a c e e l e m e n t s i n c e r t a i n m e t a l u n d e r c h a r g e d p a r t i c l e b o m b a r d m e n t h a s a l s o b e e n o b s e r v e d [ 2 4 ] .

Irradiation Set-up. T h e h a r d w a r e c o m p o n e n t s n e e d e d a t t h e i r r a d i a t i o n s i t e a r e s u m m a r i z e d i n F i g u r e 1 . T h i s s c h e m a t i c d i a g r a m s h o w s t h e s i m p l e s e t - u p f o r a s i n g l e t a r g e t [ 2 5 ] .T h e s a m p l e h o l d e r m ay b e r e p l a c e d w i t h a r o t a t i n g t a r g e t w h e e l a c c o m o d a t i n g m u l t i p l e s a m p l e s , o r a t h i n w in d o w e n d p l a t e m a y b e s u b s t i t u t e d f o r i r r a d i a t i o n s i n a i r . - T h i s l a t t e r a r r a n g e m e n t i s p a r t i c u l a r l y w e l l s u i t e d f o r w o r k w h e r e a f a s t s a m p l e t r a n s f e r s y s t e m i s n e e d e d [ 2 6 , 2 7 ] , o r f o r i n - s i t u c o u n t i n g o f v e r y s h o r t - l i v e d r a d i o i s o t o p e s [ 2 8 ] . F i n a l l y t h e t a r g e t h o l d e r m a y b e r e p l a c e d w i t h a t a r g e t c h a m b e r ( F i g u r e 2 ) w h e n p r o m p t r a d i a t i o n m e a s u r e m e n t s a r e c o u p l e d w i t h n u c l e a r a c t i v a t i o n [ 2 9 ] . T h e e x p l o i t a t i o n o f v e r y s h o r t - l i v e d n u c l i d e s ( T j , < 1 s e c ) r e q u i r e s p u l s i n g o f t h e c h a r g e d p a r t i c l e b e a m t o a c h i e v e m ax im u m b u i l d - u p o f t h e n u c l i d e s o f i n t e r e s t w h i l e m i n i m i z i n g t h e p r o d u c t i o n o f l o n g e r l i v e d s p e c i e s . C l e a r l y , i r r a d i a t i o n s m u s t b e c a r r i e d o u t u n d e r p r e c i s e l y r e p r o d u c i b l e c o n d i t i o n s . S e v e r a l m e t h o d s h a v e b e e n d e s c r i b e d f o r V a n d e G r a a f f a c c e l e r a t o r s [ 3 0 , 3 1 ] , f o r c y c l o t r o n s t h e p u l s i n g o f t h e D e e v o l t a g e w a s f o u n d t o b e v e r y s a t i s f a c t o r y [ 2 8 ] .

Counting. F o r у - r a y s p e c t r o m e t r y G e ( L i ^ d e t e c t o r s a r e u s e d , N a l ( T l ) a r e p r e f e r r e d f o r m e a s u r i n g g a n n i h i l a t i o n r a d i a t i o n b y y-y c o i n c i d e n c e c o u n t i n g . X - r a y c o u n t i n g i s c a r r i e d o u t w i t h t h i n G e ( L i ) a n d S i ( L i ) d e t e c t o r s [ 1 9 ] . S p e c i a l c o u n t i n g t e c h n i q u e s h a v e b e e n d e s c r i b e d f o r h i g h e n e r g y ß - e m i t t e r s , u s i n g C e r e n k o v d e t e c t o r s [ 3 2 ] o r t h i n p l a s t i c s c i n t i l l a t o r s [ 3 3 ] . F o r d a t a r e d u c t i o n , numerous y - r a y s p e c t r a l a n a l y s i s p rogram s a r e a v a i l a b l e ( e . g . 34 , 3 5 ) , f o r d e c ay c u r v e a n a l y s i s t h e " C L S Q " p rogram o f Cunning i s w id e l y u s e d [ 3 6 ] .

6 SC H W E IK E R T

1 . 4 INCH ID ALUMINUM P IP E FOR BEAM TRANSPORT2 . QUADROPOLE FOCUSING MAGNET3 . HIGH VACUUM TURBINE PUMP4 . ISO LA TIO N VALVE5 . GRAPHITE BEAM MONITOR FOR TOTAL BEAM CURRENT MEASUREMENT6 . ALUMINUM OXIDE BEAM VIEWER7 . T E L E V ISIO N CAMERA8 . ROUGHING PUMP9 . GRAPHITE COLLIMATOR

1 0 . SAMPLE HOLDER

F IG . I. Experim enta l configuration o f cyclotron terminal fo r activation analysis [25].

A Faraday C u p s

В Stepping Motor

С Target W h e e l

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E Graphite B e a m Collimator

F Target

G X - r a y Collimator

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J 1 Mil B e W i n d o w

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L 1 0 0 M e s h Ni Grid

F IG .2 . Set-up fo r com b ined nuclear and atom ic activation [29].

2 . 3 Q u a n t i t a t i v e A s p e c t s

D i f f e r e n t m e t h o d s h a v e b e e n d e v i s e d f o r q u a n t i t a t i v e c a l c u l a t i o n s a p p l i c a b l e t o t h i c k t a r g e t s . Am ong t h e s e i s t h e a v e r a g e c r o s s s e c t i o n ( a ) c o n c e p t o f R i c c i e t a l . [ 3 7 ] о i s d e f i n e d a s f o l l o w s :

Ra d x

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d x=

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a d x x ( 3 )

C H A R G E D -P A R T IC L E A C T IV A T IO N A N A L Y S IS 7

w h e r e a i s t h e v a l u e o f t h e c r o s s s e c t i o n a t d e p t h x i n t h e t a r g e t I n d R i s t h e r a n g e o f t h e p a r t i c l e s i n t h a t t a r g e t . F o r a c t i v a t i o n a t s a t u r a t i o n , o n e o b t a i n s :

w h e r e D i s t h e n u m b e r o f d i s i n t e g r a t i o n s p e r s e c o n d , b i s t h e i n t e n s i t y o f t h e p a r t i c l e b e a m , n i s t h e n u m b e r o f t a r g e t a t o m s i n t h e i r r a d i a t e d s a m p l e . O b v i o u s l y о i s c o n s t a n t f o r a g i v e n r e a c t i o n a t a g i v e n i n c i d e n t p a r t i c l e e n e r g y . A c o m p a r i s o n o f t h e d i s i n t e g r a t i o n r a t e s o f a s a m p l e , u , a n d a s t a n d a r d , s , b o t h i r r a d i a t e d u n d e r t h e s a m e c o n d i t i o n s g i v e s t h e n :

E q u a t i o n 5 i n d i c a t e s a l s o t h a t t h e s i m p l e r e l a t i v e c o m p a r i s o n m e t h o d c a n b e u s e d i n CPAA p r o v i d e d a n a p p r o p r i a t e s t a n d a r d i s a v a i l a b l e a n d r a n g e c o r r e c t i o n s a r e m a d e . A l t h o u g h i n h e r e n t l y s i m p l e , t h e c o m p a r i s o n m e t h o d c a n o n l y b e a p p l i e d i f t h e t r a c e e l e m e n t i s k n o w n t o b e h o m o g e n e o u s l y d i s t r i b u t e d i n t h e s a m p l e u s e d a s s t a n d a r d . T h e i n t e r n a l s t a n d a r d m e t h o d , s u b j e c t t o t h e s a m e r e q u i r e m e n t , c a n a l s o b e u s e d [ 3 8 ] .A n o t h e r a p p r o a c h w a s d e v e l o p e d b y E n g e l m a n n [ 3 9 ] . I t i s b a s e d o n t h e c o n c e p t o f a n e q u i v a l e n t t h i c k n e s s , e , o v e r w h i c h t h e s p e c i f i c a c t i v i t y o f t h e t r a c e s p e c i e s w o u l d b e c o n s t a n t . I n t h i s c a s e , r e f e r r i n g t o e q . 1 g i v e n b e f o r e , t h e t e r m

m a y b e e x p r e s s e d a s

w h e r e b i s t h e i n t e n s i t y o f t h e i n c i d e n t p a r t i c l e b e a m a n d a t h e c r o s s s e c t i o n f o r a n i n f i n i t e l y t h i n t a r g e t ( d x ) i r r a d i a t e d a t t h e i n c i d e n t e n e r g y . I t c a n b e r e a d i l y sh o w n t h a t w h e n t h e s a m p l e w e i g h t i s e x p r e s s e d a s a f u n c t i o n o f e , a d i r e c t c o m p a r i s o n b e t w e e n t h e w e i g h e d a c t i v i t i e s i n d u c e d i n a t h i n m o n i t o r f o i l o f k n o w n c o m p o s i t i o n a n d i n t h e s a m p l e c a n b e m a d e . T h e v a l u e s f o r a x , a0 a r e d e r i v e d f r o m a n e x p e r i m e n t a l l y m e a s u r e d a c t i v a t i o n c u r v e ( i . e . r e l a t i v e e x c i t a t i o n f u n c t i o n ) f o r a g i v e n n u c l e a r r e a c t i o n i n a g i v e n m a t r i x . An i m p o r t a n t f e a t u r e o f t h i s p r o c e d u r e i s t o m i n i m i z e s y s t e m a t i c e r r o r s d u e t o d i f f e r e n c e s i n b e a m c h a r a c t e r i s t i c s f o r d i f f e r e n t a c c e l e r a t o r s a n d i r r a d i a t i o n p o s i t i o n s [ 3 9 - 4 2 ] . R o o k e t a l . h a v e s h o w n t h a t t h e e q u i v a l e n t t h i c k n e s s m e t h o d i s r e a d i l y u s a b l e i n p r a c t i c e s i n c e a n a c t i v a t i o n c u r v e d e t e r m i n e d e x p e r i m e n t a l l y i n a m a t r i x c a n b e t r a n s f o r m e d i n t o t h e c o r r e s p o n d i n g a c t i v a t i o n c u r v e i n a n y o t h e r m a t r i x b y u s i n g t h e d i f f e r e n t i a l r a n g e - e n e r g y

D = b n o R ( 4 )

о


r e l a t i o n s h i p [ 4 3 ] . V a l u e s f o r r a n g e s c a n b e o b t a i n e d f r o m t h e t a b l e s o f W i l l i a m s o n e t a l . [ 4 4 ] .

An i m p o r t a n t a s p e c t o f q u a n t i t a t i v e w o r k c o n c e r n s t h e v a l i d i t y o f t h e n u m b e r s o b t a i n e d . A s f a r a s t h e a c c u r a c y i s c o n c e r n e d , t h e r e a r e tw o s o u r c e s o f a b s o l u t e e r r o r s : i n t e r f e r i n g r e a c t i o n s a n d n o n h o m o g e n e ô u s i m p u r i t y d i s t r i b u t i o n . T h e m e t h o d s o f q u a n t i t a t i o n u s e d a s s u m e a u n i f o r m i m p u r i t y d i s t r i b u t i o n . CPAA i s h o w e v e r i n h e r e n t l y v e r y s e n s i t i v e t o c o n c e n t r a t i o n g r a d i e n t s [ 4 5 ] . A n u m b e r o f d i f f e r e n t f a c t o r s a f f e c t t h e p r e c i s i o n , t h e s e w i l l o n l y b e b r i e f l y l i s t e d h e r e :

- I n t e g r i t y o f t h e s t a n d a r d s - R e s o l u t i o n o f t h e a c t i v a t i o n o r e x c i t a t i o n

f u n c t i o n s ( w e l l d e f i n e d c u r v e s a r e n e c e s s a r y f o r a c c u r a t e i n t e g r a t i o n a n d c o m p a r i s o n w i t h t h i n m o n i t o r f o i l s )

- R e s o l u t i o n o f t h e b e a m e n e r g y a n d c u r r e n t m e a s u r e m e n t s

- Q u a l i t y o f t h e p o s t - i r r a d i a t i o n e t c h - M e a s u r e m e n t e r r o r s i n t h e s a m p l e t h i c k n e s s b e f o r e

a n d a f t e r e t c h i n g - C o u n t i n g s t a t i s t i c s a n d d e c a y c u r v e a n a l y s i s . A

s e l e c t i o n h a s t o b e m a d e a m o n g a n u m b e r o f c o m p o n e n t s , w h i c h c o u l d b e p r e s e n t , t o r e t a i n o n l y t h o s e w h i c h a r e e f f e c t i v e l y m a j o r c o n t r i b u t o r s o f t h e o b s e r v e d g r o s s a c t i v i t y . ( I t m i g h t b e n o t e d h e r e t h a t e x c e l l e n t f i t s c a n b e o b t a i n e d f o r a n y d e c a y c u r v e p r o v i d e d a l a r g e e n o u g h n u m b e r o f c o m p o n e n t s a r e c o n s i d e r e d e v e n i f t h o s e c o m p o n e n t s h a v e w r o n g h a l f - l i f e v a l u e s ) .I n s u m m a r y , s a t i s f a c t o r y a c c u r a c y a n d p r e c i s i o n m a y b e a c h i e v e d w i t h r e p e a t e d a n a l y s e s u s i n g w h e n e v e r p o s s i b l e d i f f e r e n t n u c l e a r r e a c t i o n s .

T A B L E I. A C T IV A T IO N R E A C T IO N S

ActivationReaction Interfering Reactions

Maximum "Safe" Energy

l®0(p,n)18f 19р(р,рп) 10 MeV

l60(3He,p)18F 19F(3He,a)18F, 20Ne(3He,an)18Ne — >18F,

160(3He,n)18Ne — > 18F 23Na (3H e ,2a)18F , 2*Mg(3H e ,2ap)18F ,

2?Al(3He,3a) 18F , 29si(3He,14N)18F

15 MeV

160(a,pn)18F

^ ( a . n ) 18f , 19F(a,an)18F,

2ONe(a,ad)18F, 23Na(a>a2n ) 18F,

2¿Mg(a,2ad)18F, 27Al(a,3an)18F,

28Si(a,14N)18F, 29Si(a,15N)18F

< 35 MeV


3 . 0 SCOPE

A s n o t e d b e f o r e , t h e m a j o r t h r u s t o f CPAÀ h a s b e e n a n d s t i l l i s i n t h e f i e l d o f l i g h t e l e m e n t a n a l y s i s [ 1 7 , 4 6 ] , b u t g r o w i n g a t t e n t i o n h a s b e e n g i v e n t o t h e t r a c e d e t e c t i o n o f m e d i u m a n d h i g h Z e l e m e n t s , f r o m c a l c i u m t o l e a d . N o e x h a u s t i v e r e v i e w c a n b e g i v e n h e r e , u s e f u l f u r t h e r i n f o r m a t i o n c a n b e f o u n d i n r e f e r e n c e s [ 4 7 - 5 7 ] . A s f a r a s a p p l i c a t i o n s a r e c o n c e r n e d , CPAA h a s b e e n f o u n d s u i t a b l e f o r s o l v i n g m a n y p r o b l e m s i n d i f f e r e n t f i e l d s . A n u m b e r o f r e p o r t s d e a l w i t h t h é c h a r a c t e r i z a t i o n o f s e m i c o n d u c t o r m a t e r i a l s , m o r e s p e c i f i c a l l y t h e d e t e r m i n a t i o n o f t r a c e s o f l i t h i u m , b o r o n , c a r b o n , n i t r o g e n o r o x y g e n i n s i l i c o n [ 4 0 , 5 8 - 7 6 ] , i n g e r m a n i u m [ 6 3 , 7 7 - 8 0 ] , i n g a l l i u m p h o s p h i d e [ 8 0 , 8 1 ] o r i n c h a l c o g e n i d e g l a s s e s [ 8 2 ] . O t h e r w o r k h a s d e a l t w i t h t h e a s s a y o f e n v i r o n m e n t a l s a m p l e s [ 5 5 ] , b i o l o g i c a l s p e c i m e n s [ 8 3 ] , g e o l o g i c a l m a t e r i a l s [ 8 4 ] , p e t r o l e u m p r o d u c t s [ 2 0 , 2 1 ] a n d m a n y h i g h p u r i t y m e t a l s [ 1 9 , 5 2 - 5 4 , 5 7 , 8 5 - 8 7 ] . So m e a p p l i c a t i o n s o f CPAA s t u d i e d i n o u r l a b o r a t o r y a r e f u r t h e r o u t l i n e d b e l o w , f o l l o w e d b y a c o m p a r i s o n w i t h r e l a t e d t e c h n i q u e s .

3 . 1 S i n g l e E l e m e n t A n a l y s i s

T h e f l e x i b i l i t y i n e x p e r i m e n t a l p a r a m e t e r s ( i . e . t y p e a n d e n e r g y o f b o m b a r d i n g p a r t i c l e s ) c a n b e t a k e n a d v a n t a g e o f t o o p t i m i z e a n a n a l y s i s p r o c e d u r e f o r a g i v e n p r o b l e m . T h e e x a m p l e s c h o s e n i l l u s t r a t e t h e d i f f e r e n t r e q u i r e m e n t s t h a t a c c o m p a n y d i f f e r e n t a p p l i c a t i o n s : a c c u r a t e u l t r a t r a c ea n a l y s i s ( < ppm ) i n s u p p o r t o f s o l i d s t a t e r e s e a r c h , t r a c e a n a l y s i s ( > ppm ) a p p l i c a b l e t o a l a r g e n u m b e r o f v a r i e d s a m p l e s , i s o t o p i c r a t i o m e a s u r e m e n t s .

3 . 1 . 1 D e t e r m i n a t i o n o f O x y g e n

T h e a c t i v a t i o n r e a c t i o n s s u c c e s s f u l l y u s e d i n p r a c t i c e a r e s u m m a r i z e d i n T a b l e I . A l t h o u g h t h e n u m b e r o f p o s s i b l e i n t e r f e r i n g r e a c t i o n s a p p e a r s o v e r w h e l m i n g , t h e s e a r e i n a l m o s t a l l c a s e s n e g l i g i b l e f o r b o m b a r d i n g e n e r g i e s u p t o t h e m a x im u m " s a f e " e n e r g i e s i n d i c a t e d . R e a s o n s f o l l o w :

- F l u o r i n e , n e o n , s o d i u m , m a g n e s i u m a n d a l u m i n u m a r e u n l i k e l y t o b e p r e s e n t o r a t m u c h l o w e r c o n c e n t r a t i o n l e v e l s t h a n o x y g e n [ 1 7 , 6 9 ] .

- T h e s e i n t e r f e r e n c e s a r e , f u r t h e r m i n i m i z e d i n t h e c a s e o f 3He a c t i v a t i o n , w h e r e t h e 1 8 F y i e l d s f r o m t h e s e e l e m e n t s a r e o n e o r s e v e r a l o r d e r s o f m a g n i t u d e s m a l l e r t h a n t h a t r e s u l t i n g f r o m 16 0 ( 3H e , p ) 1 8 F [ 8 1 ] .

- T h e r e l a t i v e e f f e c t s o f i n t e r f e r i n g r e a c t i o n s v a r y w i d e l y w i t h d i f f e r e n t a c t i v a t i n g p a r t i c l e s . T h e p o s s i b i l i t y o f e r r o r s d u e t o i n t e r f e r e n c e s c a n t h u s a l s o b e c h e c k e d a p o s t e r i o r i b y c o m p a r i n g r e s u l t s o b t a i n e d u s i n g d i f f e r e n t a c t i v a t i o n m o d e s . T h e a g r e e m e n t o n t h e d a t a o b t a i n e d , i n t h e c a s e o f

1 0 SC H W E IK E R T

T A B L E II . O X Y G E N IN S IL IC O N 1 0 - 1 5 p p m L E V E L , a A T T H E 9 5 %

C O N F ID E N C E L E V E L

Sample Particle Energy (MeV) ' РШ CT

1 *He 34 12.0

1 ^ e 33 13.7

1 lH 12 11.9AVERAGE 1 12.5 + 1.0

2 Зне 12 15.4

lH 10 14.0

AVERAGE 2 14.7 + 0.8

T A B L E I I I . O X Y G E N IN S IL IC O N 1 - 1 0 p p m L E V E L

Sample

34 MeV a Activation

(ppm)

InfraredSpectroscopy*

(ppm)

622368 10.9 9.4

623364 8.3 7.4

623363 5.6 5.7

623352 4.7 4.8

*Data supplied by J. A. Baker, Dow Corning Corp., Hemlock, MI., USA

s i l i c o n ( s e e r e s u l t s b e l o w ) , c o n f i r m s a g a i n t h a t t h e s e e r r o r s a r e n e g l i g i b l e a t t h e o x y g e n l e v e l s m e a s u r e d t h u s f a r (>^ 1 0 p p b ) .

- I n s i l i c o n , t h e e f f e c t o f t h e " f i s s i o n " r e a c t i o n S i ( 3H e , x ) 1 8 F i s n e g l i g i b l e f o r 3.He e n e r g i e s u p t o 1 5 MeV f o r o x y g e n l e v e l s a t l e a s t a s l o w a s ' 1 p p b [ 1 7 , 6 9 ] . When a a c t i v a t i o n , (E<j > 3 5 M e V ) , i s a p p l i e d o n s i l i c o n , S i ( a , x ) 1 8 F p r e c l u d e s o x y g e n m e a s u r e m e n t s b e l o w 0 . 0 5 ppm [ 1 7 ] .

A p o s t - i r r a d i a t i o n e t c h i s a b s o l u t e l y n e c e s s a r y f o r t r a c e d e t e r m i n a t i o n s o f o x y g e n , c a r b o n o r n i t r o g e n . T h i s s t e p a n d o t h e r i t e m s t o b e c o n s i d e r e d i n s a m p l e h a n d l i n g h a v e b e e n d e s c r i b e d e a r l i e r . I n t h e c a s e o f s i l i c o n , t h e CPAA p r o c e d u r e i s n o n d e s t r u c t i v e d u e t o ( a ) t h e s h o r t - l i v e d m a t r i x a c t i v i t i e s ( T ^ < 2 . 5 m i n ) a l l o w i n g t h e d e t e c t i o n o f t r a c e s p e c i e s w i t h >_ 2 0 m i n ( e . g . 1 8 F ) , a n d ( b ) i t s h i g h p u r i t y . F o r m a n y o t n e r m a t e r i a l s ( e . g . G e , A l , c h a l c o g e n i d e g l a s s e s ) , s u b - p p m d e t e r m i n a t i o n s c a n n o t b e c a r r i e d o u t n o n - d e s t r u c t i v e l y . A p o s t - i r r a d i a t i o n c h e m i c a l s e p a r a t i o n o f 1 8F f r o m o t h e r a c t i v i t i e s g e n e r a t e d i n t h e t a r g e t i s n e e d e d . S e v e r a l a u t h o r s h a v e d e s c r i b e d s u c h p r o c e d u r e s ( e . g . 7 2 , 7 7 ) .


T A B L E IV . E X P E R I M E N T A L D E T E C T IO N L I M I T S F O R T H E D E T E R M IN A T IO N

O F O X Y G E N IN S IL IC O N ( I R R A D I A T I O N O F 2 h A N D C O U N T R A T E O F

2 5 co u n ts/ m in A T t 0 ).

ReactionBeam Current uA/cm^

Max. Irradiation Energy (MeV)

Detection Limit (ppm)

180(p,n) « F 10 12 0.06

l60(a,pn)18F 4 34 0.05

1б0(3не,р)18F 4 15 0.005

O n c e a m e t h o d i s d e v e l o p e d ( i n c l u d i n g c o u n t i n g a n d q u a n t i t a t i o n s t e p s ) , i t s s u c c e s s f u l a p p l i c a t i o n d e p e n d s o n a c a r e f u l e v a l u a t i o n o f t h e a c c u r a c y , p r e c i s i o n a n d d e t e c t i o n l i m i t t h a t c a n b e a c h i e v e d u n d e r e x p e r i m e n t a l c o n d i t i o n s .T h e c a s e o f o x y g e n a n a l y s i s i n s i l i c o n c a n b e u s e d a s a n i l l u s t r a t i o n . On s a m p l e s c o n t a i n i n g o x y g e n a t t h e 1 0 ppm l e v e l , c o m p a r i s o n s o f r e s u l t s f r o m d i f f e r e n t a c t i v a t i o n m o d e s a n d f r o m t h e t o t a l l y i n d e p e n d e n t I R a b s o r p t i o n s p e c t r o s c o p y a r e p o s s i b l e . D a t a f r o m d u p l i c a t e a n a l y s e s o n s o m e s a m p l e s a r e g i v e n i n T a b l e s I I a n d I I I . R e s u l t s i n T a b l e I I v e r i f y t h a t f o r t h e c o n c e n t r a t i o n r a n g e c o n s i d e r e d , e r r o r s d u e t o i n t e r f e r i n g r e a c t i o n s a r e m i n i m i z e d . T h e a g r e e m e n t b e t w e e n t h e r e s u l t s b y a a c t i v a t i o n a n d b y I R v a l i d a t e s t h e a c t i v a t i o n m e t h o d a s a w h o l e a t t h e 1 0 ppm - l e v e l . F o r o x y g e n c o n c e n t r a t i o n s b e l o w 0 . 5 ppm o n l y CPAA c a n b e a p p l i e d . W i t h r e p e a t e d d e t e r m i n a t i o n s o n e a c h s a m p l e , a v e r a g e d e v i a t i o n s o f + 30% t o + 50% w e r e o b t a i n e d f o r o x y g e n l e v e l s r a n g i n g f r o m 1 t o 0 . Ü 5 ppm [ 1 7 ] . F o r c o n c e n t r a t i o n s a t o r b e l o w t h e 1 0 ppm l e v e l ( T a b l e I V ) , t h e a v e r a g e d e v i a t i o n m a y r e a c h + 1007» d u e t o t h e p o o r c o u n t i n g s t a t i s t i c s . I t m u s t b e e m p K a - s i z e d t h a t t h e CPAA d a t a r e f l e c t s a c c u r a t e l y t h e u l t r a t r a c e l e v e l s , i m p r o v e m e n t s i n t h e p r e c i s i o n c a n b e a n t i c i p a t e d a s m o r e a n a l y s e s o n s u c h u l t r a p u r e m a t e r i a l s a r e m a d e .

3 . 1 . 2 Q u á s i - p r o m p t A c t i v a t i o n

I d e a l l y , a n a n a l y s i s t e c h n i q u e s h o u l d b e s e n s i t i v e , s e l e c t i v e , r a p i d a n d r e q u i r e a m in im u m o f s a m p l e h a n d l i n g . Q u a s i - p r o m p t a c t i v a t i o n , i . e . t h e a n a l y t i c a l e x p l o i t a t i o n o f s h o r t - l i v e d n u c l i d e s ( T ^ < 1 s e c ) i s , a p r i o r i , a n a t t r a c t i v e a p p r o a c h f o r m e e t i n g t h e s e r e q u i r e m e n t s : o n l ys h o r t i r r a d i a t i o n s a r e r e q u i r e d , m a t r i x a c t i v i t i e s s h o u l d b e m i n i m i z e d (m a n y e l e m e n t s y i e l d l i t t l e o r n o v e r y s h o r t l i v e d a c t i v i t y w i t h c h a r g e d p a r t i c l e b e a m s ) , t h e s a m p l e s r e m a i n i n t a c t , m a x im u m s e n s i t i v i t y c a n b e o b t a i n e d w i t h r e p e t i t i v e i r r a d i a t i o n - c o u n t i n g c y c l e s , t h e c o s t p e r s a m p l e a n a l y z e d s h o u l d b e l o w d u e t o s h o r t b e a m t i m e r e q u i r e d a n d t h e e l i m i n a t i o n o f a n y p o s t - i r r a d i a t i o n s a m p l e h a n d l i n g . S e v e r a l l i g h t e l e m e n t s y i e l d v e r y s h o r t l i v e d s p e c i e s u n d e r c h a r g e d p a r t i c l e b o m b a r d m e n t . A m ong t h e s e , o n l y t h o s e t h a t a r e p r o d u c t s o f i n t e r f e r e n c e - f r e e r e a c t i o n s w i t h a d e q u a t e c r o s s s e c t i o n s a r e o f i n t e r e s t . An a d d i t i o n a l r e q u i r e m e n t

T A B L E V . Q U A S I - P R O M P T A C T I V A T I O N 3

R e a c t i o n TJ¿ m s e c Eßm ax

I n t e r f e r e n c e - f r e e D e t e c t i o n L i m i t s

ppmb )

E x p e r i m e n t a l D e t e c t i o n L i m i t s

p p m c )

1 2 C ( p , n ) 12 N 1 1 1 6 . 4 0 . 2 5 5 i n i r o n

1 4 N ( p , 2 n ) 1 3 0 8 . 7 16 2 0 0 0

2l+M g ( p , n ) 21tmA l 1 2 9 1 3 . 3 3 5 0

2 8 S i ( p , n ) 2 8 P 2 7 0 1 1 . 5 2 5

3 2 S ( p , n ) 3 2 C l 2 9 7 9 . 4 3 0 7 5 i n p e t r o l e u m p r o d u c t s

lt6T i ( p , n ) l t6V 4 8 6 6 . 6 3 0 0

5 0 C r ( p , n ) 5 0 Mn 2 8 8 6 . 1 4 5 0

5 4 F e ( p , n ) 5t+Co 1 9 4 7 . 3 5 0 0

7L i ( d , p ) 8L i 8 5 0 1 3 . 0 0 . 1 5 0 . 5 i n g e r m a n i u m

u B ( d , p ) 1 2 B 2 0 1 3 . 4 0 . 2 5 S e e T a b l e V I

4 0 C a ( d , n ) l+1S c 6 0 0 5 . 5 1 0 0 0

a ) l n c i d e n t p a r t i c l e e n e r g y 4 0 MeV f o r 12C ( p , n ) 12N a n d 11+N ( p , 2 n ) 130 r e a c t i o n s . A l l o t h e r r e a c t i o n s s t u d i e d a t 2 0 MeV.

b )

c )

B a s e d o n 1 0 r e p e t i t i v e i r r a d i a t i o n s a t 2 УА a n d 1 0 0 c o u n t s f o r a c o u n t t i m e e q u a l t o Ti^ o f t h e p r o d u c t n u c l i d e .S a m e i r r a d i a t i o n c o n d i t i o n s a s f o r b ) .

C H A R G E D -P A R T IC L E A C T IV A T IO N A N A L Y S IS 1 3

T A B L E V I . R E S U L T S O F B O R O N D E T E R M IN A T IO N S A N D E X P E R I M E N T A L

L I M I T S O F D E T E C T IO N W IT H D E U T E R O N A C T IV A T IO N [ 2 8 ]

SampleAmount found (ppm) Experim ental

detection limit (ppm)Quasi-prompt Other methods

NBS glass SRM 610 352 ± 7 a 3 5 1 b 2 - 3

NBS glass SRM 612 38 ± 5a 3 2 b 2 - 3

Si IX 225 73 ± 5a 77° 0.5

Si IX 252 11 ± 2a I I e 0.5

Si IX 253 2.3 ± 0 .7 a 1 - 2 ° 0.5

NBS SRM 1571 (orchard leaves)

3 6 ± 5 33 ± 3b 0 .75

a Average deviation based on 10 determinations, k Non-certified value, National Bureau o f Standards.c Data supplied by F.A . Thrum bore, Bell Laboratories, Murray Hill, N J, USA.

i s t h a t t h e r a d i o n u c l i d e m u s t e m i t c h a r a c t e r i s t i c -у- r a y s o r b e a h i g h e n e r g y 8 - e m i t t e r . T h i s i s n e c e s s a r y f o r i t s s e l e c t i v e i d e n t i f i c a t i o n w h i c h i s b a s e d o n t h e t y p e a n d e n e r g y o f t h e e m i s s i o n a n d t h e h a l f - l i f e . A c o r o l l a r y i s t h a t n o o t h e r n u c l i d e s o f s i m i l a r o r c l o s e d e c a y c h a r a c t e r i s t i c s s h o u l d b e p r o d u c e d c o n c u r r e n t l y . A s u r v e y o f q u a s i p r o m p t a c t i v a t i o n r e a c t i o n s y i e l d i n g n u c l i d e s w i t h 1 0 <. T ^ <. 1 0 0 0 m s e c a n d m e e t i n g t h e a b o v e c r i t e r i a i s g i v e n i n T a b l l V . B a s e d o n t h e t h i c k t a r g e t y i e l d s , p r o t o n o r d e u t e r o n a c t i v a t i o n a p p e a r s f e a s i b l e f o r m e a s u r i n g l i t h i u m , b o r o n , c a r b o n , s i l i c o n o r s u l f u r . T h e r e l a t i v e e x c i t a t i o n f u n c t i o n s f o r 7L i ( d , p ) 8L i a n d 1 1 B ( d , p ) 1 1 B s h o w l o w t h r e s h o l d a n d p e a k e n e r g i e s , t h u s t h e s e r e a c t i o n s c a n a l s o b e u s e d w i t h l o w e r e n e r g y a c c e l e r a t o r s ( E ¿ = 5 MeV f o r e x . ) . S p e c i a l e x p e r i m e n t a l r e q u i r e m e n t s , b e a m p u l s i n g a n d s e l e c t i v e a n d e f f i c i e n t d e t e c t i o n o f h i g h e n e r g y ß p a r t i c l e s m u s t b e m e t . T h e s e h a v e b e e n a d d r e s s e d e a r l i e r . A s e r i e s o f r e s u l t s o f b o r o n d e t e r m i n a t i o n s a n d e x p e r i m e n t a l d e t e c t i o n l i m i t s a r e g i v e n i n T a b l e V I . W i d e l y d i f f e r e n t k i n d s o f s a m p l e s w e r e a s s a y e d a n d b e c a u s e o f t h e s h o r t b e a m t i m e n e e d e d f o r e a c h s a m p l e , l a r g e n u m b e r s o f s p e c i m e n s c a n b e p r o c e s s e d r e a d i l y . A t y p i c a l d e c a y c u r v e f r o m t h e a n a l y s i s o f b o r o n i n s i l i c o n i s g i v e n i n F i g u r e 3 . I t s h o w s t h a t l i t t l e m a t r i x a c t i v i t y i s p r o d u c e d i n s i l i c o n w i t h a 2 0 m s e c b o m b a r d m e n t w i t h 2 0 MeV d e u t e r o n s . I n t h e c a s e o f l i t h i u m , pp m s e n s i t i v i t y c a n a l s o b e a c h i e v e d i n v e r y d i f f e r e n t m a t r i c e s . Q u a s i - p r o m p t p r o c e d u r e s f o r c a r b o n , s u l f u r a n d s i l i c o n a r e l e s s s e n s i t i v e w i t h i n t e r f e r e n c e s b e t w e e n s u l f u r a n d s i l i c o n . A m ong t h e a p p l i c a t i o n s r e p o r t e d a r e t h e d é t e r m i n a t i o n o f c a r b o n i n s t e e l [ 2 8 ] a n d o f s u l f u r i n p e t r o l e u m p r o d u c t s [ 2 0 , 2 1 ] .


T IM E (m s )

F IG .3 . Q uasi-prom pt activation. D e ca y curve fo r boron in silicon (11.2 ppm Bj.

3 . 1 . 3 I s o t o p i c R a t i o M e a s u r e m e n t s

A m ong t h e e l e m e n t s w i t h v a r y i n g n a t u r a l . i s o t o p i c a b u n d a n c e s , l e a d h a s r e c e i v e d p a r t i c u l a r a t t e n t i o n . B a s e d o n l e a d i s o t o p e r a t i o s a g e d e t e r m i n a t i o n s i n g e o l o g i c a l s p e c i m e n s c a n b e m a d e [ 5 6 1 , p a t t e r n s o f e n v i r o n m e n t a l p o l l u t i o n c a n b e e s t a b l i s h e d w i t h l e a d " f i n g e r p r i n t s " [ 5 5 ] . T h i s d i s c u s s i o n r e v i e w s o n l y p r o c e d u r e s f o r d e t e r m i n i n g 2 0 4 P b / 2 0 6 P b . D i f f e r e n t a c t i v a t i o n r e a c t i o n s m a y b e u s e d f o r t h i s p u r p o s e [ 5 7 ] . E x a m p l e s o f s o m e r e l e v a n t p r o t o n a n d d e u t e r o n r e a c t i o n s a r e l i s t e d i n T a b l e V I I . T h e a c t i v a t i o n m e t h o d c a n n o t m a t c h t h e p r e c i s i o n o f m a s s s p e c t r o m e t r y . B u t f o r a p p l i c a t i o n s w h e r e a p r e c i s i o n o f a f e w p e r c e n t i s s u f f i c i e n t , CPAA i s p r e f e r r e d b e c a u s e o f i t s s i m p l i c i t y ( l i t t l e o r n o s a m p l e p r e p a r a t i o n ) a n d i n h e r e n t a c c u r a c y ( f r e e d o m f r o m r e a g e n t b l a n k s ) . S t a b l e t r a c e r a n d e n v i r o n m e n t a l s t u d i e s a r e a m o n g s u c h a p p l i c a t i o n s [ 5 5 ] . I t s h o u l d b e n o t e d t h a t m a n y c h a r g e d p a r t i c l e a c t i v a t i o n r e a c t i o n s a r e a v a i l a b l e f o r t o t a l l e a d d e t e r m i n a t i o n s , d e t e c t i o n l i m i t s a s l o w a s 1 p p b h a v e b e e n r e p o r t e d [ 8 8 ] . L i k e i n t h e c a s e o f t h e l o w Z e l e m e n t s , CPAA p r o v i d e s f o r s o m e o f t h e h e a v i e s t s p e c i e s , T l , P b , B i , a u n i q u e c o m b i n a t i o n o f s e n s i t i v i t y , s e l e c t i v i t y a n d a p p l i c a b i l i t y t o m a n y d i f f e r e n t k i n d s o f s a m p l e s [ 1 8 ] .


T A B L E V I I . R E A C T IO N S U S E D F O R L E A D IS O T O P E R A T IO

M E A S U R E M E N T S [ 5 5 ]

ReactionThresholdenergy(M eV)

Half-lifePrincipal gamma rays (keV )

Interfering reactions

204P b(p ,n)204Bi ~ 10 11.2 h 376 206P b(p ,3n)204Bi if E p > 2 0 MeV

206Pb(p ,2n) Bi ~ 12 15.3 d 703 207P b(p ,3n )205Bi if Ep > 18.5 MeV

204P b(d ,2n )204Bi ~ 8 11.2 h 376 206Pb(d ,4n )204Bi if E d > 22 MeV

206P b(d ,2n )206Bi ~ 8 6 .2 4 d 8 0 3 , 8 8 0 207P b(d ,3n )206Bi if Ed > 13.5 MeV

3 . 2 N o n d e s t r u c t i v e M u l t i e l e m e n t A n a l y s i s

P r o t o n a c t i v a t i o n h a s t h e i n h e r e n t p o t e n t i a l f o r m e e t i n g t h e r e q u i r e m e n t s o f m u l t i e l e m e n t a n a l y s i s : b r o a d e l e m e n t a lc o v e r a g e , s e n s i t i v i t y (p p m a n d s u b - p p m r a n g e ) a n d s e l e c t i v i t y . M o r e o v e r f o r n o n d e s t r u c t i v e a n a l y s i s , d i s c r i m i n a t i o n o f i m p u r i t y v s . m a t r i x a c t i v a t i o n i s p o s s i b l e i n m a n y c a s e s , b a s e d o n t h e d i f f e r e n c e s i n Q - v a l u e s f o r p r o t o n i n d u c e d r e a c t i o n s . P r o t o n s o f 1 0 t o 1 5 MeV e n e r g y a r e m o s t a d v a n t a g e o u s , y i e l d i n g h i g h s p e c i f i c a c t i v i t i e s f o r ( p , n ) r e a c t i o n s . P o s s i b l e i n t e r f e r i n g r e a c t i o n s [ ( p , a ) , ( p , 2 n ) , ( p , p n ) ,( p , d ) ] h a v e l o w c r o s s s e c t i o n s a t t h e s e p r o t o n e n e r g i e s a n d a r e t h u s ( u s u a l l y ) n e g l i g i b l e s o u r c e s o f e r r o r s . M u l t i e l e m e n t d e t e r m i n a t i o n s c a n a l s o b e a c c o m p l i s h e d b y 3He a c t i v a t i o n [ 4 9 ] . F u r t h e r , m e n t i o n s h o u l d b e m a d e o f p r o c e d u r e s u t i l i z i n g f a s t n e u t r o n s p r o d u c e d b y a h i g h e n e r g y d e u t e r o n b e a m i m p i n g i n g o n a b e r y l l i u m t a r g e t [ 8 9 ] .

T h e f e a s i b i l i t y o f n o n d e s t r u c t i v e m u l t i e l e m e n t a n a l y s i s m u s t b e e v a l u a t e d o n a c a s e b y c a s e b a s i s . S e v e r a l i t e m s a f f e c t t h e p e r f o r m a n c e o f CPAA: ( a ) A c t i v i t i e s d u e t o m a j o r ,m i n o r a r i d / o r t r a c e c o m p o n e n t s o f t h e m a t r i x i m p o s e t h e a c t u a l d e t e c t i o n l i m i t s t h a t c a n b e a c h i e v e d w i t h y - r a y o r x - r a y s p e c t r o m e t r y . T h e s e a r e a l s o d e p e n d e n t o n t h e n u m b e r o f c o u n t i n g s , i . e . o n t h e e x t e n t t h a t d e t e c t i o n c o n d i t i o n s c a n b e o p t i m i z e d g i v e n t h e h a l f l i v e s o f t h e d i f f e r e n t r a d i o n u c l i d e s i n v o l v e d , ( b ) G a m m a - r a y i n t e r f e r e n c e s m a y o c c u r [ 5 0 , 5 2 ] . E r r o r s c a n b e a v o i d e d b y i d e n t i f y i n g p e a k s v i a d e c a y c u r v e a n a l y s i s a n d / o r b y u s i n g s e v e r a l у - r a y s t o m e a s u r e t h e s a m e n u c l i d e , ( c ) T h e m a g n i t u d e o f n u c l e a r i n t e r f e r e n c e s c a n b e r e a d i l y a s s e s s e d e x p e r i m e n t a l l y . T h r e e e x a m p l e s o f a p p l i c a t i o n s a r e d e s c r i b e d b e l o w , t h e y i l l u s t r a t e t h e d i v e r s i t y o f t r a c e a n a l y s i s p r o b l e m s t h a t c a n b e h a n d l e d b y CPAA.

T A B L E V I I I . N O N D E S T R U C T I V E T A N T A L U M A N A L Y S I S [ 5 0 ] a

Gamma-ray Interferences^ ExperimentalActivation Data Mode of Detection Limit

Activation Reaction ' ÏÇ - Ey, MeV Ey, MeV Nuclide Production in Ta, ppm

,+ 8Ti(p,n)1*8V 16.0 d 0.9831.312

0.02

56Fe(p,n)56Co 77.3 d 0.8471.2381.7712.598

0.850 96Tc 4.3 d 9®Mo(p,n)96Tc

0.1

90Zr(p,n)90Nb 14.6 h 1.1292.3190.141

1.127 96Tc 4.3 d 96Mo(p,n)96Tc0.005

93Nb(p,n)93mMo 6.9 h 0.2630.6851.477

0.265 lS2Ta 115 d 181Та(п,у)182Та0.3

9l*Mo(p,n)9l*Tc 4.8 h 0.8500.7020.871

0.847 56Co 77.3 d 56Fe(p,n)56Co not measured

95Mo(p,n)96Tc 4.3 d 0.7780.8500.812

0.847 56Co 77.3 d 56Fe(p,n)56Co0.2

182W(p,n)182Re 13 h 1.1221.222

1.1221.222

182Ta182Ta

115 d 115 d

181Ta(n,y)182Ta not meaningful

182W(p,2n)181Re 19 h 0.366 0.5

a-'Irradiation with 15 MeV protons.b^For Ep = 15 MeV, nuclear interferences were found to be negligible in practice.c)2.5 hour bombarding tlgne with a beam intensity of 2 pA, followed by a cooling period of 10 to 50 hours.


3 . 2 . 1 D e t e r m i n a t i o n o f Som e T r a c e E l e m e n t s i n H i g h P u r i t y T a n t a l u m a n d N i o b i u m

I n t a n t a l u m , t h e i m p u r i t i e s o f i n t e r e s t i n c o n n e c t i o n w i t h m e t a l l u r g i c a l a n d p h y s i c a l s t u d i e s a r e t i t a n i u m , i r o n , z i r c o n i u m , n i o b i u m , m o l y b d e n u m a n d t u n g s t e n . I t h a s b e e n s h o w n t h a t p r o t o n a c t i v a t i o n i s t h e o n l y n o n d e s t r u c t i v e t e c h n i q u e a v a i l a b l e t o d a t e c a p a b l e o f d e t e c t i n g . t h e s e e l e m e n t s i n h i g h p u r i t y t a n t a l u m [5 2 ] . P e r t i n e n t d a t a o n t h i s a n a l y s i s i s g i v e n i n T a b l e V I I I . F o r a n a l y s e s , i r r a d i a t i o n s o f 2 t o 4 h o u r s ( 1 5 MeV, 1 t o 2 iiA b e a m ) w e r e s u f f i c i e n t t o d e t e r m i n e t h e s i x t r a c e e l e m e n t s o f i n t e r e s t w i t h a p r e c i s i o n o f b e t t e r t h a n 20% e v e n a t t h e 0 . 1 ppm l e v e l .

N i o b i u m h a s s e v e r a l s p e c i a l p r o p e r t i e s w h i c h a r e , o r m ay b e , u t i l i z e d i n v a r i o u s s c i e n t i f i c a n d i n d u s t r i a l f i e l d s . O f p a r t i c u l a r i n t e r e s t i s t h e s u p e r c o n d u c t i v i t y e x h i b i t e d b y n i o b i u m a n d i t s c o m p o u n d s . I n t h i s c o n t e x t , t h e p u r i t y g r a d e o f n i o b i u m p l a y s a n i m p o r t a n t r o l e . I m p u r i t i e s o f i n t e r e s t i n c l u d e : t a n t a l u m , t u n g s t e n , m o l y b d e n u m , z i r c o n i u m , i r o n ,t i t a n i u m , v a n a d i u m , c h r o m i u m a n d h a f n i u m . T h e s e n i n e t r a c e e l e m e n t s c a n b e d e t e r m i n e d s i m u l t a n e o u s l y u s i n g 1 2 MeV p r o t o n a c t i v a t i o n [ ( p , n ) r e a c t i o n s ] f o l l o w e d - b y y - r a y a n d x - r a y c o u n t i n g o f t h e r a d i o n u c l i d e s . E x c e p t f o r t a n t a l u m , s u b - p p m d e t e c t i o n l i m i t s w e r e o b t a i n e d i n a l l c a s e s [ 8 7 ] . I t i s i n t e r e s t i n g t o n o t e t h a t p r o t o n a c t i v a t i o n n o t o n l y p r o v i d e s a n o n d e s t r u c t i v e m u l t i e l e m e n t c a p a b i l i t y b u t i n t h e c a s e o f n i o b i u m , i t i s , e x c l u d i n g m a s s s p e c t r o m e t r y , t h e m o s t s e n s i t i v e t e c h n i q u e f o r t h e d e t e r m i n a t i o n o f t i t a n i u m , v a n a d i u m , i r o n , z i r c o n i u m a n d m o l y b d e n u m .

3 . 2 . 2 A p p l i c a t i o n o f P r o t o n A c t i v a t i o n i n B i o l o g i c a l S a m p l e s

T h e i m p o r t a n c e o f t r a c e e l e m e n t s i n b i o l o g i c a l s y s t e m s h a s b e e n r e c o g n i z e d f o r m a n y y e a r s . So m e e l e m e n t s a r e kn o w n t o p l a y a n e s s e n t i a l r o l e i n t h e l i f e p r o c e s s , w h i l e o t h e r s a r e k n o w n t o b e t o x i c a n d t h e r e f o r e t h e i r c o n c e n t r a t i o n i n b i o l o g i c a l m a t e r i a l s , i n c l u d i n g f o o d , a r e o f g r e a t i m p o r t a n c e I n t h i s c o n t e x t t h e a n a l y t i c a l t a s k i s f o r m i d a b l e , c o n s i d e r i n g t h e m u l t i p l i c i t y o f t r a c e e l e m e n t s o f k n o w n o r p o t e n t i a l i n t e r e s t , t h e v a r y i n g l e v e l s o f c o n c e n t r a t i o n a n d t h e n u m b e r o f s p e c i m e n s t o b e a s s a y e d t o v a l i d a t e a n y f i n d i n g s . T h e s e r e q u i r e m e n t s c a n n o t b e f u l l y m e t b y a n y c u r r e n t a n a l y t i c a l t e c h n i q u e i n c l u d i n g p r o t o n a c t i v a t i o n , b u t i t c a n p r o v i d e a u s e f u l a n a l y s i s c a p a b i l i t y . A s u r v e y o f 1 2 MeV p r o t o n a c t i v a t i o n i n b o t a n i c a l s p e c i m e n s a n d a n i m a l t i s s u e s h a s s h o w n t h a t 1 0 e l e m e n t s c a n b e d e t e r m i n e d a t t h e ppm l e v e l w i t h a 1 h o u r 0 . 5 uA i r r a d i a t i o n [ 8 3 ] . I n t h i s n o n d e s t r u c t i v e a p p r o a c h o n l y n u c l i d e s w i t h h a l f - l i v e s l o n g e r t h a n a f e w h o u r s a r e u s e f u l a s a n a l y t i c a l s i g n a l s . A d e c a y t i m e o f a t l e a s t 1 0 h o u r s i s r e q u i r e d d u e t o t h e h i g h l e v e l s o f m a t r i x a c t i v i t i e s , t h e m o s t i m p o r t a n t o f t h e s e b e i n g 18 F ( T ^ : . 1 0 9 . 7 m i n ) w h i c h , i s f o r m e d b y t h e r e a c t i o n 1 8 0 ( p , n ) 1 8 F . T h e m a j o r l i m i t a t i o n f o r b i o l o g i c a l s a m p l e s i s t h e h e a t g e n e r a t i o n i n t h e s a m p l e w h i c h p r e v e n t s t h e u s e o f h i g h b e a m i n t e n s i t i e s ( e . g . 1 uA) a n d t h u s l i m i t s s e n s i t i v i t i e s . D e s p i t e . t h i s , t h e d e t e c t i o n l i m i t s ( T a b l e I X ) a r e a d e q u a t e f o r s e v e r a l e s s e n t i a l t r a c e e l e m e n t s ( C u , F e ,

1 8 S C H W E IK E R T

T A B L E IX . 12 -M e V P R O T O N . A C T IV A T IO N A P P L IE D IN B IO L O G IC A L

S A M P L E S [ 8 1 ]

Reaction Half-lifey-rays

Used, MeV

Experimental Detection in

Orchard Leaves ppma

75As(p,n)75Se 120 d 0.280; 0.264 3

lt‘4Ca(p,n)'(‘*mSc 2.44 d 0.271 500

65Cu(p,n)65Zn 243 d 1.115 3

56Fe(p,n)56Co 77.3 d 1.240; 0.849 6

96Mo(p,n)96Tc 4.3 d 0.813; 0.778 2

206Pb(p,n)206Bi 6.2 d 0.881; 0.803 10

88Sr(p,n)88Y 107 d 1.836 10

48Ti(p,n)1,8V 16 d 1.311; 0.980 0.2

66Zn(p,n)66Ga 9.5 h 2.760; 0.839 2

92Zr(p,n)9®Nb 14.6 h 2.319; 1.129 2.5

a)One hour irradiation of 0.6 yA followed by a cooling period of 10 h.

Mo a n d Z n , s e e 3 . 3 b e l o w ) . T h e s e p o s s i b i l i t i e s c o u l d b e s u b s t a n t i a l l y e n l a r g e d w i t h c h e m i c a l s e p a r a t i o n s , l e a d a n d s t r o n t i u m w o u l d b e a m o n g t h e e l e m e n t s w h i c h c o u l d b e d e t e c t e d w i t h g r e a t s e n s i t i v i t y .

-An a l t e r n a t e a p p r o a c h w o u l d c o n s i s t o f u s i n g o n l y v e r y s h o r t i r r a d i a t i o n s a n d b a s i n g t h e ' a n a l y s i s o n s h o r t l i v e d r a d i o i s o t o p e s . D e b r u n e t a l . [ 2 7 ] h a v e i n v e s t i g a t e d s u c h p o s s i b i l i t i e s b a s e d o n t h e m e a s u r e m e n t o f y - r a y e m i t t e r s w i t h h a l f l i v e s r a n g i n g f r o m 1 s e c . t o 1 m i n . One o r s e v e r a l o f t h e f o l l o w i n g e l e m e n t s , S e , B r , Y , Z r , L a , P r , Dy a n d N d , c o u l d b e d e t e c t e d w i t h . 1 0 t o 1 7 MeV p r o t o n i r r a d i a t i o n s o f a f e w s e c o n d s . T h e r e s p e c t i v e d e t e c t i o n l i m i t s v a r y w i t h t h e m a j o r , m i n o r a n d t r a c e c o m p o n e n t m a k e - u p o f t h e m a t r i x .I n t h e c a s e o f b i o l o g i c a l m a t e r i a l s , t h e y a r e e s t i m a t e d t o b e i n t h e t e n s t o h u n d r e d s o f pp m .

3 . 2 . 3 M u l t i e l e m e n t A s s a y s W i t h X - r a y C o u n t i n g

T h e p o s s i b i l i t i e s o f CPAA a n d y - r a y s p e c t r o m e t r y c a n b e e x p a n d e d w i t h d e l a y e d x - r a y c o u n t i n g . I n d e e d m o s t o f t h e r a d i o n u c l i d e s o f m e d i u m a n d h i g h Z e l e m e n t s , p r o d u c e d b y c h a r g e d p a r t i c l e r e a c t i o n s , d e c a y p r i n c i p a l l y b y i n t e r n a l c o n v e r s i o n o r e l e c t r o n c a p t u r e a n d a r e t h u s p r e d o m i n a n t l y x - r a y e m i t t e r s . T h e m a i n f e a t u r e s o f n u c l e a r a c t i v a t i o n f o l l o w e d b y n o n d i s p e r s i v e x - r a y s p e c t r o m e t r y ( u s i n g a S i ( L i ) o r t h i n G e ( L i ) d e t e c t o r ) a r e : t h e d i r e c t r e l a t i o n s h i p b e t w e e n t h e x - r a y e n e r g y a n d t h e a t o m i c n u m b e r o f t h e p e r t i n e n t e l e m e n t ; t h e r e l a t i v e l y s i m p l e s t r u c t u r e o f x - r a y s p e c t r a ( i n c o m p a r i s o n w i t h у - r a y s p e c t r a ) ; t h e a v a i l a b i l i t y o f

T A B L E X . 2 0 -M e V P R O T O N A C T IV A T IO N W IT H X - R A Y C O U N T IN G [ 1 9 ]


Q-Value, . X-ray detected Interference-free®Reaction Half-life MeV (energy in keV) detection limits,

S6Fe(p ,n )s6Co 7 8 .5 days —5.4 Fe Ka (6 .4 ) 1 .0 X 1 0 -î69Ga(p,n)69Ge 120 days - 1 .7 Ga Ka (9 .2 ) 4 .0 x 1 0 _l7SAs(p,n)7SSe 19 .5 hr - 5 . 4 As Ka (1 0 .5 ) 1 .9 X 1 0 -276Se(p ,n)76Br 4 .4 hr —2.7 Se Ka (1 1 .2 ) 2 .6 x 1 0 “28SR b(p ,n)85mSr 67 .7 min - 2 . 0 Sr Kor (1 4 .1 ) 4.4 x 1 0 -3e6Sr(p ,7 )87 Y 87Sr(p ,n)87Y 80 .3 hr > 0

- 2 . 5 Sr Ka (1 4 .1 ) 6.4 X 1 0 “486Sr(p,n)86m Y 1 2;6 hr - 6 .1 Y Ka (1 4 .9 ) 3 .8 X 1 0 -289Y(p,n)*9Zr 78 .5 hr - 3 . 6 Y K a (1 4 .9 ) 7.4 X 1 0 -492M o(p,7 )93Tc 94M o(p,2n)93Tc 2.7 hr > 0

- 1 3 .6 Mo Ka (1 7 .5 ) 4 .2 X 1 0 “394M o(p,7 )9STc 95M o(p,n)9STc 20 .0 hr > 0

- 2 . 4 Mo Ka (1 7 .5 ) 1.0 x 1 0 " 4l03Rh (p ,p ')l03mRh 5 7 .0 min 0 Rh Ka (2 0 .1 ) 3.4l02Pd(p,7 ),03Ag 67 min > 0104Pd(p,2n)*03Ag - 1 3 .4 Pd Ka (2 1 .1 ) 2 .8 x 1 0 ' 1104Pd(p,n)l04Ag 68 min - 5 .1‘07Ag(pln)*07Cd 6.5 hr - 2 . 2 Ag Ka (2 2 .1 ) 1.1 X 1 0 -2"-0Cd(p,n)noIn 4 .9 hr - 4 . 7 Cd Ka. (2 3 .1 ) 1.0 x 1 0 " ‘" 0Cd(p,7 ) lu In > 0MlCd(p,n), l 4n 2.8 days - 2 . 8 Cd Ka (2 3 .1 ) 5.8 X 10 “4,12Cd(p,2n),MIn - 1 1 .21 l3In(p ,n)‘ 3mSn " sIn (p ,p ')1,5mIn

20 .0 min - 1 . 8 Sn Ka (2 5 .1 ) .8 .6 x 1 0 * 2. 4 .4 hr 0 .0 In Ka (2 4 .1 ) 2.3 x 1 0 ”2

1,ftSn(p,7 )‘ l7Sb > 0“ 7S n (p ,n )"7Sb 2.8 hr - 2 . 6 • Sn Ka (25 .1 ) 8.4 X 1 0 “4ll8Sn (p ,2n )ll7Sb - 1 1 .9" 8Sn(p,7 ) " 9Sb > 0M9S n (p ,n )"9Sb 3 8 .0 hr - 1 . 4 Sn Ka (2 5 .1 ) 3 .2 X 10~4l20Sn (p ,2n )ll9Sb - 7 . 8‘2* Sb(p ,n )n iTe 17 days - 2 .1 Sb Ka (2 6 .3 ) 2.4 x 1 0 " 'l33Cs(p,n)133mBa 38 .9 hr - 1 . 3 Cs Ka (3 0 .8 ) 4 .5 X 1 0 -3l39La(p,n)139Ce 137 days - 1 .1 La Ka (3 3 .3 ) 5.8 x 1 0 _l,40C e(p ,2n)‘39Pr 4.4 hr 11 .8 Ce Ka (3 4 .5 ) 2.2 x 1 0 * 3l4IC e(p,n)142Pr 19 .6 hr - 1 .7 Ce Ka (3 4 .5 ) 7 .8 x 1 0 " 4

• 14,Pr(p ,2n)‘40Nd l4lPr(p,n)14lNd

3.37 days 10.5 Pr Ka (3 5 .8 ) 1.5 x 1 0 -42.5 hr - 2 . 6 Pr Ka (3 5 .8 ) 9 .7 x 1 0 -4

15lEu(p ,n)lslGd 120 days - 1 . 2 Eu Ka (4 1 .3 ) 6 .6 X 10 "4153E u (p ,p ')ls3mEu 96 min 0 Eu Ka (4 1 .3 ) 8.6 x 10~ 3*ssG d(p,n),ssTb 5.1 days - 1 . 7 Gd Ka , , (4 2 .3 ; 43 .0 )IS6G d(p,n)‘S6Tb 5.3 days - 3 . 2 Gd Ka (4 2 .3 ; 43 .0 ) 2.1 X 1 0 " 4,60D y(p,n)‘60mHol6lD y(p,2n)l6omHo 4 .8 hr - 4 .1

- 1 0 .5 Ho Ka , ,2 (4 6 .7 ; 4 7 .5 ) 4 .3 X 1 0 " 3,6SHo(p,pn),64mHo 37 min 8.04 Ho Ka , , (4 6 .7 ; 4 7 .5 ) 3 .5 x 1 0 -2■6SH o(p,n)*65Er 10 .3 hr - 1 . 2 H oK a ù (46 .7 4 7 .5 ) 3 .0 x 1 0 -4l64E r(p ,2n )l63Tm 1.8 hr 11.8 Er Ka (4 8 .2 ; 4 9 .1 ) 1.5 X 1 0 " 3,66Er(p,7 )167Tm > 0l61Er(p ,n)167Tm 0.3 days —1.7 Er Ka (4 8 .2 ; 4 9 .1 ) 1.5 x 1 0 “4l68E r(p ,2n )l67Tm - 9 . 5l71Yb(p ,n)l7lLul72Y b (p ,2n )l7‘Lu 8 .2 days —3.7

—10.5 Yb Ka li3 (5 1 .3 ; 5 2 .3 ) 1.2 x 1 0 " ',7SLu(p,n)17SHf 70 days - 1 . 7 Lu Ka , 2 (5 2 .9 ; 54 .1 ) 3.3 x 10 “4177H f(p,n)l77Ta 56 .4 hr - 1 . 9 Hf Ka *a (5 4 .6 ; 5 5 .8 ) 2.1 x 1 0 -4l78H f(p,n)178Ta 2.2 hr —2.7 Hf Ka ’2 (5 4 .6 ; 5 5 .8 ) 1.3 x 10~3I82W (p,n)l83Re 6 4 .0 hr - 3 . 7 W Ka, (5 7 .9 ; 59 .3 ) 1.6 X 1 0 * 4,870 s(p,7 )188Ir ,88 0 s (p ,n )l88Ir 4 1 .0 hr > 0

- 3 . 6 Os Ka ,, (6 1 .4 ; 63 .0 ) 1.9 x 1 0 " 3l90O s(p,n)190Ir19IIr(p ,n )19iPt

3 .2 hr - 2 . 8 - Os Ka 3 (6 1 .4 ; 6 3 .0 ) 5 .6 X 1 0 “43 .0 days - 1 . 5 Ir Ka, ’ (6 3 .3 ; 6 4 .9 ) 1 .6 X 1 0 “3

,9 ,Ir(p ,pn),90Ir 3.1 hr - 7 . 6 Ir K a, 3 (6 3 .3 ; 6 4 .9 ) 8 .2 X 1 0 -4l94Pt(p ,n),94Au 39 .0 hr - 3 . 3 Pt Ka j (65 .1 ; 6 6 .8 ) 1.4 x 1 0 -3197A u(p,n)l97Hg 64.1 hr —1.6 Au Ka i , (6 7 .0 ; 6 8 .8 ) 7.7 X 1 0 " 3203,П (р,п)203РЬ 52.1 days' - 1 . 6 Tl Ka ; (7 0 .8 ; 72 .8 ) 7 .3 x 1 0 " 3204Pb(p,p ')204mPb 6 6 .9 niin 0 Pb Ka (7 2 .8 ; 7 4 .9 ) 2 .6 x 1 0 -2206Pb(p,n)í06B¡ 6 .2 days - 4 .4 . Pb Ka (7 2 .8 ; 7 4 .9 ) 8 .4 x 1 0 ’ 4

a)Based on a 3 yA irradiation for 3 hours or one half-life of the product nuclide (whichever is shorter) and a count time of one half-life of the product nuclide.


r a d i o a c t i v e d e c a y r a t e s a s a n a d d i t i o n a l c r i t e r i o n o f i d e n t i f i c a t i o n . T h e s e a d v a n t a g e s m u s t b e w e i g h e d a g a i n s t p o s s i b l e l i m i t a t i o n s a r i s i n g f r o m s e l f - a b s o r p t i o n a n d e n h a n c e m e n t e f f e c t s common t o a l l x - r a y t e c h n i q u e s . An a d d i t i o n a l l i m i t a t i o n p r o p e r t o n o n d i s p e r s i v e x - r a y c o u n t i n g o f r a d i o a c t i v e s a m p l e s a r i s e s w h e n 6 a c t i v i t y i s p r e s e n t , w h i c h r e s u l t s i n i n c r e a s e d b a c k g r o u n d . T h i s b r e m s S t r a h l u n g b a c k g r o u n d c a n b e r e d u c e d w i t h a b s o r b e r s ( a c c o m p a n i e d b y a c o r r e s p o n d i n g l o s s i n l o w e n e r g y x - r a y s ) o r e l i m i n a t e d b y m a g n e t i c d e f l e c t i o n o f t h e g p a r t i c l e s [90 ] . 2 0 MeV p r o t o na c t i v a t i o n s h o w s b r o a d e l e m e n t a l c o v e r a g e a n d c a p a b i l i t y : 3 3 _ e l e m e n t s c a n b e m e a s u r e d w i t h d e t e c t i o n l i m i t s f r o m 1 0 S g t o 1 y g ( T a b l e X ) . T h e s e v a l u e s a r e f o r i n t e r f e r e n c e - f r e e c o n d i t i o n s . N u c l e a r i n t e r f e r e n c e s a r e , i n m o s t c a s e s , s m a l l ( < 10%) e v e n w i t h b o m b a r d i n g e n e r g i e s o f 2 0 MéV, m a n y c a n b e t o t a l l y e l i m i n a t e d b y r e d u c i n g t h e b o m b a r d i n g e n e r g y t o 1 0 MeV. X - r a y i n t e r f e r e n c e s c a n o c c u r , i . e . o v e r l a p s a m o n g x - r a y s f r o m a d j a c e n t e l e m e n t s o r b e t w e e n К a n d L x - r a y s o f s i m i l a r e n e r g i e s . H o w e v e r , t h e h a l f - l i v e s o f t h e n u c l i d e s i n v o l v e d a r e d i f f e r e n t a n d p r o p e r a s s i g n m e n t s c a n b e m a d e v i a d e c a y c u r v e a n a l y s i s . P o s s i b l e l i m i t a t i o n s d u e t o 8 - a c t i v i t y a n d r e m e d i e s h a v e a l r e a d y b e e n m e n t i o n e d .

T h e a p p l i c a b i l i t y o f t h i s t e c h n i q u e f o r m u l t i e l e m e n t t r a c e a n a l y s i s h a s b e e n t e s t e d o n NBS g l a s s s a m p l e s . T h e s e g l a s s e s a r e d o p e d w i t h 6 1 t r a c e e l e m e n t s a n d t h e i r m a j o r c o n s t i t u e n t s ( S i 0 2 , C aO , N a 2 0 a n d А 1 2 0 з ) y i e l d i n t e n s e , l o n g - l i v e d 8 a n d ’ у " a c t i v i t y f o l l o w i n g p r o t o n b o m b a r d m e n t .T h e y c a n t h u s b e v i e w e d a s f a r i l y r e p r e s e n t a t i v e o f a v a r i e t y o f d i f f i c u l t m a t r i c e s i n c l u d i n g g e o l o g i c a l a n d b i o l o g i c a l s p e c i m e n s . I n t h e c a s e o f a g l a s s w i t h a n a v e r a g e t r a c e e l e m e n t c o n c e n t r a t i o n o f ' 4 0 ppm (N B S SRM 6 1 2 ) , 26 e l e m e n t s w e r e d e t e r m i n e d b y x - r a y s p e c t r o m e t r y f o l l o w i n g a 3 0 - m i n . i r r a d i a t i o n a t 0 . 5 pA. S p e c t r a f r o m a g l a s s w i t h t r a c e e l e m e n t c o n c e n t r a t i o n s i n t h e 3 0 0 t o 4 0 0 ppm r a n g e (N B S SRM 6 1 0 ) o b t a i n e d a f t e r d i f f e r e n t d e c a y t i m e s a r e g i v e n i n F i g u r e 4 . T h e y i l l u s t r a t e t h e c o m b i n a t i o n o f x - r a y s p e c t r o m e t r y a n d d e c a y r a t e s , w h i c h y i e l d s q u a n t i t a t i v e d a t a o n a t r u l y l a r g e n u m b e r o f e l e m e n t s . T h e a p p l i c a t i o n s b e s t s u i t e d f o r t h i s p r o c e d u r e w o u l d i n c l u d e t h e d e t e r m i n a t i o n o f m e d iu m a n d h i g h Z i m p u r i t i e s i n s a m p l e s w i t h m a j o r c o n s t i t u e n t s c o m p o s e d o f l o w Z e l e m e n t s ( 1 <. Z <. 1 4 , e . g . B ,B e , С , A l , S i , t h e i r c a r b i d e s , n i t r i d e s , o x i d e s , r o c k s , o r g a n i c c o m p o u n d s , b i o l o g i c a l m a t e r i a l ) .

3 . 3 C o m p a r i s o n o f CPAA W i t h O t h e r T e c h n i q u e s

A s CPAA a p p l i c a t i o n s a r e d e v e l o p e d , t h e y m u s t b e v a l i d a t e d a n d j u s t i f i e d o n a p e r f o r m a n c e b a s i s . So m e c o m m e n t s o n h ow CPAA d o e s a n d m i g h t f i t i n t o t h e b r o a d a r s e n a l o f t r a c e a n a l y s i s t o o l s f o l l o w .

CPAA h a s r e c e i v e d m o s t a t t e n t i o n b e c a u s e o f i t s h i g h s e n s i t i v i t y a n d a c c u r a c y f o r l i g h t e l e m e n t s , p a r t i c u l a r l y В , C , N , 0 . A f u r t h e r d i s t i n c t i v e t r a i t o f CPAA i s i t s a p p l i c a b i l i t y a s a n i n - d e p t h ( b u l k ) c h a r a c t e r i z a t i o n m e t h o d . " C l a s s i c a l " m e t h o d s a d e q u a t e a l s o f o r b u l k p r o b i n g a n d a p p l i c a b l e t o В , C , N o r 0 i n c l u d e i n f r a r e d s p e c t r o s c o p y , v a c u u m f u s i o n , m a s s s p e c t r o m e t r y a n d e l e c t r i c a l r e s i s t i v i t y

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irradiation with 2 0 -M e V p ro to n s. Count times were 2 0 m in [19].

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t e c h n i q u e s . A s a g e n e r a l r u l e t h e s e a r e a d e q u a t e f o r l i g h t e l e m e n t c o n c e n t r a t i o n s a s l o w a s 1 0 pp m . F r o m t h a t p o i n t o n t h e y a r e h a m p e r e d b y i n c r e a s i n g d i f f i c u l t i e s a s t r a c e l e v e l s d e c r e a s e , ( b l a n k s , e r r o r s d u e t o s u r f a c e a d s o r p t i o n , l i m i t a t i o n s i n s i g n a l - t . o - n o i s e r a t i o s ) . A t s u b - p p m l e v e l s , p h o t o n a c t i v a t i o n c o m p e t e s w e l l w i t h CPAA f o r c a r b o n a n d n i t r o g e n [ 9 1 ] . B o t h m e t h o d s a r e f r e e f r o m r e a g e n t b l a n k s a n d e r r o r s d u e t o s u r f a c e c o n t a m i n a n t s . CPAA i s i n h e r e n t l y m o r e s e n s i t i v e , b u t t h e r e a r e m o r e i n t e r f e r e n c e s t h a n w i t h p h o t o n a c t i v a t i o n [ 7 2 ] . F o r o x y g e n , 3H e , a n d p r o b a b l y t r i t o n a c t i v a t i o n ( s e e b e l o w ) , f e a t u r e s u p e r i o r s e n s i t i v i t y . P h o t o n a c t i v a t i o n c a n n o t c o m p e t e a t l e v e l s b e l o w 0 . 5 ppm d u e t o t h e s h o r t h a l f - l i f e o f 1 5 0 ( T ^ : 2 . 0 3 m i n . ) p r o d u c e d b y 16 0 ( у , п ) 1 5 0 , a n d d u e t o t h e n e c e s s i t y o f a r a d i o c h e m i c a l s e p a r a t i o n o f 1 5 0 f o r p r o p e r i d e n t i f i c a t i o n [ 9 2 ] . I n s m ir a r y i t i s f e l t t h a t CPAA h a s c u r r e n t l y a u n i q u e c a p a b i l i t y f o r t h e i n - d e p t h d e t e r m i n a t i o n o f s u b - p p m l e v e l s o f o x y g e n a n d b o r o n . H y d r o g e n m i g h t b e a d d e d i f h e a v y i o n a c t i v a t i o n i s i n c l u d e d ( s e e b e l o w ) . A t l e v e l s a b o v e t h e pp m , CPAA i s w e l l s u i t e d ( a c c u r a c y , t h o r o u g h s e l e c t i v e a c t i v a t i o n , f r e e d o m f r o m r e a g e n t b l a n k s a n d s u r f a c e c o n t a m i n a t i o n , w i d e d y n a m i c r a n g e ) a s a r e f e r e n c e t e c h n i q u e f o r c a l i b r a t i n g o t h e r m e t h o d s .

When a p p r a i s i n g CPAA a s a m u l t i e l e m e n t t e c h n i q u e , i t s f e a t u r e s s h o u l d b e c o m p a r e d w i t h t h o s e o f o t h e r m e t h o d s a l s o c a p a b l e o f d e t e r m i n i n g s i m u l t a n e o u s l y s e v e r a l m e d iu m a n d / o r h i g h Z e l e m e n t s , e . g . : p h o t o n o r p a r t i c l e i n d u c e d x - r a y e m i s s i o n a n a l y s i s , e m i s s i o n s p e c t r o g r a p h y , m a s s s p e c t r o m e t r y , n e u t r o n o r p h o t o n a c t i v a t i o n a n a l y s i s . S u c h c o m p a r i s o n s m u s t b e m a d e a n d a r e o n l y v a l i d f o r a g i v e n a n a l y t i c a l p r o b l e m . D e p e n d i n g o n t h e r e q u i r e m e n t s o f t h e a n a l y s i s , t h e i m p o r t a n c e o f v a r i o u s c r i t e r i a o f e v a l u a t i o n v a r i e s , e . g . i m p o r t a n c e o f r e a g e n t b l a n k s , s p e e d o f a n a l y s i s , s a m p l e s i z e n e e d e d . A s a n e x a m p l e , t h e f i n d i n g s o f a c o m p a r i s o n o f i n s t r u m e n t a l p r o t o n a n d n e u t r o n a c t i v a t i p n a n a l y s i s a p p l i e d t o b i o l o g i c a l s a m p l e s a r e s u m m a r i z e d h e r e [ 8 1 ] . A t t h e o u t s e t , a c t i v a t i o n c o n d i t i o n s h a d t o b e d e f i n e d . T h e s e w e r e s e t a s f o l l o w s : a o n e h o u r i r r a d i a t i o n o f 0 . 6 y A / c m 2w i t h 1 2 MeV p r o t o n s a n d a o n e m i n u t e a n d a 1 4 h o u r i r r a d i a t i o n s w i t h 4 x 1 0 12 n / c m 2 / s e c o n a 5 0 0 mg s a m p l e . T h e s e c o n d i t i o n s w e r e a r b i t r a r y b u t d e e m e d r e p r e s e n t a t i v e o f w h a t c a n b e a c h i e v e d i n p r a c t i c e . T h e d a t a o b t a i n e d o n o r c h a r d l e a v e s s a m p l e s (N BS SRM 1 5 7 1 ) i s p r e s e n t e d i n F i g u r e 5 , g i v i n g s e n s i t i v i t i e s f o r v a r i o u s e l e m e n t s . C l e a r l y , n e u t r o n a c t i v a t i o n c a n d e t e r m i n e a g r e a t e r n u m b e r o f e l e m e n t s w i t h h i g h s e n s i t i v i t y . ( M o r e o v e r i m p r o v e m e n t s i n i t s p e r f o r m a n c e b y u s i n g l o n g e r i r r a d i a t i o n s , h i g h e r f l u x e s , f a s t o r r e s o n a n c e n e u t r o n s c a n b e r e a d i l y e n v i s i o n e d ) . On t h e o t h e r h a n d , w i t h p r o t o n a c t i v a t i o n , e l e m e n t s w h i c h a r e n o t a c c e s s i b l e b y t h e n e u t r o n t e c h n i q u e c a n b e m e a s u r e d : t i t a n i u m , s t r o n t i u m a n d l e a d . F o r o t h e r s l i k e i r o n , c o p p e r a n d z i n c , b e t t e r d e t e c t i o n l i m i t s a r e o b t a i n e d e v e n w i t h a . r e l a t i v e l y s h o r t a c t i v a t i o n . (One m i g h t n o t e a l s o , t h a t b e y o n d p r o t o n a c t i v a t i o n , o t h e r c h a r g e d p a r t i c l e b e a m s m ay b e a d v a n t a g e o u s l y u s e d f o r t h e d e t e c t i o n o f s p e c i f i c t r a c e e l e m e n t s , e . g . L i , B , C a , P , P b ) . A m o r e q u a n t i t a t i v e c o m p a r i s o n c a n s t i l l b e a t t e m p t e d b y c o m p u t i n g a f i g u r e o f m e r i t f o r e a c h e l e m e n t . T h i s f i g u r e o f m e r i t w a s d e f i n e d


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a s t h e r a t i o o f t h e a v e r a g e c o n c e n t r a t i o n o f a g i v e n e l e m e n t i n t h e h u m an b o d y d i v i d e d b y i t s d e t e c t i o n l i m i t . A l a r g e n u m e r i c a l v a l u e w o u l d t h u s i n d i c a t e t h a t t h e e l e m e n t i n q u e s t i o n c a n r e a d i l y b e m e a s u r e d b y t h e m e t h o d s e l e c t e d . C o n v e r s e l y , a f i g u r e o f m e r i t b e l o w u n i t y w o u l d i n d i c a t e t h a t t h e r e s p e c t i v e e l e m e n t c o u l d p r o b a b l y i n m a n y t i s s u e s a m p l e s n o t b e d e t e c t e d . W i t h n e u t r o n a c t i v a t i o n , 9 e l e m e n t s h a d f i g u r e s o f m e r i t g r e a t e r t h a n u n i t y , w i t h p r o t o n a c t i v a t i o n t h i s w a s t h e c a s e f o r 4 e l e m e n t s . T h i s c a n r e a d i l y b e u n d e r s t o o d w h e n c o n s i d e r i n g t h e s h o r t i r r a d i a t i o n t i m e u s e d i n p r o t o n a c t i v a t i o n . A g r a p h i c a l i l l u s t r a t i o n o f t h e s e f i g u r e s o f m e r i t i s g i v e n i n F i g u r e 6 .

An i n t e r e s t i n g o b s e r v a t i o n d e a l s w i t h t h e c o m p a r a t i v e c a p a b i l i t i e s f o r t h e d e t e r m i n a t i o n o f k n o w n e s s e n t i a l e l e m e n t s . F i g u r e s o f m e r i t e q u a l o r g r e a t e r t h a n u n i t y w e r e o b t a i n e d w i t h n e u t r o n a c t i v a t i o n f o r c o b a l t , i r o n a n d z i n c ;

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w i t h p r o t o n a c t i v a t i o n f o r c o p p e r , i r o n a n d z i n c . T h i s w o u l d i n d i c a t e t h a t b o t h a c t i v a t i o n m o d e s a r e a p p r o x i m a t e l y e q u i v a l e n t a s f a r a s t h e n u m b e r o f e s s e n t i a l e l e m e n t s t h a t c a n b e d e t e c t e d i s c o n c e r n e d . I t s h o u l d b e e m p h a s i z e d a g a i n t h a t t h e s e c o n c l u s i o n s a r e b a s e d o n a r b i t r a r y e x p e r i m e n t a l c o n d i t i o n s . T h e a b o v e c o n s i d e r a t i o n s e x e m p l i f y t h e n e e d f o r a n i n - d e p t h e v a l u a t i o n a s b a s i s f o r c o m p a r i s o n s .

4 . 0 CURRENT TRENDS AND PRO SPECTS

T h e p r o g r e s s m a d e i n CPAA i n r e c e n t y e a r s i s l e a d i n g t o n u m e r o u s n ew o p p o r t u n i t i e s f o r f u r t h e r s t u d i e s a n d d e v e l o p m e n t s . Am ong t h e p o s s i b i l i t i e s r e a d y t o b e e x p l o r e d a n d a p p l i e d a r e t h o s e a r i s i n g f r o m h e a v y i o n i n t e r a c t i o n s , t r i t o n a c t i v a t i o n , u l t r a s h o r t h a l f - l i v e s , t h e c o m b i n a t i o n o f d e l a y e d a n d p r o m p t m e t h o d s a n d t h e r e f i n e m e n t o f p r e c i s i o n a n d a c c u r a c y .

4 . 1 N o v e l a n d I m p r o v e d C a p a b i l i t i e s

4 . 1 . 1 H e a v y I o n A c t i v a t i o n A n a l y s i s

T h e i n c r e a s i n g a v a i l a b i l i t y o f h e a v y i o n b e a m s o p e n s n e w p o s s i b i l i t i e s . I n p a r t i c u l a r t h e d e t e r m i n a t i o n o f h y d r o g e n , d e u t e r i u m a n d h e l i u m a p p e a r s f e a s i b l e v i a " i n v e r s e " n u c l e a r r e a c t i o n s . T h i s a p p r o a c h c a n b e b r i e f l y d e s c r i b e d w i t h t h e f o l l o w i n g e x a m p l e : i n s t e a d o f m e a s u r i n gt h e X1B c o n t e n t i n a s a m p l e v i a 1 ‘ B ( d , p ) 1 2 B , d e u t e r i u m w o u l d b e m e a s u r e d v i a t h e " i n v e r s e " r e a c t i o n 2H ( X^ B , p ) 1 2 B . T h e m e r i t s o f t h i s a p p r o a c h t e r m e d HIAA ( H e a v y I o n A c t i v a t i o n A n a l y s i s ) c a n b e s e e n b y c o n s i d e r i n g a f e w i n t e r e s t i n g p o i n t s : ( a ) t h e r e a r e . a n u m b e r o f w e l l k n o w n " f o r w a r d "r e a c t i o n s , e . g . ( p , n ) , ( d , p ) w h i c h c o u l d b e " i n v e r t e d " ; t h e r e a r e o n l y a f e w m e t h o d s a v a i l a b l e f o r t h e d e t e r m i n a t i o n o f h y d r o g e n , d e u t e r i u m o r h e l i u m a n d t h e s e a r e o f l i m i t e d c a p a b i l i t y ( s e n s i t i v i t y , s i z e o f s a m p l e a n a l y z e d ) ; ( c ) t h e r e a c t i o n p r o d u c t s a r e r a d i o a c t i v e s p e c i e s , t h u s , e x c e p t f o r v e r y s h o r t - l i v e d n u c l i d e s , t h e i n t e r f e r e n c e o f s u r f a c e c o n t a m i n a t i o n c a n b e a v o i d e d b y p o s t - i r r a d i a t i o n e t c h i n g .A s t u d y o f HIAA h a s b e e n u n d e r t a k e n b y M c G i n l e y e t a l . [ 9 3 ] , i n f o r m a t i o n o n t h e r e a c t i o n s s t u d i e d t o d a t e i s p r e s e n t e d i n T a b l e X I . Among t h e s e 7L i a n d 1 0 B a c t i v a t i o n h a v e b e e n a p p l i e d f o r t h e d e t e r m i n a t i o n o f h y d r o g e n ( F i g u r e 7 ) . C l e a r l y t h e HIAA t e c h n i q u e s h o u l d b e r e a d i l y a p p l i c a b l e a l s o t o t h e d e t e r m i n a t i o n o f h e l i u m . F u r t h e r p o s s i b i l i t i e s m i g h t e x i s t f o r o t h e r v e r y l o w Z n u c l i d e s .

4 . 1 . 2 T r i t o n A c t i v a t i o n

T h e f a v o r a b l e c h a r a c t e r i s t i c s o f 1 60 ( 3H , n ) 1 8F , h i g h , c r o s s s e c t i o n , c o m p l e t e f r e e d o m f r o m i n t e r f e r e n c e s , h a v e b e e n p o i n t e d o u t b y s e v e r a l a u t h o r s [ 9 4 - 9 6 ] . R e c e n t l y , B o r d e r i e e t a l . [ 9 5 ] h a v e m e a s u r e d t h e s p e c i f i c a c t i v i t i e s i n d u c e d b y 3 . 5 MeV t r i t o n a c t i v a t i o n o n 2 0 e l e m e n t s w i t h Z < 3 4 . S e n s i t i v e ( < ppm ) a n d s e l e c t i v e d e t e r m i n a t i o n s o f

CH A R G E D -P A R T IC L E A C T IV A T IO N A N A L Y S IS 2 5

T A B L E X I . R E A C T IO N S O F I N T E R E S T F O R H E A V Y IO N A C T IV A T IO N

A N A L Y S I S

Reaction Ti1

E/3max(MeV)

E7(keV)

Thick

target yield

(cps/^g)a

Interference-

free detection

limit (ppm)

*H(7Li,n)7Be 53 d — 480 0.051 0.1b

2H(7Ii,p)8Li 0.85 s 13.0 - 5.79 X 103 0.1c

‘Щ'^.п)1̂ 19s 1.9' 720 -

*H(10B,a)7Be 53 d - 480 0.0261 0.5b

^ ( " B . n /’C 20 min 0.96 511 (ß+ ) -

2H(n B,p)12B 0.02 s 13.0 4400 6.9 X 104 0.1c

10 min 1.19 511. (í¡+) -

*H(19F,n)19Ne 18s 2.23 511 (ß+ ) -

2H(19F,p)20F 11 s 5.4 1630 1.24 X 102 1.2C

2H(22Ne,p)23Ne 38 s 4.4 440 -

a Irradiated at 1 дА for 1 hour or 1 half-life, whichever is shorter.

b Irradiated at 3 дА for 3 hours, counted for 12 hours following a 1-week delay. E(7Li) = 74 M e V

and E(‘°B) = 64 MeV.

c 20 repetitive irradiations at 1 ß A , count time equal to the half-life. E(U B) = 60 M e V and

E(19F)= 109 MeV.

S N BS-353 td * 85d

m 7jj Activation tc * !2 h

01¡ñ

оо

V(/><->>

со ю 00 Л 'Т

500 1000 E N E R G Y (keV)

1500

F IG . 7. Gam m a-ray spectrum o f titanium sam ple ( N B S 3 5 3 ) irradiated with 40-M e V 10B ions

(1 0 0 0 sec, 1 fiA bom bardm ent). H yd rogen is determ ined via lH ( 10B, a ) 1 Be.


B , N, 0 , F , N a , M g, S , C l , V a n d Mn a r e p o s s i b l e n o n d e - s t r u c t i v e l y i n m e d iu m a n d h i g h Z t a r g e t s . T h e w o r k s o f a r h a s b e e n l i m i t e d b y t h e l o w t r i t o n e n e r g i e s . F u r t h e r s t u d i e s w i t h h i g h e r e n e r g y b e a m s s h o u l d b e o f g r e a t i n t e r e s t .

4 . 1 . 3 Q u a s i - p r o m p t A c t i v a t i o n

T h e p o s s i b i l i t i e s o f q u a s i - p r o m p t a c t i v a t i o n w i t h v e r y s h o r t - l i v e d y - r a y e m i t t e r s ( T ^ £ 1 m s e c ) h a s n o t y e t b e e n e x p l o r e d . I t c a n b e p o i n t e d o u t t h a t a n a c c e l e r a t o r i s i d e a l l y s u i t e d f o r s u c h w o r k g i v e n t h e i n h e r e n t s i m p l i c i t y o f p r o d u c i n g s h o r t b e a m b u r s t s a n d g i v e n t h a t d e t e c t o r s c a n b e p l a c e d d i r e c t l y a t t h e i r r a d i a t i o n s i t e . A s e r i e s o f i s o m e r i c s t a t e s a r e k n o w n w i t h h a l f l i v e s f r o m 1 y s e c t o1 m s e c . C y c l i c a c t i v a t i o n p r o c e d u r e s w i t h y - r a y s p e c t r o m e t r y c a n t h u s b e e n v i s i o n e d .

4 . 2 C o m b i n a t i o n o f N u c l e a r a n d A t o m i c A c t i v a t i o n

H e a v y i o n b e a m s a r e o f i n t e r e s t n o t o n l y f o r n u c l e a r a c t i v a t i o n o f t h e h y d r o g e n i s o t o p e s a n d h e l i u m b u t a l s o a s a m e a n s f o r e x c i t i n g c h a r a c t e r i s t i c x - r a y s f r o m m e d iu m a n d h i g h Z e l e m e n t s v i a i o n - a t o m c o l l i s i o n s ( a t o m i c a c t i v a t i o n ) . S t u d i e s c a r r i e d o u t b y Z e i s l e r [ 9 8 ] a n d C r o s s [99 ] h a v e sh o w n t h a t h i g h e n e r g y (E >_ 0 . 2 M e V /am u ) , h e a v y i o n (Z > ._3) i n d u c e d x - r a y e m i s s i o n f e a t u r e s h i g h s e n s i t i v i t y ( 1 0 1 0 - 1 0 11 g o n s a m p l e s i z e s o f 1 0 5 - 1 0 4 g ) • a n d s e l e c t i v i t y a s w e l l a s a p p l i c a b i l i t y t o a w i d e r a n g e o f e l e m e n t s . W i t h a p r o p e r l y s e l e c t e d i o n b e a m , o n e c a n t h u s e n v i s i o n a u n i q u e m u l t i e l e m e n t a n a l y s i s c a p a b i l i t y , d e t e r m i n a t i o n o f * H , 2H, 4 He v i a n u c l e a r a c t i v a t i o n a n d o f m e d i u m a n d h i g h Z e l e m e n t s v i a t h e i r c h a r a c t e r i s t i c К a n d L x - r a y s e m i t t e d d u r i n g b o m b a r d m e n t . One c a n a n t i c i p a t e s u b s t a n t i a l f u r t h e r d e v e l o p m e n t s m a x i m i z i n g t h e a n a l y t i c a l i n f o r m a t i o n ( p r o m p t a n d d e l a y e d e m i s s i o n s ) t h a t c a n b e d e r i v e d f r o m i o n b o m b a r d m e n t .

4 . 3 I m p r o v e m e n t s i n P r e c i s i o n a n d A c c u r a c y

T h e w i d e r a n g i n g a n d i n c e r t a i n c a s e s u n i q u e c a p a b i l i t i e s o f CPAA r e m a i n t o b e f u l l y e v a l u a t e d - w i t h r e s p e c t t o s o u r c e s o f e r r o r s . T h e a i m s h o u l d b e t o b r i n g CPAA p r o c e d u r e s t o t h e l e v e l o f p e r f o r m a n c e o f a r e f e r e n c e m e t h o d , p a r t i c u l a r l y f o r t h o s e e l e m e n t s w h e r e p o s s i b i l i t i e s w i t h o t h e r t e c h n i q u e s a r e r e s t r i c t e d . So m e i t e m s i n n e e d o f c o n s i d e r a t i o n i n c l u d e :

- y - r a y i n t e r f e r e n c e s . T y p i c a l a r e t h o s e a r i s i n g f r o m n u c l i d e s p r o d u c e d c o n c u r r e n t l y b y p r o t o n a n d f a s t n e u t r o n r e a c t i o n s . T h e y a r e d i f f i c u l t o r i m p o s s i b l e t o d e t e c t w h e n o c c u r r i n g a t l e v e l s c l o s e t o t h e e x p e r i m e n t a l d e t e c t i o n l i m i t s a n d a r e t h u s l i k e l y t o g o u n n o t i c e d . L i t t l e d a t a s e e m s t o b e a v a i l a b l e o n t h i s t o p i c i n t h e f o r m a t n e e d e d f o r CPAA.

- P r e s e r v a t i o n o f s a m p l e i n t e g r i t y d u r i n g i r r a d i a t i o n .

- L a c k o f a d e q u a t e s t a n d a r d s a t t h e s u b - p p m l e v e l .


T h e t o p i c s o u t l i n e d a b o v e a r e d i f f i c u l t t o h a n d l e e x p e r i m e n t a l l y : t h e e f f e c t s t o b e i n v e s t i g a t e d , a l t h o u g h i m p o r t a n t a t t h e l e v e l o f t r a c e a n a l y s i s , a r e v e r y s m a l l i n a b s o l u t e t e r m s .

5 . 0 CONCLUSIONS

CPAA h a s a b r o a d r a n g e o f a p p l i c a t i o n s e i t h e r a s a h i g h p e r f o r m a n c e s i n g l e e l e m e n t t e c h n i q u e o r a s a t o o l f o r m u l t i e l e m e n t a s s a y s . Am ong t h e a d v a n t a g e s a r e t h e s e n s i t i v i t y ( 1 0 _ 6 - 1 0 10 g ) , t h e s e l e c t i v i t y (m a n y a c t i v a t i o n r e a c t i o n s a r e i n t e r f e r e n c e - f r e e ) a n d t h e a p p l i c a b i l i t y t o v i r t u a l l y a l l s t a b l e e l e m e n t s . L i m i t a t i o n s a r i s e f r o m t h e h a r d w a r e r e q u i r e m e n t s ( i . e . i o n b e a m s o f s e v e r a l M e V /am u a r e n e e d e d ) a n d t h e g e n e r a t i o n o f h e a t i n t h i c k s a m p l e s . CPAA d e t e c t s p a r t i c u l a r l y w e l l t h e v e r y l i g h t a n d v e r y h e a v y e l e m e n t s a n d t h u s c o m p l e m e n t s t h e o t h e r t r a c e a n a l y s i s m e t h o d s f o r w h i c h t h e p r e f e r r e d d o m a i n o f a p p l i c a t i o n i s i n t h e m e d i u m Z r e g i o n . T h e c a p a b i l i t i e s o f CPAA a r e f a r f r o m b e i n g f u l l y a s s e s s e d . N o v e l a n a l y t i c a l p r o c e d u r e s w i t h s i g n i f i c a n t p r o b l e m s o l v i n g a b i l i t i e s c a n b e e n v i s i o n e d , b a s e d o n u l t r a s h o r t - l i v e d n u c l i d e s , h e a v y i o n i n t e r a c t i o n s o r c o m b i n a t i o n s o f d e l a y e d a n d p r o m p t m e t h o d s .

A C K N O W L E D G E M E N T S

I t i s a p l e a s u r e a n d a p r i v i l e g e t o a c k n o w l e d g e t h e n u m e r o u s h e l p f u l d i s c u s s i o n s , i d e a s a n d o b s e r v a t i o n s o f my c o - w o r k e r s p r e s e n t a n d p a s t : J . N . B e c k , T . D . B u r t o n , J . B .C r o s s , S . A . D a b n e y , J . - L . D e b r u n , G . F r a n c i s , S . M. K o r m a l i , V . K r i v a n , J . R . M c G i n l e y , L . R . N o v a k , D. C . R i d d l e , H. L . R o o k , G . J . S t o c k , D . L . S w i n d l e , J . - P . T h o m a s , R . Z e i s l e r , a n d L . Z i k o v s k y .

T h a n k s a r e a l s o d u e t o t h e c y c l o t r o n o p e r a t i o n s p e r s o n n e l , T e x a s A&M U n i v e r s i t y .

T h e f i n a n c i a l s u p p o r t b y t h e N a t i o n a l S c i e n c e F o u n d a t i o n f o r t h e s t u d i e s c a r r i e d o u t a t T e x a s A&M U n i v e r s i t y i s g r a t e f u l l y a c k n o w l e d g e d .

R EFER EN C E S

[ 1 ] SEABORG, G . T . , LIVIN G OOD , J . J . , J . Am. Chem . S o c . 60 ( 1 9 3 8 ) 1 7 8 4 .

[ 2 ] S U E , P . , C . R . A c a d . S e i . P a r i s Ç 2 4 2 ( 1 9 5 6 ) 7 7 0 .[ 3 ] A L B E R T , P h . , M o d e r n T r e n d s i n A c t i v a t i o n A n a l y s i s ( P r o c .

1 9 6 1 I n t . C o n f . C o l l e g e S t a t i o n ) , T e x a s A&M U n i v e r s i t y ( 1 9 6 1 ) 7 8 .

[ 4 ] G I L L , R . A . , P r o t o n A c t i v a t i o n A n a l y s i s i n t h e D e t e r - m i n i a t i o n o f S u b m i c r o g r a m A m o u n t s o f B o r o n i n S i l i c o n , AERE R e p o r t C / R 2 7 5 8 ( 1 9 5 8 ) .

[ 5 ] S A I T O , K . , NOZAKI, T . , TANAKA, S . , FURUKAWA, M . , CHENG,H . , I n t . J . A p p l . R a d i a t , I s o t . 1 4 ( 1 9 6 3 ) 3 5 7 .

[ 6 ] MARKOWITZ, S . S . , MAHONY, J . D . , " 5 n a l . C hem . 3 4 ( 1 9 6 2 ) 3 2 9 .


[ 7 ] P I E R C E , T . B . , S p a t i a l l y S e n s i t i v e A n a l y t i c a l T e c h n i q u e s , t h e s e p r o c e e d i n g s .

[ 8 ] VALKOVIC, V . , X - r a y F l u o r e s c e n c e , t h e s e p r o c e e d i n g s .[ 9 ] LANGE, J . , MUNZEL, H . , K E L L E R , K . A . , PF E N N IG , G . ,

E x c i t a t i o n F u n c t i o n s f o r C h a r g e d P a r t i c l e I n d u c e d R e a c t i o n s , L a n d o l t - B ö r n s t e i n , N e u e S e r i e , G r u p p e I ,B a n d 5 b , N u c l e a r R e a c t i o n s , S p r i n g e r , B e r l i n ( 1 9 7 3 ) .

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[ 1 4 ] A JZ E N B E R G -S E L O V E , F . , N u c l . P h y s . A 1 9 0 ( 1 9 7 2 ) 1 .[ 1 5 ] ENDT, P . M . , VAN DER L E U N , C . , N u c T T T h y s . A 1 0 5 ( 1 9 6 7 )

1 . .

[ 1 6 ] KRIVAN, V . , A p p l i c a t i o n s o f N u c l e a r D a t a i n S c i e n c ea n d T e c h n o l o g y ( P r o c . S y m p . , P a r i s 1 9 7 3 ) , IA E A , V i e n n a ( 1 9 ) .

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[ 1 9 ] M cG IN E L Y , J . R . , SCHWEIKERT, E . A . , A n a l . C hem . 4 8( 1 9 7 6 ) 4 2 9 . —

[ 2 0 ] THOMAS, J . P . , SCHWEIKERT, E . A . , N u c l . I n s t r . M e t h . 9 9( 1 9 7 2 ) 4 6 1 . —

[ 2 1 ] BURTON, T . D . , SWINDLE, D . L . , SCHWEIKERT, E . A . , R a d i o - c h e m . R a d i o a n a l . L e t t . 13^ ( 1 9 7 3 ) 1 9 1 .

[ 2 2 ] DeM ICHELE, <D. W . , F A R E S , Y . , GOESCHL, J . D . , B A L T U SK O N IS , D . A . , P l a n t P h y s i o l o g y ( 1 9 7 7 ) i n p r e s s .

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[ 2 4 ] A L B E R T , P h . , p e r s o n a l c o m m u n i c a t i o n ( 1 9 7 6 ) .[ 2 5 ] ROOK. H. L . , T h e D e t e r m i n a t i o n o f T r a c e L e v e l s o f

O x y g e n a n d C a r b o n b y C h a r g e d P a r t i c l e A c t i v a t i o n A n a l y s i s , P h . D . D i s s e r t a t i o n , T e x a s A&M U n i v e r s i t y , C o l l e g e S t a t i o n ( 1 9 6 9 ) .

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[ 2 8 ] Mc G IN E C Y , J . R . , SCHWEIKERT, E . A . , A n a l . Chem . 4 7 ( 1 9 7 5 ) 2 4 0 3 .

[ 2 9 ] M c G IN L E Y , J . R . , STOCK, G . J . , SCHWEIKERT, E . A . , C R O S S , J . B . , Z E I S L E R , R . , Z IK O V S K Y , L . , J . R a d i o a n a l . Chem.( 1 9 7 7 ) i n p r e s s .

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p a r a c t i v a t i o n d a n s l e s p a r t i c u l e s c h a r g e e s e t l e sp h o t o n s d e 35 MeV, D i s s e r t a t i o n U n i v . o f P a r i s ( 1 9 6 9 ) .

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3 7 1 . —


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[ 6 8 ] ENGELMANN, C h . , G O S S E T , J . , L O E U I L L E T , M . , MARSCHAL, A . , O SSA R T , P . , B O I S S I E R , M . , M o d e r n T r e n d s i n A c t i v a t i o n A n a l y s i s ( P r o c . 1 9 6 8 I n t . C o n f . G a i t h e r s b u r g ) , NBS S p e c . P u b l . 3 1 2 ( 1 9 6 9 ) , V o l . I I , 8 1 9 .

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[ 7 2 ] ENGELMANN, C h . , C o n t r i b u t i o n a l ' e t u d e d e l ' a n a l y s e p a r a c t i v a t i o n a n m o y e n d e s p a r t i c u l e s c h a r g e e s e t d e s p h o t o n s gam m a, d i s s e r t a t i o n U n i v . o f P a r i s ( 1 9 7 0 ) .

[ 7 3 ] L IG E O N , E . , BONTEMPS, A . , J . R a d i o a n a l . Chem . 1 2 ( 1 9 7 2 ) 3 3 5 .

[ 7 4 ] GIROUX, J . , TALVAT, M . , THOMAS, J . - P . , T O U S S E T , J . ,B u l l . S o c . C h im . F r a n c e 2 ( 1 9 7 1 ) 7 0 6 .

[ 7 5 ] ENGELMANN, C h . , MARSCHAL, A . , R a d i o c h e m . R a d i o a n a l .L e t t . 6 ( 1 9 7 1 ) 1 9 5 .

[ 7 6 ] VALLADON, M . , DEBRUN, J . - L . , M o d e r n T r e n d s i n A c t i v a t i o n A n a l y s i s ( P r o c . 1 9 7 6 I n t . C o n f . M ü n c h e n ) , t o a p p e a r i n J . R a d i o a n a l . C hem . ( 1 9 7 7 ) .

[ 7 7 ] ALEKSANDROVA, G. I . , DEMIDOV, A . M . , KOTELNIKOV, G . A . ,PLESHAKOVA, G . P . , SUKHOV, G . V . , CHOPOROV, D . Y . , SHMANENKOV, G . I . , S o v i e t A t o m i c E n e r g y 2 3 ( 1 9 6 7 ) 7 8 7 .

[ 7 8 ] HOLM, D. M. , B R I S C O E , W . , PARKER, J . , SANDERS, W. M . ,L A S L R e p o r t D C - 8 8 6 6 ( 1 9 6 6 ) .

[ 7 9 ] K IM , C . K . , R a d i o c h e m . R a d i o a n a l . L e t t . 2 ( 1 9 6 9 ) 5 3 .[ 8 0 ] K IM , C . K . , A n a l . C h im . A c t a 5 4 ( 1 9 7 1 ) 4 Ö 7 .[ 8 1 ] KRASNOV, N . N . , S o v i e t A t o m i c E n e r g y 2 6 ( 1 9 6 9 ) 2 8 4 .[ 8 2 ] SWINDLE, D . L . , SCHWEIKERT, E . A . , T a l a n t a 2 2 ( 1 9 7 5 ) 8 4 .[ 8 3 ] ZIK O V SK Y , L . , SCHWEIKERT, E . A . , J . R a d i o a n a T . Chem .

39 ( 1 9 7 7 ) i n p r e s s .[ 8 4 ] S I P P E L , R . F . , GLOVER, E . D . , N u c l . I n s t r . M e t h . 9

( 1 9 6 0 ) 3 7 .[ 8 5 ] KRIVAN, V . , A n a l . C hem . 4 7 ( 1 9 7 5 ) 4 6 9 .[ 8 6 ] KORMALI, S . M . , SCHWEIK ERT, E . A . , J . R a d i o a n a l . Chem .

2 2 ( 1 9 7 4 ) 1 3 9 .[ 8 7 ] KRIVAN , V . , J . R a d i o a n a l . C hem . 2 6 ( 1 9 7 5 ) 1 5 1 .[ 8 8 ] PARSA, B . , MARKOWITZ, Si. S . , А п а Г Г Chem . 4 6 ( 1 9 7 4 ) 1 8 6 .[ 8 9 ] KRIVAN, V . , MUNZEL, H . , J . R a d i o a n a l . Chem . 15 ( 1 9 7 3 )

5 7 5 .[ 9 0 ] A M IE L , S . , MANTEL, M . , A L F A S S I , Z . B . , M o d e r n T r e n d s i n

A c t i v a t i o n A n a l y s i s ( P r o c . 1 9 7 6 I n t . C o n f . M ü n c h e n )t o a p p e a r i n J . R a d i o a n a l . Chem . ( 1 9 7 7 ) .


[ 9 1 ] ENGLEMANN, C h . , R a d i o c h e m i c a l M e t h o d s o f A n a l y s i s ( P r o c . S y m p . S a l z b u r g 1 9 6 4 ) , V o l . I . , I A E A , V i e n n a( 1 9 6 5 ) 3 4 1 .

[ 9 2 ] H I S L O P , J . S . , S T E V E N S , J . R . , WOOD, D . A . , M o d e r nT r e n d s i n A c t i v a t i o n A n a l y s i s ( P r o c . 1 9 7 6 I n t . C o n f . M ü n c h e n ) , t o a p p e a r i n J . R a d i o a n a l . Chem . ( 1 9 7 7 ) .

[ 9 3 ] M cG IN L E Y , J . R . , Z IK O V SK Y , L . , SCHWEIKERT, E . A . ,M o d e r n T r e n d s i n A c t i v a t i o n A n a l y s i s ( P r o c . 1 9 7 6 I n t . C o n f . M ü n c h e n ) , t o a p p e a r i n J . R a d i o a n a l . Chem .( 1 9 7 7 ) .

[ 9 4 ] S M A L E o , A . A . , A n n . R e p . P r o g r . Chem . 4 6 ( 1 9 4 9 ) 2 9 0 .[ 9 5 ] BARRANDON, N . . A L B E R T , P h . , M o d e m T r e n d s i n A c t i v a t i o n

A n a l y s i s ( P r o c . 1 9 6 8 I n t . C o n f . G a i t h e r s b u r g ) , NBS S p e c . P u b l . 3 1 2 , V o l . I I ( 1 9 6 9 ) 7 9 4 .

[ 9 6 ] R E V E L , G . , L I N C K , I . , DA CUNHA BELO , M . , P A ST O L , J . L . , KRAU S, L . , R a d i o c h e m . R a d i o a n a l . L e t t . 2 7 ( 1 9 7 6 ) 1 9 1 .

[ 9 7 ] B O R D E R IE , B . , BARRANDON, J . N . , DEBRUN, 7 7 - L . , M o d e r nT r e n d s i n A c t i v a t i o n A n a l y s i s ( P r o c . 1 9 7 6 I n t . C o n f . M ü n c h e n ) , t o a p p e a r i n J . R a d i o a n a l . C hem . ( 1 9 7 7 ) .

[ 9 8 ] Z E I S L E R , R . , C R O S S , J . . B . , SCHWEIKERT, E . A . , A n a l .Chem . 4 8 ( 1 9 7 6 ) ' 2 1 2 4 .

[ 9 9 ] C R O SS , J . B . , Z E I S L E R , R . , SCHWEIKERT, E . A . , N u c l . I n s t r . M e t h . 1 4 2 ( 1 9 7 7 ) 1 1 1 .

S P A T I A L L Y S E N S I T I V E

A N A L Y T I C A L T E C H N I Q U E S

T .B . P I E R C E

A to m ic E n e rg y R e s e a rc h E s ta b lis h m e n t ,

H arw ell, O x fo r d s h ire ,

U n ite d K in g d o m

Abstract

SPA TIA LLY SE N SIT IV E AN A LYTICA L TECHNIQUES.The properties o f many materials are dependent not only upon the overall composition,

but also upon the way the elemental constituents are distributed throughout the sample. The paper reviews the theoretical background and the experimental requirements o f analytical methods which exploit the manoeuvrability and focusing ability o f the ion beams in particle accelerators to induce nuclear reactions in the selected part o f the sample. By detecting and analysing the products o f a nuclear reaction, quantitative results on the sample constituents and their spatial distribution are obtained. Examples o f profitable practical applications of the techniques are presented.

1. IN T R O D U C T IO N

T h e g ro w in g a p p r e c ia t io n o f th e im p o r ta n c e o f th e e f fe c t o f lo c a t io n as w ell

as c o n c e n tr a t io n o n th e p r o p e r tie s o f m a n y m a te r ia ls h a s s t im u la te d in te r e s t in

s p a tia lly se n sitiv e a n a ly t ic a l te c h n iq u e s . F o r m a te r ia ls se n s itiv e to e le m e n ta l

d is tr ib u t io n s , a g ro ss a n a ly s is o f th e c o m p o s it io n o f a b u lk sa m p le o f t e n d o e s n o t

p ro v id e a d e q u a te in fo r m a t io n fo r su b s e q u e n t in te r p r e ta t io n an d m o re d e ta ile d

m e a s u r e m e n ts a re re q u ire d . In o r d e r to m e e t th is g ro w in g n e e d , su b s ta n tia l

a t te n t io n h a s b e e n a llo c a te d to th e d e v e lo p m e n t o f s p a tia lly se n sitiv e a n a ly t ic a l

te c h n iq u e s an d th e m o s t p o s s ib le m e th o d s o f m e a s u r e m e n t, b o t h n o v e l and

e s ta b lis h e d , h av e b e e n e x a m in e d to a ssess w h e th e r so m e d e g re e o f sp a tia l sen si

t iv ity c a n b e c o n fe r r e d u p o n th e m . T w o ty p e s o f a p p r o a c h hav e b e e n m o st

w id e ly in v e s tig a te d . O n e o f th e s e re lie s o n g e n e r a l i llu m in a t io n o f th e sam p le

s u r fa c e w ith r a d ia t io n a n d e x p lo its a p o s itio n -s e n s itiv e d e te c to r to id e n t i fy th e

lo c a t io n o f th e s o u rc e o f so m e p r o d u c t e m it te d as a re s u lt o f th e in te r a c t io n o f

th e sa m p le w ith th e p r im a ry r a d ia tio n b e a m ; a u to ra d io g r a p h y an d c e r ta in ty p e s

o f m ic r o s c o p y fa ll in to th is c a te g o r y . T h e o t h e r a p p r o a c h c o n s is ts o f in te rr o g a tin g

th e sa m p le w ith a b e a m o f r a d ia t io n o f c a r e fu lly c o n tr o lle d g e o m e tr y w h ic h

in te r a c ts o n ly w ith th a t p a r t ic u la r p a rt o f th e sa m p le th a t, is o f in te r e s t and

m o n ito r in g th e p r o d u c ts o f in te r a c t io n w ith a d e te c to r w h ic h , in th is c a se , n eed

3 3

3 4 P IE R C E

n o t n e c e s s a r ily b e sp a tia lly se n sitiv e . T h is m ic r o p r o b e -ty p e a p p r o a c h h as

e x p lo ite d ir ra d ia tio n w ith e le c t r o n s , lo w -e n e rg y io n s an d p h o to n s and th e

p r o d u c ts m ea su red hav e in c lu d e d s e c o n d a ry io n s , X -ra y s and lin e s in th e v is ib le

s p e c tr u m [1 ]. B e s t k n o w n o f th e m ic r o p r o b e s is th e e le c t r o n m ic r o p r o b e , w h ic h

h a s n o w b e e n v ery w id e ly a p p lie d , and s o p h is t ic a te d c o m m e rc ia l e q u ip m e n t

h a s b e e n d e v e lo p e d p o ssessin g s u b s ta n tia l a n a ly t ic a l and im a g e -p ro ce ss in g

c a p a b ilit ie s . U n til c o m p a r a tiv e ly r e c e n t ly , h o w e v e r, n u c le a r -a n a ly t ic a l te c h n iq u e s

hav e m a d e o n ly a re la t iv e ly m in o r c o n t r ib u t io n to th is im p o r ta n ta n d e x p a n d in g

a re a o f a n a ly t ic a l s c ie n c e , r e p o r te d a p p lic a t io n s b e in g re s tr ic te d m a in ly to a u to

ra d io g ra p h y an d tr a c k -c o u n tin g m e th o d s . O n e re q u ire m e n t w h ic h e x is t in g

m ic r o a n a ly t ic a l te c h n iq u e s d o n o t a lw a y s m e e t s a t is fa c to r i ly , h o w e v e r, is th e

q u a n tita t iv e d e te r m in a t io n o f th e d is tr ib u t io n o f lig h t e le m e n ts s in ce th e s e are

o f t e n d if f ic u lt to d e te r m in e b y e x is t in g m e th o d s . O v er r e c e n t y e a rs , c o n s id e ra b le

a t te n t io n h as b e e n d e v o te d to e v a lu a tin g and a p p ly in g a n a ly t ic a l te c h n iq u e s ,

e x p lo it in g m e a su re m e n t o f th e p ro m p t r a d ia t io n e m it te d as a re s u lt o f n u c le a r

in te r a c t io n r a th e r th a n th e in d u ce d r a d io a c t iv ity p ro d u c e d as a re s u lt o f n u c le a r

d e c a y . S u c h p ro m p t-ra d ia tio n te c h n iq u e s are w ell ap p lied to th e d e te r m in a t io n

o f lig h t e le m e n ts s in c e p ro m p t p a r t ic le s o r d e -e x c ita t io n g a m m a r a d ia tio n ca n

b e p ro d u c e d f r o m lig h t e le m e n ts a t in c id e n t io n en e rg ie s w h ic h a re su b s ta n tia lly

lo w e r th a n th o s e re q u ire d to p e n e tr a te th e C o u lo m b b a r r ie r o f in te r m e d ia te o r

h e a v y e le m e n ts . R e a c t io n s c a n th e r e fo r e b e p r e fe r e n t ia l ly g e n e ra te d w ith lig h t

e le m e n ts w ith o u t e x c it in g r a d ia t io n f ro m h e a v ie r e le m e n ts w h ic h m ig h t c o m p lic a te

t h e d e te r m in a t io n . P ro m p t-r a d ia t io n m e a su r e m e n t c a n b e e x p lo ite d to p e rm it

c o r r e la t io n o f a n a ly t ic a l d a ta w ith lo c a t io n b y tw o ro u te s . F ir s t ly , th e c h a r a c

t e r is t ic s o f n u c le a r in te r a c t io n ca n b e u sed to g ive in fo r m a t io n a b o u t s u b -su rfa c e

c o m p o s it io n s an d , se c o n d ly , th e in c id e n t io n -b e a m ca n b e re d u ce d in size b y c o ll im a t io n o r b y fo c u ss in g to p ro v id e a m ic r o p r o b e w h ic h is th e n u c le a r a n a lo g u e

o f th e e le c t r o n m ic r o p r o b e ; th is p a p e r is c o n c e r n e d so le ly w ith th e la t te r a p p ro a c h .

In g e n e r a l, d e te r m in a t io n s w ith th e n u c le a r m ic r o p r o b e re q u ire m e a s u r e m e n ts o f

p ro m p t p a r t ic le s o r p h o to n s re s u ltin g f r o m n u c le a r r e a c t io n p ro c e s s e s , p a r t ic le s

p ro d u c e d as a re s u lt o f e la s t ic s c a t te r in g o r X -ra y s . T h e c h a r a c te r is t ic s o f a n a ly t ic a l

p ro c e d u re s e x p lo it in g e a c h o f th e s e so u rc e s o f ra d ia tio n w ill b e co n s id e re d b r ie f ly

in th is p a p e r b e fo r e m e th o d s o f p r o d u c t io n o f sm a ll-d ia m e te r io n b e a m s and

a p p lic a t io n s a re d iscu ssed .

2 . A N A L Y T IC A L M E T H O D S

T h e m o s t fa m ilia r a n a ly t ic a l te c h n iq u e s e x p lo it in g sa m p le ir ra d ia tio n w ith

n e u tr o n s , ch arg ed p a r t ic le s o r g a m m a ra y s a re based a lm o s t e x c lu s iv e ly u p o n

c o u n tin g th e in d u ce d r a d io a c t iv ity fo r m e d in th e sa m p le as a re s u lt o f n u c le a r

r e a c tio n . T h e s e a c t iv a tio n te c h n iq u e s hav e b e e n e x te n s iv e ly in v e s tig a te d b y

S P A T IA L L Y S E N S IT IV E T E C H N IQ U E S 3 5

- I N C I D E N T I O N S ------------------------------------► ■«------------------------------------------------------P R O D U C T S -

A C C E L E R A T E D

C H A R G E D P A R T I C L E S

F IG .f . D iagram m atic representation o f princip les o f m icroprobe operation.

m a n y w o rk e rs fo llo w in g th e d e m o n s tr a t io n o f n e u tr o n a c t iv a tio n b y H e v esy [2 ]

and c h a rg e d -p a rtic le te c h n iq u e s b y S e a b o r g [ 3 ] . N e u tr o n a c t iv a tio n a n a ly s is n o w

h a s an e s ta b lish e d p la c e as a h ig h -s e n s itiv ity a n a ly t ic a l m e th o d w h ile ch a rg ed -

p a r t ic le ir ra d ia tio n h a s b e e n p r im a rily re se rv e d f o r th e d e te r m in a t io n o f th o s e

lig h t e le m e n ts to w h ic h n e u tr o n a c t iv a tio n c a n n o t b e c o n v e n ie n t ly a p p lie d ; in

a lm o s t a ll a p p lic a t io n s , th e p u rp o s e o f th e e x p e r im e n ta l p r o c e d u r e h as b e e n to

p ro v id e a n a ly t ic a l d a ta re p re s e n ta t iv e o f t h e b u lk c o m p o s it io n o f th e sa m p le .

I n e x te n d in g n u c le a r -a n a ly t ic a l te c h n iq u e s to p ro v id e in fo r m a t io n w ith g o o d

sp a tia l r e s o lu t io n , a p r im a ry re q u ir e m e n t is f o r h ig h in tr in s ic s e n s it iv ity o f th e

a n a ly t ic a l m e th o d s in c e , in th e e n d , th e v o lu m e o f sa m p le f ro m w h ic h re le v a n t

a n a ly t ic a l in fo r m a t io n is a v a ila b le is l ik e ly to b e v ery sm a ll. F o r tu n a te ly , m a n y

o f th e p ro m p t-r a d ia t io n te c h n iq u e s , in c o m m o n w ith c o n v e n tio n a l a c t iv a tio n

m e th o d s , d o o f f e r g o o d “ b e s t -c a s e ” s e n s it iv ity a lth o u g h th e a c tu a l lo w e st

c o n c e n tr a t io n th a t c a n a c tu a lly b e a ch ie v e d w h e n in v e s tig a tin g a “ r e a l” sam p le

w ill b e less an d w ill b e a fu n c t io n o f c o m p o s it io n . N e v e rth e le ss , th e sev eral

te c h n iq u e s d o hav e th e ir o w n p a r t ic u la r c h a r a c te r is t ic s an d th e s e a re su m m a riz ed

in tu r n b e lo w w h ile t h e b a s ic m ic r o p r o b e p ro c e d u re is su m m a riz e d d ia g ra m -

m a tic a l ly in F ig . 1. T h e s e e m it te d r a d ia tio n s in c lu d e d in F i g . l b u t n o t d iscu ssed

in th e p a p e r a re e x c lu d e d o n ly b e c a u s e th e y hav e n o t so fa r p ro v id e d th e b a s is

fo r m a jo r m ic r o p r o b e a p p lic a t io n s .

( a ) G a m m a r a d ia tio n

E x c i te d n u c le a r s ta te s are p ro d u c e d as a re s u lt o f n u c le a r r e a c t io n and

ro u te s m a y b e o p e n f o r th e s e s ta te s to lo s e th e ir e x c i ta t io n e n e rg y b y g am m a-

ra y e m is s io n . In m a n y c a se s , p h o to n s a re e m it te d w ith in p ic o s e c o n d s o f th e

N E U T R O N S

E L E C T R O N S

P A R T I C L E S F R O M S C A T T E R I N G

Î i - R A Y S

P A R T I C L E S F R O M R E A C T I O N S

X - R A Y S ' ' ,

L U M I N E S C E N C E

3 6 P IE R C E

in it ia l in te r a c t io n o f . t h e in c id e n t io n so th a t a d e te c to r m u st b e in p o s it io n and

c o u n tin g d u rin g th e ir ra d ia tio n fo r th e m to b e m easu red - T h e s e g a m m a ra y s are

o f a s p e c if ic e n e rg y and th e r e fo r e ca n b e co n s id e re d to b e c h a r a c te r is t ic o f a

p a r t ic u la r n u c le u s ; th e y th e r e fo r e o f f e r a m e a n s o f id e n tify in g in d iv id u a l

e le m e n ts a n d , in a p p r o p ria te ca se s , p e rm it m u lti-e le m e n t a n a ly sis o f c o m p le x

sa m p le s . F u r th e r , s in ce su c h lin es ca n re s u lt f ro m s ta b le - to -s ta b le t r a n s itio n s ,

a ra d io a c tiv e p ro d u c t is n o lo n g e r n e e d e d fo r m e a s u r e m e n t. T h is g re a tly e x te n d s

th e ra n g e o f m e th o d s a v a ila b le f o r l ig h t-e le m e n t d e te r m in a t io n o v e r th o s e o f fe r e d

b y c o n v e n tio n a l c h a rg e d -p a r tic le a c t iv a tio n a n a ly s is , w h ic h g e n e ra lly p ro d u c e s

p o s itr o n e m it te r s th a t c a n n o t b e d is tin g u ish ed fr o m e a c h o th e r b y e s ta b lish e d

m e th o d s o f g a m m a -ra y s p e c tr o s c o p y . A s a s p e c if ic e x a m p le o f th e p r o d u c t io n

o f c h a r a c te r is t ic g a m m a r a d ia tio n fro m s ta b le n u c le i , a 3 .0 9 -M e V g a m m a ra y is

e m it te d fr o m c a r b o n -1 3 p ro d u c e d b y th e r e a c t io n 12C ( d ,p ) 13C w h en a c a rb o n -

c o n ta in in g m a te r ia l is ir ra d ia te d w ith d e u te ro n s o f a b o u t 1 M e V , a lth o u g h b o t h

c a r b o n - 1 2 and c a r b o n -1 3 are s ta b le .

M e a s u re m e n t o f c a r b o n h as fig u red la rg e ly in a n a ly t ic a l a p p lic a t io n s o f

p ro m p t-ra d ia t io n te c h n iq u e s an d v a rio u s m e th o d s hav e b e e n d e v e lo p e d to m e e t

p a r t ic u la r re q u ire m e n ts . P r o to n ir ra d ia tio n p ro v id es a 2 .1 3 -M e V g a m m a ra y fo r

m e a s u r e m e n t [ 4 ] b y th e 12C (p ,7 ) 13N r e a c tio n . H ig h er s e n s it iv ity ca n b e a tta in e d

b y m e a n s o f th e 12C (d ,p )13C r e a c tio n as d e sc r ib e d a b o v e [ 5 ] and o th e r r e a c t io n s

a v a ila b le f o r c a r b o n in c lu d e th o s e in d u ce d b y h e liu m -3 io n s [6 ] and in e la s t ic

s c a t te r in g r e a c t io n s . W h e re a n u m b e r o f a lte r n a tiv e r o u te s fo r g e n e ra tin g p ro m p t

g a m m a r a d ia tio n are a v a ila b le , th e a n a ly s t h as so m e o p p o r tu n ity to c h o o s e lin es

w h ic h a p p e a r in c o n v e n ie n t re g io n s o f th e g a m m a -ra y s p e c tru m and are a w ay

fr o m th o s e e n erg ies w h ich are l ik e ly to b e s u b je c t to in te r fe r e n c e d u e t o o th e r

ta rg e t e le m e n ts . H o w ev er, th e p o te n tia l o f th is a p p r o a c h te n d s to b e r e s tr ic te d

b y th e s e n s it iv ity an d c o n v e n ie n c e o f th e v a r io u s .r e a c tio n s .

A n a ly tic a l te c h n iq u e s hav e b e e n d e sc r ib e d fo r m o s t o f th e lig h t e le m e n ts

and a n u m b e r o f e x a m p le s are g iven in T a b le I. G a m m a -ra y m e a su re m e n t has

th e a t t r a c t io n th a t s p e c tr a d o n o t e x p e r ie n c e th e sam e s e n s it iv ity to sa m p le

th ic k n e s s as, fo r e x a m p le , p a r t ic le p ea k sh a p e , and c a n b e in te rp r e te d b y

c o n v e n tio n a l te c h n iq u e s o f g a m m a -ra y s p e c tr o s c o p y . G a m m a -ra y d e te c t io n

c lo s e to an a c c e le r a to r o f t e n s u ffe r s fro m a h ig h b a c k g r o u n d w h ic h te n d s to

l im it th e s e n s it iv ity w ith w h ic h p a r t ic u la r e le m e n ts ca n b e d e te r m in e d . T h e

a v a ila b le s e n s it iv ity m a y b e d egrad ed s t i ll fu r th e r b y th e u su a l l im ita t io n s o f

g a m m a -ra y s p e c tr o s c o p y i f s p e c tr a are c o m p le x an d w a n te d lin e s o c c u r in re g io n s

o f C o m p to n e v en ts . F u r th e r , n u c le a r in te r fe r e n c e , fa m ilia r in a c t iv a tio n a n a ly s is ,

c a n a lso b e e x p e r ie n c e d d u rin g p ro m p t g a m m a -ra y m e a su r e m e n t w h en a

p a r t ic u la r e x c ite d lev e l w h o se d e -e x c ita t io n ra d ia tio n is b e in g c o u n te d c a n b e

fo rm e d b y tw o ro u te s , o n e f ro m th e w a n ted e le m e n t and o n e fro m a n e a r

n e ig h b o u r in th e p e r io d ic ta b le .


T A B L E I. E X A M P L E O F A N A L Y T IC A L D E T E R M IN A T IO N S

E X P L O I T I N G T H E M E A S U R E M E N T O F P R O M P T G A M M A

R A D IA T IO N

Element Reaction Ref.

Lithium 7L i(p ,7 ) 8Be [7]

Beryllium 9B e (p ,7 ) 10B [V]

' 9B e (a ,n )12C [8]

Boron n B (p ,T )12C [V]

Carbon 12C(d, p )13C [5]

12C (p ,7 )13N [4]

Nitrogen 14N (d ,p )I5N [9]

lsN (p ,a )I2C [10]

Oxygen 160 ( d ,p ) 170 [9]

I80 (p ,Y )19F [П ]

Fluorine 19F (p ,a )160 [ 7 ,8 ,1 2 ]

Magnesium 24M g(p ,p ')24Mg [13]

Aluminium 27A l(p ,7 )28Si [14]

27A l(p ,p ')27Al [13]

Silicon 28S i(p ,p ')28Si [15]

Sulphur 32S (p ,p ')32S [16]

( b ) P a rt ic le s f r o m n u c le a r r e a c t io n s

C h arg ed p a r t ic le s p ro d u c e d b y n u c le a r r e a c t io n s su c h as (d ,p ) , ( a ,p ) and

( p ,a ) p ro v id e a p o w e r fu l b a s is f o r a n a ly t ic a l m e a s u r e m e n t. T h e s e p a r t ic le s a re

e m it te d as a re s u lt o f p a r t ic u la r n u c le a r m e c h a n is m s an d hav e en e rg ie s w h ich

ca n b e re la te d to th e tr a n s it io n o c c u rr in g . E n e rg y a n a ly s is o f in d iv id u a l p a rt ic le s

th e r e fo r e p ro v id es a r o u te to id e n tify in g th e r e a c t io n w h ich h a s ta k e n p la c e and

a lso o f fe r s a m e a n s o f d is tin g u ish in g p a r t ic le s e m it te d fro m d if fe r e n t n u c le i .

A m a jo r d if fe r e n c e b e tw e e n ch a rg ed p a r t ic le s and g am m a p h o to n s a rises fro m th e

e n e rg y lo ss e x p e r ie n c e d b y ch a rg ed p a r t ic le s as th e y p ass th ro u g h m a tte r w ith th e

re s u lt th a t p a r t ic le p e a k -sh a p e s a re c r i t ic a lly d e p e n d e n t o n sa m p le th ic k n e s s .

W h ilst th is ca n c o m p lic a te in te r p r e ta t io n o f s p e c tr a l fe a tu r e s , i t a lso o f fe r s a

m e a n s o f e x a m in in g th e d e p th b e n e a th th e s u r fa c e a t w h ic h th e r e a c t io n h as

o c c u rr e d and so ca n p ro v id e th e b a s is fo r “ s u b -s u r fa c e ” a n a ly s is . P a r t ic le m e a su re

m e n t a lso p o ssesses tw o fu r th e r a d v a n ta g es w h ic h are n o t a s s o c ia te d w ith g am m a-

3 8 P IE R C E

T A B L E II . S O M E A N A L Y T IC A L M E T H O D S E X P L O IT IN G

C H A R G E D -P A R T IC L E D E T E C T IO N

Element , Reaction Ref.

Lithium 6L i(d ,a )4He [19]

7L i(d ,a )sHe

Beryllium 9B e (d ,p )10Be [20]

Boron i0B (d ,p )n B [21]

Carbon 12C (d ,p )13C [ 1 7 ,2 2 ]

Nitrogen 14N (d ,p )lsN [22]

Oxygen 160 ( d ,p ) 170 [17 , 22 , 23]

160 ( d ,a ) I4N [24]

180 ( p ,a ) lsN [25]

Fluorine 19F (p ,a )160 [19]

Sulphur 32S (d ,p )33S [26]

ra y m e a s u r e m e n t: ( a ) th e n a tu ra l p a r t ic le b a c k g r o u n d is v e ry lo w an d ( b ) s u r fa c e

b a r r ie r d e te c to r s u sed f o r p a r t ic le m e a s u r e m e n t a re re la t iv e ly in s e n s itiv e to b e ta

and g a m m a ra d ia tio n n o rm a lly e m it te d d u rin g ra d io a c tiv e d e c a y . T h is la t t e r

c h a r a c te r is t ic e n a b le s th e m ic r o p r o b e to b e ap p lied to h ig h ly ra d io a c tiv e sam p les

w h ic h c a n b e o f v a lu e w h en e x a m in in g m a te r ia ls used in n u c le a r r e a c to r s p ar

t ic u la r ly a f te r ir ra d ia tio n . S p e c tr a l in te r p r e a tio n c a n , o f c o u rs e , o n ly b e a ch iev ed

w ith in th e c o n s tr a in ts im p o se d b y s p e c tr a l c o m p o s it io n , an d a c le a r s ig n a l is

n e e d e d fr o m th e c o m p o n e n t o f in te r e s t fo r r e lia b le q u a n t ita t iv e in te r p r e ta t io n

to b e fe a s ib le . A la rg e n u m b e r o f r e a c t io n s p ro d u c in g p a r t ic le s is a v a ila b le fo r

l ig h t-e le m e n t d e te r m in a t io n s and th e s e hav e b e e n u sed fo r a ra n g e o f a n a ly t ic a l

m e a s u r e m e n ts [ 1 7 ] . S u c h r e a c t io n s n o t o n ly o f f e r a m e a n s o f m ea su rin g th e

p re s e n c e o f in d iv id u a l e le m e n ts in a v a r ie ty o f m a tr ic e s b u t a lso p e r m it th e

m o v e m e n t o f p a r t ic u la r e le m e n ts to b e tra c e d b y d o p in g w ith r e a c ta n ts e n r ic h e d

in a p a r t ic u la r s ta b le is o to p e [ 1 8 ] . T a b le I I in c lu d e s e x a m p le s o f a n a ly t ic a l

te c h n iq u e s w h ic h a re b a se d o n p a r t ic le m e a s u r e m e n t. . . ■

W hilst th e r e a c tio n s g iv en in T a b le I I e x p lo it c o u n tin g o f ch a rg ed p a r t ic le s , .

in p r in c ip le , d e te c t io n o f n e u tr o n s p ro d u c e d b y , f o r e x a m p le , th e (d ,n ) r e a c t io n

c o u ld a lso b e used fo r a n a ly t ic a l d e te r m in a t io n . H o w ev er, th e m o re lim ite d

c a p a b il i ty o f e x is t in g n e u tr o n -d e te c t io n te c h n iq u e s h a s r e s tr ic te d n e u tr o n c o u n tin g

to a fe w a p p lic a t io n s w h e re th e p a r t ic u la r a d v a n ta g e s a re w o r th w h ile ; an e x a m p le

is th e d e te c t io n o f b e ry ll iu m [ 2 0 ] .


( c ) E la s t ic s c a t te r in g

E la s t ic s c a t te r in g o f ch a rg ed p a r t ic le s o f fe r s a n a lte r n a tiv e m e a n s o f an a ly s in g

s u r fa c e s and p ro v id es in fo r m a t io n w h ich is p r im a rily a fu n c t io n o f th e m a ss o f

th e ta rg e t n u c le u s . In te r a c t io n s w h ic h o c c u r d u rin g th e s c a t te r in g p ro c e s s are

a n a lo g o u s to a “ b il l ia r d -b a ll” ty p e c o ll is io n an d r e s u lt w h en a p o s itiv e ly ch a rg ed

p r o je c t i le is s c a t te r e d f ro m a p o s itiv e ta rg e t n u c le u s . T h e a m o u n t o f e n e rg y

re ta in e d b y th e p r o je c t i le a f te r s c a t te r in g is d e p e n d e n t u p o n th e q u a n t i ty o f

e n e rg y lo s t b y im p a r tin g r e c o i l to th e s c a t te r in g n u c le u s a n d , s in c e e n e rg y lo ss

is g re a te r th e lig h te r th e ta rg e t n u c le u s , th e m e th o d p ro v id es a te c h n iq u e o f

m ass a n a ly s is w ith th e lo w e s t e n e rg y io n s s c a t te r e d fro m th e lig h t e le m e n ts .

T h e r a t io o f th e en e rg ie s o f th e in c id e n t p a r t ic le (m a ss m ) b e fo r e ( E 0) and a f te r

(Ел ) c o ll is io n w ith a ta rg e t n u c le u s (m a ss M ) is g iv en b y

w h ere в = th e s c a t te r in g an g le .

W h en ta rg e t th ic k n e s s e s b e c o m e a p p r e c ia b le c o m p a re d w ith th e ra n g e o f

p e n e tr a t io n o f th e p a r t ic le in th e sa m p le m a tr ix , th e sp read o f en e rg ie s o f th e

s c a t te r e d p a r t ic le s w ill in c r e a s e s in c e s o m e p a r t ic le s lo s e e n e rg y m o v in g th ro u g h

th e sa m p le to an d f ro m th e s c a t te r in g s ite . T h u s , fo r a th ic k ta rg e t , th e s p e c tru m

w ill t a k e th e fo rm o f th e tr a d it io n a l R u th e r fo r d p la te a u w ith a n ed g e c h a r a c te r is t ic

o f s c a t te r in g m a te r ia l. I n te r p r e t a t io n 'o f d is tr ib u t io n o f e n e rg ie s o f p a r t ic le s

s c a t te r e d f ro m t h ic k ta rg e ts c a n b e v a lu a b le s in c e it m a y p ro v id e a m e a n s o f

d eriv in g in fo r m a t io n a b o u t s u b -s u r fa c e e le m e n ta l c o n c e n tr a t io n s . H o w ev er, w h en

a th in -s u r fa c e la y e r is b e in g e x a m in e d in th e p re s e n c e o f a th ic k s u b s tr a te ,

d if f ic u lt ie s ca n arise i f th e s u b s tr a te is o f g re a te r m ass th a n th e s u r fa c e f ilm s in ce

th e p e a k fro m th e s u r fa c e c o m p o n e n t w ill b e p re s e n t o n th e p la te a u f ro m th e

s u b s tr a te . E la s t ic s c a t te r in g is th e r e fo r e b e s t a p p lied to th e a n a ly s is o f a th in

f ilm o f a h eav y e le m e n t p re s e n t o n a l ig h te r s u b s tr a te . P ro v id in g th a t th e m ass

o f th e s u r fa c e f ilm is s u f f ic ie n t ly d if f e r e n t f ro m th a t o f th e s u b s tr a te , a v ery

se n sitiv e a n a ly t ic a l m e th o d is a v a ila b le .

S in c e th e d if fe r e n t ia l c r o s s -s e c t io n fo r p a r t ic le s c a t te r in g is a fu n c t io n o f

th e sq u a re o f th e a to m ic n u m b e r o f th e s c a t te r in g n u c le u s , th e g ro ss p a r t ic le y ie ld

f ro m a th ic k ta rg e t c a n b e u til iz e d to p ro v id e in fo r m a t io n a b o u t th e m e a n a to m ic

n u m b e r o f th e ta rg e t a re a b e in g e x a m in e d . T h is e n a b le s p a r t ic le s c a t te r in g to b e

u sed to p ro v id e a m e a n s o f rap id e x a m in a t io n o f sa m p le s u r fa c e s b e fo r e m o re

d e ta ile d e x a m in a t io n o f s e le c te d re g io n s o f p a r t ic u la r in te r e s t [ 2 7 ] . T h e te c h n iq u e

p ro v id es a v a lu a b le c o m p le m e n t to o t h e r a p p lic a t io n s o f fe r in g m o re s p e c if ic

a n a ly t ic a l in fo r m a t io n s in c e i t e n a b le s s e m i-q u a n tita tiv e in fo r m a t io n to b e

c o lle c te d ra p id ly f r o m la rg e areas .

4 0 P IE R C E

W hen sa m p les c a n b e p re p a red w h ich are s u ff ic ie n t ly th in to avo id a p p r e c ia b le

e n erg y lo ss in th e sa m p le , th e R u th e r fo r d p la te a u is a b s e n t and a m ass a n a ly s is o f

th e ta rg e t c o m p o s it io n ca n s o m e tim e s b e c a rr ie d o u t , p a r t ic u la r ly a t lo w m asses

w h e re th e b e s t m ass s e p a ra t io n is a ch ie v e d . T h e te c h n iq u e is v a lu a b le fo r th e

e x a m in a t io n o f b io lo g ic a l sa m p le s s in ce e s ta b lis h e d m e th o d s o f sp e c im e n p re

p a ra tio n e n a b le th in ta rg e ts to b e p ro d u c e d .

(d ) X -ra y g e n e r a tio n

T h e p assage o f ch a rg ed p a rt ic le s th ro u g h m a tte r lea d s to th e g e n e r a tio n o f

X -ra y s w h ich o f fe r s y e t a n o th e r m ean s o f o b ta in in g c h a r a c te r is t ic in fo r m a t io n

a b o u t th e e le m e n ta l c o n te n t o f in d iv id u a l sa m p le s . S in c e X -ra y s are e m itte d

s im u lta n e o u s ly w ith p ro d u c ts o f n u c le a r in te r a c t io n , th e tw o ty p e s o f ra d ia tio n

c a n b e m ea su red s im u lta n e o u s ly to o b ta in m o re d e ta ile d in fo r m a t io n a b o u t

c o m p o s it io n i f re q u ire d . A n a ly tic a l m e a su r e m e n ts re q u ir in g o n ly X -ra y c o u n tin g

fo llo w in g c h a rg e d -p a rt ic le ir ra d ia tio n w ith th e n u c le a r m ic r o p r o b e are o f te n hard

to ju s t i f y s in ce th e m a jo r ity o f su c h d e te r m in a t io n s a re l ik e ly to b e a b le to be

ca rr ie d o u t m o re c h e a p ly and m o re e f fe c t iv e ly w ith th e e le c t r o n m ic r o p r o b e ,

b u t th e u se o f X -ra y m e a su r e m e n t to s u p p le m e n t th e in fo r m a t io n o b ta in e d fro m

n u c le a r re a c t io n s ca n b e v ery v a lu a b le . T h e p o te n tia l o f X -ra y m e th o d s is

d iscu ssed e lse w h e re in th e s e P ro ce e d in g s an d th e r e fo r e is n o t co n s id e re d in a n y

d e ta il h e re , b u t a n u m b e r o f a s p e c ts o f th e a p p r o a c h re le v a n t to th e m ic r o p r o b e

are w o r th su m m a riz in g . A d e q u a te a c c e s s to th e d e te c to r fo r ra d ia tio n e m itte d

f ro m th e ta rg e t is n e e d e d in sp ite o f th e lim ite d sp a c e a v a ila b le a ro u n d th e p o in t

o f in te r a c t io n o f th e io n b e a m w ith th e sa m p le , and th is ca n b e d if f ic u l t to

a c h ie v e , p a r t ic u la r ly i f h ig h c o l le c t io n e f f ic ie n c y is re q u ire d . In g e n e ra l, e n e rg y

d isp ersiv e d e te c to r s h av e b e e n p re fe rre d fo r X -r a y c o u n tin g , s in ce th e ir h ig h

e f f ic ie n c y p e rm its g o o d c o u n t-r a te s to b e a ch iev ed fro m m a n y m a te r ia ls ev en

w h en b e a m c u rre n ts are lo w . H o w ev er, th e l im ita t io n s im p o sed b y th e re la tiv e ly

p o o r r e s o lu t io n o f s o lid -s ta te d e te c to r s re s u lts in th e n eed fo r c r y s ta l s p e c tr o

m e te r s fo r p a r t ic u la r a p p lic a t io n s and a c c e s s to b o t h ty p e s o f d e te c to r is re q u ire d

fro m t im e to t im e .

T h e o v era ll c a p a b il i ty o f X -ra y m e a s u r e m e n t h as b e e n su m m a riz e d in

p ro c e e d in g s o f c o n fe r e n c e s w h ic h have co n s id e re d th e to p ic in d e p th [ 2 8 ] . M an y

o f th e a p p lic a t io n s r e p o r te d have c e n tr e d o n th e e x a m in a t io n o f th e c o m p o s it io n

o f e i th e r th in ta rg e ts o r o f sa m p le s p o ssessin g m a tr ic e s o f lo w Z -n u m b e rs . W h ilst

th is c o n f ig u r a t io n is d e s ira b le to e n su re a lo w B re m s stra h lu n g c o m p o n e n t in th e

b a c k g r o u n d , in p r a c t ic e , m a n y sa m p les d o n o t c o n fo r m to th is ty p e o f c o m p o s it io n .

A p p lic a tio n o f X -ra y m e a s u r e m e n t to less s a t is fa c to r y m a tr ic e s c o m p lic a te s

d e te c t io n and re d u c e s th e o v e ra ll s e n s it iv ity o f th e d e te r m in a t io n . H o w ev er,

m e a s u r e m e n t o f th e h ig h y ie ld o f X -ra y s f ro m m a jo r e le m e n ts in m e ta l sam p les

p e r m its th e d is c o n tin u ity b e tw e e n th e m o u n tin g m a te r ia l and th e sa m p le to b e

S P A T I A L L Y S E N S IT IV E T E C H N IQ U E S 4 1

c le a r ly id e n tif ie d an d c o m p le m e n ts o p t ic a l a ss e ssm e n t o f sa m p le p o s it io n w h ic h

is n o t a lw a y s a d e q u a te to p e r m it a c c u r a te a lig n m e n t o f th e io n b e a m w ith a

p a r t ic u la r re g io n o f in te r e s t . X -ra y re s u lts w h e n c o r r e la te d w ith in fo r m a t io n

fr o m lig h t e le m e n ts p e r m it th e p re s e n c e o f s p e c if ic c o m p o u n d s to b e id e n tif ie d

in c e r ta ih ty p e s o f m e ta llu r g ic a l sa m p le (e .g . c h ro m iu m c a rb id e o r m e ta l o x id e s ) .

3 . P R O D U C T IO N O F S M A L L -D IA M E T E R IO N B E A M S

S m a ll-d ia m e te r io n b e a m s w ere in it ia l ly p ro d u ce d fo r th e m ic r o p r o b e b y

c o ll im a t io n [ 2 9 ] . B e a m c o ll im a to r s y s te m s a re c o m p a r a tiv e ly in e x p e n s iv e and

are d esig n ed to p ro d u c e io n b e a m s w ith d ia m e te rs o f 2 0 —3 0 ¡ x m w ith o u t to o

m u c h d if f ic u l ty . C o llim a tio n in fa c t m a y b e th e p re fe rre d m e a n s o f m ic r o b e a m

p r o d u c t io n i f sm a lle s t d ia m e te rs a re n o t re q u ire d . E x a m in a t io n o f th e sam p le

w ith a sm a lle r b e a m th a n w a rra n te d b y th e d e f in it io n re q u ire d is n o t a d v isa b le ,

s in c e b e a m c u rre n t is o f t e n r e s tr ic te d , t im e t a k e n fo r th e a n a ly s is lo n g and d a ta

g e n e ra te d u n n e c e s s a r ily d e ta ile d . M o re th a n o n e c o ll im a t in g s to p is u su a lly

n e e d e d to re m o v e th e e d g e -s c a tte re d c o m p o n e n t and a d ju s ta b le s to p s an d s to p -

p o s itio n s aid a lig n m e n t p ro c e d u re s w h ic h c a n b e te d io u s i f th e c o llim a tin g sy s te m

c o n ta in s tw o o r th r e e f ix e d s to p s p ie rc e d w ith h o le s a fe w te n s o f m ic r o m e tr e s in

d ia m e te r . F o c u s s in g th e io n b e a m s b y m e a n s o f su ita b le m a g n e ts d o e s o f c o u rs e

p ro v id e a m o re f le x ib le e q u ip m e n t an d p e r m its m o re e f f ic ie n t u se o f th e

a v a ila b le io n b e a m . V a r io u s w ay s hav e n o w b e e n r e p o r te d to a c h ie v e sm all

fo c u sse d p a r t ic le b e a m s a n d d ia m e te rs o f 2 o r 3 f i m ca n b e a t ta in e d . T h e re la tiv e ly

h ig h b e a m d e n s it ie s th a t c a n b e o b ta in e d b y fo c ü ss in g p e r m it th e s e b e a m s to b e

u s e fu lly e m p lo y e d fo r a n a ly s is an d th e s ta b i l i ty o f b e a m p o s it io n ( in sp ite o f th e

re la t iv e ly lo n g f lig h t-p a th b e tw e e n th e b e a m d e fin in g s to p s and ta r g e t ) e n a b le s

re p r o d u c ib le m e a s u r e m e n ts to b e c a rr ie d o u t . T h e sy s te m c u r r e n tly in u se a t

A E R E , H arw ell h as n o w b e e n r o u t in e ly e m p lo y e d fo r a n u m b e r o f y e a rs and

e x p lo it s a s y s te m o f le n s e s in th e fo r m o f a R u ss ia n Q u a d ru p le t [ 3 0 ] to p ro v id e

b e a m d ia m e te rs o f th e o r d e r o f 3 ^ m . T h e s y s te m w as c o n s tr u c te d fro m fo u r

s p e c ia lly d esig n ed q u a d r u p o le le n s e s , th e d is ta n c e s b e tw e e n o p p o s ite p o les

d iffe r in g b y ± 0 . 0 1 3 m m w h ile th e v a r ia t io n in th e s h o r te s t d is ta n c e b e tw e e n

n e ig h b o u rin g p o le s o n a sin g le m a g n e t w as o n ly ± 0 . 0 4 5 m m in 1 4 .2 5 m m .

Q u a d ru p o le s w ere a lig n ed o n th e sa m e g e o m e tr ic a l a x is w ith an a c c u r a c y o f a b o u t

± 5 0 /um and m a g n e tic c e n tr e s o f th e fo u r q u a d r u p o le s c o in c id e d w ith th e

g e o m e tr ic a l c e n tr e s to w ith in a b o u t ± 13 0 / л п [ 3 1 ] . In m a n y o f th e a p p lic a t io n s

e x a m in e d w ith th is m ic r o p r o b e , sa m p le s a re m o v ed m e c h a n ic a lly re la tiv e to th e

p o s it io n o f th e io n b e a m , b u t in c e r ta in c a se s , p a r t ic u la r ly th o s e w h e re rap id

sca n n in g o f th e s u r fa c e is n e e d e d to p ro v id e a v isu a l im a g e o r to re d u c e th e h e a t-

d am ag e in th e sa m p le , e le c t r o s ta t ic d e f le c t io n is e m p lo y e d . A b e r r a t io n s ca u sed

b y b e a m m o v e m e n t m u st b e k e p t to a m in im u m to p reserv e r e s o lu t io n an d fo r th e

4 2 P IE R C E

MICROSCOPE PARTICLE

F IG . 2. B lo c k diagram o f the nuclear m icroprobe.

I B I S s y s te m th e y w ere fo u n d to b e a b o u t 5 .5 % o f th e la te r a l d is p la c e m e n t in th e

X d ir e c t io n and 6 .7 % in th e Y d ir e c t io n w h en a p o te n tia l w as a p p lied to a sin g le

d e f le c to r p la te , b u t w h en e q u a l an d o p p o s ite p o te n tia ls w ere ap p lied to b o th

Y p la te s , a b e r r a t io n w as re d u c e d to 2 .7 % o f th e d is p la c e m e n t.

4 . E X P E R I M E N T A L C O N F IG U R A T IO N O F T H E M I C R O P R O B E

T h e c o m p o n e n t p a rts o f th e m ic r o p r o b e in c lu d e (a ) a s o u rc e o f e n e rg e tic

ch a rg e d p a r t ic le s , ( b ) a b e a m h a n d lin g and fo cu ss in g sy s te m to d ir e c t th e io n

b e a m o n th e ta rg e t , ( c ) a ta rg e t c h a m b e r w ith a ss o c ia te d sa m p le h a n d lin g fa c ilit ie s

and ( d ) r a d ia tio n d e te c to r s an d th e a ss o c ia te d c o u n tin g , c o n t r o l and d a ta p ro ce ss in g

e q u ip m e n t n e e d e d fo r c o l le c t io n an d in te r p r e ta t io n o f th e a n a ly t ic a l signals.

T h e e x p e r im e n ta l a rra n g e m e n t o f th e n u c le a r m ic r o p r o b e h as b e e n d e scrib e d in

d e ta il e lse w h e re [ 3 2 ] and c o n s e q u e n tly o n ly th e m a in c o m p o n e n ts o f th e s y s te m

are c o n s id e re d h e r e ; an o u t lin e d iag ram is g iv en in F ig .2 . T h e e q u ip m e n t i ts e lf ,

a lth o u g h in use fo r sev era l y e a rs , m u st s t i ll b e re g a rd ed as e x p e r im e n ta l s in c e it

h as n o t b e n e f ite d f ro m th e le v e l o f in s tr u m e n ta l d e v e lo p m e n t th a t h as b e e n d e v o te d

to o t h e r m ic r o a n a ly t ic a l te c h n iq u e s , fo r e x a m p le th e e le c t r o n m ic r o p r o b e .

C o n s e q u e n t ly , fa c i li t ie s a v a ila b le a re ru d im e n ta ry b y m ic r o a n a ly t ic a l s ta n d a rd s and

c o rre s p o n d m u c h m o re c lo s e ly to a re s e a rc h c o n f ig u r a t io n th a n to a fu lly d e v e lo p e d

p ie c e o f a n a ly t ic a l in s tr u m e n ta tio n . T h e e q u ip m e n t is c e n tr e d o n th e 3 -M V e le c t r o

s ta t ic g e n e r a to r I B I S an d th e m ic r o b e a m lin e is se c u re d to a su b s ta n tia l s te e l

s u p p o rtin g b e a m to e n su re th e a b s o lu te r ig id ity o f lin e th e c o m p o n e n ts n e ed to


av o id m o v e m e n t d u rin g u se. A n a d ju s ta b le b e a m -d e fin in g s to p a c ts as th e o b je c t iv e

fo r th e io n -o p t ic s and th e io n b e a m is fo c u sse d d o w n o n th e p la n e o f th e sa m p le -

ta b le in th e c h a m b e r so m e 1 2 f t ( 3 .6 6 m ) a w a y . T h e ta b le c a n b e m o v e d in th r e e

d im e n s io n s b y s te p p in g m o to r s u n d e r r e m o te c o n tr o l fro m o u ts id e th e c h a m b e r .

In p r a c t ic e , m e c h a n ic a l m o v e m e n t o f th e ta b le is p re fe rre d fo r lin e -sc a n s , th e ta b le

b e in g m o v ed d is c o n tin u o u s ly w ith ir ra d ia tio n s o f e a c h p o in t o f th e sa m p le s u r fa c e

b e in g c o n tr o l le d a u to m a t ic a lly to a g iv en io n d o se and th e sa m p le b e in g m o v ed to

a n e w p o s itio n re la tiv e to th e io n b e a m b e tw e e n ir ra d ia tio n s . T h e io n b e a m ca n

a lso b e d e fle c te d e le c t r o s ta t ic a l ly , and th is m o d e is p re fe rre d w h e n a h ig h -sp eed

ra s te r sca n is n e e d e d to g e n e r a te a p h o to g ra p h ic im a g e o r w h en th e h e a t d issip ate d

in th e sa m p le is to b e sp rea d o v e r a la rg e a re a to re d u c e d a m a g e . T h e sa m p le ta b le

i t s e l f h as s p a c e fo r tw o sa m p le d iscs u p to 1| in ( 2 .8 6 c m ) in d ia m e te r as w ell as

a p o s it io n fo r a q u a r tz d isc used d u rin g b e a m a lig n m e n t p ro c e d u re s . A m ic r o s c o p e

v iew s th e p o in t o f in te r a c t io n o f th e io n b e a m w ith th e sa m p le an d e n a b le s sam p les

to b e p o s itio n e d in it ia l ly so th a t th e re g io n o f in te r e s t is ir ra d ia te d . A se c o n d

m ic r o s c o p e a t th e re a r o f th e ta rg e t c h a m b e r p e r m its th e q u a r tz d is c to b e e x a m in e d

d u rin g in it ia l se ttin g -u p p ro c e d u re s so th a t b o th th e sh ap e an d size o f th e b e a m c a n

b e e x a m in e d . G a m m a -ra y , p a r t ic le an d X -r a y d e te c to r s v iew th e p o in t o f in te r

a c t io n o f th e io n b e a m and p e r m it th e a p p r o p r ia te ra d ia tio n s to b e c o u n te d .

D e te c to r o u tp u t is fed to a p u lse -h e ig h t a n a ly sin g s y s te m a f te r c o n v e n tio n a l

a m p lif ic a t io n and c o u n tin g s y s te m s a re u su a lly k e p t as s im p le as p o ss ib le to re d u c e

th e a m o u n t o f d a ta h a n d lin g n e e d e d . P u lses a re n o rm a lly fe d t o o n e o r m o r e s in g le

c h a n n e l a n a ly se rs and c o u n ts are c o lle c te d fo r a k n o w n io n d o se fa llin g o n th e

sa m p le . A n a ly se r o u tp u t c a n a lso b e fe d to an o s c i llo s c o p e o p e ra tin g in se q u e n c e

w ith e le c t r o s ta t ic d e f le c t io n o f th e io n b e a m to p ro v id e a v isu a l r e p r e s e n ta t io n o f

e le m e n ta l d is tr ib u t io n s .

A n im p o r ta n t p a rt o f th e m ic r o p r o b e te c h n iq u e is th e p r e p a ra tio n o f sa m p les

in a s u ita b le fo rm fo r ir r a d ia tio n an d p re fe rr e d p ro c e d u re s g e n e r a lly fo llo w a

s ta n d a rd m e ta llo g r a p h ic te c h n iq u e . T h e m a te r ia ls o f in te r e s t a re p la c e d in so m e

m o u n tin g m a te r ia l and a re p o lish e d w ith s u c c e ss iv e ly f in e r g rad es o f a b ra s iv e u n til

a g o o d f in is h is a c h ie v e d . S a m p le s a re n o rm a lly h e ld in W o o d s M e ta l to re d u c e th e

t o t a l q u a n t i ty o f lig h t e le m e n ts p re se n t in th e m o u n t and to re d u c e th e c h a n c e o f

c o n ta m in a t io n . S in c e sa m p le s o f fa ir ly w e ll-d e fin e d c o m p o s it io n s h av e g e n e ra lly

b e e n e x a m in e d w ith th e m ic r o p r o b e , th e p re fe rre d m e th o d o f c a l ib r a t io n has

b e e n w ith s ta n d a rd s and fo r th is p u rp o s e sev e ra l sta n d a rd m a te r ia ls , a n a ly sed b y

e s ta b lis h e d a n a ly t ic a l te c h n iq u e s , h av e b e e n p re p a red in th e sa m e m o u n t and

irra d ia te d to p e r m it a p p r o p r ia te c a l ib r a t io n cu rv es to b e g e n e r a te d .

5 . A P P L IC A T IO N S

T h e a p p lic a t io n s th a t h av e b e e n r e p o r te d fo r th e n u c le a r m ic r o p r o b e hav e

b e e n re s tr ic te d b y th e lim ite d n u m b e r o f in s ta lla tio n s so fa r in re g u la r u se. T h e

4 4 P IE R C E

p o te n tia l o f th e te c h n iq u e c a n , h o w e v e r, b e assessed b y c o n sid e rin g a n a ly t ic a l

uses fo u n d f o r b e a m s o f larg e d ia m e te r , b e a rin g in m in d th e p a r t ic u la r a d v an tag es

th a t a c c r u e fro m b e in g a b le to ir ra d ia te a sm a ll v o lu m e o f sa m p le . S in c e d e v e lo p

m e n t o f th e m ic r o p r o b e h as b e e n c e n tr e d o n n u c le a r la b o r a to r ie s , a p p lic a t io n s

o f p r im a ry in te r e s t hav e n o t u n n a tu r a lly b e e n th o s e o f c o n c e r n to su c h e s ta b lis h

m e n ts . T h u s , m e ta llu r g ic a l s p e c im e n s hav e b e e n m o s t f r e q u e n t ly in v e s tig a te d ,

b u t a s u ff ic ie n t ly w id e ran g e o f o th e r m a tr ic e s hav e n o w b e e n e x a m in e d to ■

p ro v id e so m e id ea o f th e o v e ra ll p o te n t ia l o f th e m e th o d .

s0.3

о 50 . 100

OUTER INNERSURFACE SURFACE

% OF WALL THICKNESS -----------►

F IG .3 . ' Typical carbon profile obtained with the nuclear microprobe.

(a ) M e ta llu rg ic a l a n a ly s is

In v e st ig a tio n in to d is tr ib u t io n o f lig h t e le m e n ts in m e ta llu rg ic a l sam p les

ca n b e im p o r ta n t , n o t o n ly to o b ta in in fo r m a t io n a b o u t e f fe c ts a t g ra in b o u n d a r ie s ,

b u t a lso to id e n tify th e b e h a v io u r o f m e ta ls w h e n s u b je c t to p a r t ic u la r p ro c e ss e s .

C o n s id e ra b le e f fo r t h a s b e e n d e v o te d to g e n e ra tin g c a r b o n [ 3 2 ] p ro f ile s to m o n ito r

th e p ic k u p o f th e e le m e n ts d u rin g h ig h -te m p e ra tu re e x p e r im e n ts . M ic r o p ro b e

re s u lts hav e b e e n sh o w n to a g re e w ell w ith th o s e o b ta in e d b y m ic r o m a c h in in g th e

sa m p le s u r fa c e a n d a n a ly s in g th e s e p a ra te d fra g m e n ts . A lth o u g h m a c h in in g did

n o t o f f e r a sp a tia l r e s o lu t io n w h ic h c o u ld m a tc h th e m ic r o p r o b e , re s u lts w ere

a d e q u a te to sh o w th a t th e re s u lts w ere s im ila r . A n im p o r ta n t c h a r a c te r is t ic o f th e

n u c le a r m ic r o p r o b e is th e a b il i ty to d is tin g u ish b e tw e e n e le m e n ts p re se n t o n and


b e n e a th th e s u r fa c e . T h is p e r m its s u r fa c e c o n ta m in a t io n to b e id e n tif ie d w h ic h ,

is p a r t ic u la r ly v a lu a b le d u rin g c a r b o n a n a ly se s w h e n c a r b o n m a y b e d e p o s ite d

fro m th e m a c h in e v a cu u m s y s te m [ 3 3 ] . T h e s e n s it iv ity o f th e d e te r m in a t io n ca n

th u s b e im p ro v e d [ 3 4 ] . A n e x a m p le o f a c a rb o n -d iffu s io r i p r o f ile , ty p ic a l o f th e

m a n y g e n e r a te d , is g iv en in F ig .3 . C o r r e la t io n o f th e p o s it io n o f sev e ra l lig h t

e le m e n ts c a n s o m e tim e s .b e a ch ie v e d b y d isp la y in g c o u n ts se le c te d f ro m a p p r o p ria te

re g io n s in th e c o lle c te d s p e c tr u m , an d X -ra y s e m it te d fr o m m e ta ls h av e b e e n

c o m p a re d w ith c a r b o n , o x y g e n , n itro g e n an d b o r o n d is tr ib u t io n s [ 3 5 ] . T h e

c o n c e n tr a t io n o f n itro g e n h a s b e e n co m p a r e d w ith th a t o f t i ta n iu m d is tr ib u t io n s

in p a r t ia lly n itr id e d , 2 0 -c h r o m iu m -2 5 n ic k e l t ita n iu m -s ta b iliz e d s te e l ; n itro g e n

w as d e te r m in e d b y c o u n tin g p r o to n s f ro m th e (d ,p ) r e a c t io n and K a X -ra y s

p ro v id ed a m e a su re o f t ita n iu m p re s e n t [ 2 0 ] . N u c le a r r e a c t io n s p ro v id e in fo r

m a tio n a b o u t is o to p ic c o m p o s it io n a n d , in c e r ta in c a se s , i s o to p ic a n a ly s is ca n b e

ca rr ie d o u t d u rin g s c a n n in g . A s a s p e c if ic e x a m p le , th e is o to p ic c o m p o s it io n o f

b o r o n h as b e e n d e te r m in e d b y th e ( a ,p ) r e a c t io n , s in ce p r o to n s f r o m b o t h th e

10B + a and n B + a r e a c t io n s c a n b e d is tin g u ish e d s e p a ra te ly [ 3 6 ] . T h e p re s e n c e

o f o x y g e n a lso p la y s a s ig n if ic a n t p a rt in m a n y m e ta llu r g ic a l p ro c e ss e s an d a

n u m b e r o f p a p e rs h av e r e p o r te d e x a m in a t io n o f th e d is tr ib u t io n o f o x y g e n .

H o w ev er, d e te r m in a t io n o f s p e c if ic is o to p e s c a n b e e x te n d e d to t r a c e r s tu d ie s

and th e d is tr ib u t io n ,o f o x y g e n in z irc o n iu m w eld s w as m ea su red b y d e te rm in in g

th e y ie ld o f r e a c t io n 180 ( p , a ) 15N [ 3 7 ] . N e u tr o n c o u n tin g h a s b e e n e x p lo ite d to

p e r m it th e d if fu s io n o f b e ry ll iu m in c o p p e r to b e fo llo w e d [ 2 0 ] .

(b ) M inerals and allied m aterials

T h e lo w c o n d u c t iv ity o f m in e ra l sa m p les h a s n o t p re c lu d e d th e u se o f th e

n u c le a r m ic r o p r o b e fo r th e ir e x a m in a t io n an d b o t h n u c le a r r e a c t io n s an d X -ra y

e m is s io n hav e p ro v id ed a p p r o p r ia te e le m e n ta l in fo r m a t io n . G a m m a ra d ia tio n

f ro m in e la s t ic s c a t te r in g p ro c e ss e s h a s p e r m it te d th e s im u lta n e o u s d e te r m in a t io n

o f e le m e n ts su c h as a lu m in iu m , s i l ic o n and ir o n [ 2 9 ] w h ile X -r a y s hav e b e e n

u sed to p ro v id e a n a ly t ic a l in fo r m a t io n f r o m lu n a r sa m p le s , m o n a z ite c ry s ta ls ,

m ic a fo ils and m e te o r ite s [ 3 8 ] . O b s e rv a tio n o f th e c h a r a c te r is t ic X -ra y s e m itte d

f ro m slag d u rin g p r o to n ir r a d ia tio n h a s p e r m it te d d is tr ib u t io n s o f s i l ic o n , iro n ,

z in c , c o p p e r and lead [ 3 9 ] to b e id e n tif ie d . T h e d e te r m in a t io n o f p ro f ile s o f

o x y g e n an d s il ic o n a c ro s s s i l ic o n n itr id e h a s a lso p ro v ed p o s s ib le [ 4 0 ] .

(c ) B io lo g ica l sam ples

T h e m e d ic a l a p p lic a t io n s o f th e n u c le a r m ic r o p r o b e are o f c o n s id e ra b le

p o te n t ia l b u t hav e n o t so fa r b e e n e x te n s iv e ly in v e s tig a te d . H o w ev er, th e v alu e

o f c h a rg e d -p a r tic le e x c i te d X -ra y s fo r th e e x a m in a t io n o f b io lo g ic a l sa m p le s h as

a lre a d y b e e n d e m o n s tr a te d w ith la rg e -d ia m e te r b e a m s an d th e r e is n o re a s o n to

4 6 P IE R C E

a ssu m e th a t a r e d u c t io n in b e a m d ia m e te r is l ik e ly to hav e a n y m a jo r e f fe c t ,

e x c e p t th a t o f re d u c in g th e b e a m c u rre n t in c id e n t o n th e sa m p le s . In m a n y ca se s ,

th e X -ra y y ie ld is s u ff ic ie n t ly h ig h to m e a n th a t th is is n o t an u n d u e r e s tr ic t io n

u p o n p o ss ib le u se. S p e c im e n s e x a m in e d w ith m a c ro b e a m s in c lu d e b lo o d , se ru m ,

liv e r and a u to p s y sa m p le s . T h e a p p lic a t io n o f th e m ic r o p r o b e to b io lo g ic a l

a n a ly s is h as b e e n d e m o n s tra te d [4 1 ]. O f p a r t ic u la r in te r e s t is th e p o s s ib il ity o f

a p p ly in g th e m ic r o p r o b e to sa m p le s th a t h av e b e e n p la ced in a ir [ 4 2 ] . T h is h as

b e e n a cn ie v e d b y m o u n tin g th e sa m p le o n a th in w in d o w th ro u g h w h ic h th e b e a m

is e x t r a c te d fro m th e a c c e le r a to r . S a m p le s w h ich c a n n o t b e e x a m in e d in v a cu o

ca n b e in v e s tig a te d b y th is te c h n iq u e .

R E F E R E N C E S

[1] A N D ERSEN , C.A., Ed., Microprobe Analysis, Joh n Wiley & Sons, New York (1 9 7 3 ).[2 ] H E V E SY , G ., LEV I, H., Dan. Vidensk. Selsk., M at.-Fys. Medd. 14 3 (1 9 3 6 ).[3 ] SEA BO RG , G .T ., LIVINGGOOD, J . J . , J . Am. Chem. Soc. 60 (1 9 3 8 ) 1784.[4] PIERC E, T .B ., PECK, P .F ., H EN RY, W.M., Nature (London) 2 0 4 4 9 5 8 (1 9 6 4 ) 571.[5] PIERC E, T .B ., PECK, P .F ., H EN RY, W.M., Analyst 90 1071 (1 9 6 5 ) 339.[6] P IER C E, T .B ., PECK, P.F ., Proc. SAC Conference, Nottingham, 1965 (SH A L L IS, P.W., Ed.),

Heffer and Sons Ltd., Cambridge (1 9 6 5 ) 159.[7 ] DZEM ARD’YAN, Y .A ., MIKH AILOV, G .I., TA RCH IK , L .P .S ., Ind. Lab. 37 (1 9 7 1 ) 708.[8] SIPPEL, R .F ., G LO V ER , E .D ., Nucl. Instrum. Methods 9 (1 9 6 0 ) 37.[9] M ACEY, D .J., G IL BO Y , W .B., Nucl. Instrum. Methods 9 2 (1 9 7 1 ) 501 .

[10] RICCI, E ., Private comm unication.[1 1 ] ZELEN SK I, V .F ., KH A R’KO V, O.N., K U LA KO V, V .S ., SKAKUN, N.A., Prot. Met.

6 (1 9 7 0 ) 235.[12] M Ö LLER, E ., ST A R F E L T , N., T IE B E L A G E T , A .K ., Atomenergi, Sweden, Report 237

(1 9 6 6 ).[13] P IER C E, T .B ., Proc. 2nd Conf. Practical Aspects o f Activation Analysis with Charged

Particles, Liège, 1967 (E B E R T , H.G., Ed.), Report EU R 3 8 9 6 , d-f-e, Brussels (1 9 6 8 ) 389.[1 4 ] DECONNINCK, G ., D EM O R TIER , G ., Nuclear Techniques in the Basic Metal Industries

(Proc. Symp. Helsinki, 1972), IA EA , Vienna ( 1973) 573.[15] P IER C E, T .B ., PECK, P .F ., C U FF, D .R .A ., Anal. Chim. Acta 39 (1 9 6 7 ) 433 .[16]. CHEMIN, J .F . , R O TU R IER , J . , SA BO YA , B., P ET IT , G .Y ., J . Radioanal. Chem. 12

(1 9 7 2 ) 221.[17 ] AM SEL, G ., NADAI, J .P ., D’A RD EM A RE, E., DAVID, D., G IR A R D , E ., MOULIN, J . ,

Nucl. Instrum. Methods 9 2 ( 1 9 7 1 ) 481 .[1 8 ] ROBIN , R ., COOPER, A .R ., H EU ER, A.H., J . Appl. Phys. 44 (1 9 7 3 ) 3770.[1 9 ] P R E TO R IU S, R ., C O ETZEE, P., J . Radioanal. Chem. 12 (1 9 7 2 ) 301.[2 0 ] McMILLAN, J.W ., H EA RST, P.M., PUM M ERY, F.C.W ., HUDDLESTON , J . , P IER C E, T .B .,

paper presented to 3rd Int. Conf. Ion Beam Analysis, Washington (1 9 7 7 ).[2 1 ] O L IV IE R , C., PEISACH, M., J . Radioanal. Chem. 1 2 (1 9 7 2 ) 313.[2 2 ] W EBER, G., QUAGLIA, L ., J . Radioanal. Chem. 12 (1 9 7 2 ) 232.[23] C U Y PERS, M., QUAGLIA, L., R O BA Y E, G ., DUMONT, P., BARRAN DON, J.N .,

Proc. 2nd Conf. Practical Aspects o f Activation Analysis with Charged Particles,Liège, 1967 (E B E R T , H.G., Ed.), Report EU R 3 8 9 6 , d-fre, Brussels (1 9 6 8 ) 371.


TU RO S, A., W IELU N SKI, L., BA RCZ, A., Nucl. Instrum. Methods 111 (1 9 7 3 ) 605. LIG H TO W LERS, E .C ., NORTH, J .C ., JO RD A N , A .S., D ERIC K , L., M ERZ, J . , J . Appl. Phys. 4 4 ( 1 9 7 3 ) 4 758 .W OLICKI, E .A ., KNUDSON, A .R ., Int. J . Appl. Radiat. Isot. 18 (1 9 6 7 ) 429 .PIER C E, T .B ., PECK, P .F ., C U FF, D .R .A ., Analyst 97 171 (1 9 7 2 ) 1152.Nucl. Instrum. Methods 142 (1 9 7 7 ) 1/2.PIER C E, T .B ., PECK, P .F ., C U FF, D .R .A ., Nucl. Instrum. Methods 67 (1 9 6 9 ) 1. DYM N IKOV, A.D., FISH K O V A , T ., YA V O R , S., Sov. Phys. - Tech. Phys. 10 (1 9 6 5 ) 340. COOKSON, J .A ., P ILLIN G , F .D ., United Kingdom Atom ic Energy Report, A ER E R 6 3 0 0(1 9 7 0 ).PIER C E, T .B ., McMILLAN, J.W ., PECK, P .F ., JO N E S, I.G ., Nucl. Instrum. Methods 118 (1 9 7 4 ) 115.McMILLAN, J.W ., PIER C E, T .B ., Proc. Int. Conf. Karlsruhe, Ion Beam Surface Layer Analysis (K A PPLER , F ., Ed.), Plenum Press, New York (1 9 7 6 ).McMILLAN, J.W ., PUM M ERY, F.C.W ., United Kingdom Atom ic Energy Report,A ER E R 8 4 5 7 (1 9 7 6 ).PIER C E, T .B ., HUD DLESTON , J . , United Kingdom Atom ic Energy Report, A ER E R 8 5 1 4 (1 9 7 7 ).O L IV ER , C., McMILLAN, J.W ., P IER C E, T .B ., J . Radioanal. Chem. 31 (1 9 7 6 ) 515.PRIC E, P.B., B IR D , J .R . , Nucl. Instrum. Methods 277 (1 9 6 9 ) 69.NOBILING, R ., T R A X E L , K ., BOSCH, F ., C IV ELEK O G LU , Y „ M ARTIN , B „ PO VIY, B„ SCHWALM, D., Nucl. Instrum. Methods 142 (1 9 7 7 ) 49 .COOKSON, J .A ., FERG U SO N , A .T .G ., PILLIN G, F .D ., J . Radioanal. Chem. 39 (1 9 7 2 ) 12. COOKSON, J . , PILLIN G, F .D ., Thin Solid Films 19 (1 9 7 3 ) 381.COOKSON, J .A ., LEG G E, G .J.N ., Proc. Anal. Div. Chem. Soc. 2 2 5 (1 9 7 5 ) 12.COOKSON, J . , Phys. Med. Biol. 21 (1 9 7 6 ) 965.

D E T E C T I O N O F C H A R A C T E R I S T I C X - R A Y S

M e t h o d s a n d a p p l i c a t io n s

V . V A L K O V IC

R ic e U n iv e rs ity ,

H o u s to n , T e x a s ,

U n ite d S ta te s o f A m e r ic a

and

In s t itu t “R u d e r B o s k o v ic ” ,

Z a g reb ,

Y u g o sla v ia

A bstract

DETECTIO N OF CH A RA C TERISTIC X -R A Y S: M ETHODS AND APPLICATIONS.Fundamental processes in production, absorption and detection o f X-rays are discussed.

The application o f X-ray emission spectroscopy to the analysis o f elements is presented. Sample excitation by X-ray tube, radioactive source and charged-particle beams from accelerators are described. Several experimental arrangements are discussed, and results on sensitivity, accuracy and precision are reviewed. The detection o f X-rays characteristic for an element enables trace amounts o f several elements to be detected simultaneously. Many applications in material sciences, industry, geology, archeology and forensic, science are described. The m ost interesting studies done by X-ray emission spectroscopy involve the complex movement o f elements in nature and their effects on health and diseases. There are numerous interesting applications in medicine, biology and environmental sciences. O f special interest are the biologically essential trace elements in fossil fuels and agriculture.

1. INTRODUCTION

It usually takes a decade, or even several decades, from a discovery to its practical applications. This was not the case with X-rays. They were widely applied in medical and industrial radiography within a year of their discovery (Röntgen, 1895). The milestones in the development of X-ray spectrometry are:

1895 W.C. Röntgen1896 A.W. Wright1896 J. Perrin

1909-1911 C.G. Barkla

1912 M. von Laue, W. Friedrich, and E.P. Knipping

1912 J. Chadwick

1913 W.L. Bragg, andW.H. Bragg

discovery’ of X-raysfirst photographic paper röntgenogramX-ray intensity measurement with anair ionization chamberdiscovery of absorption edges andemission line seriesdiffraction of X-rays by crystals

detection of characteristic X-rays induced by a-particles Bragg X-ray spectrometer

4 9

5 0 V A L K O V I C

1913 H.G.J. Moseley e s tab l ish ed the r e la t io n s h ip between wavelength o f X-ray l in e s and atomic numbermodel o f the atomh o t - f i l a m e n t , high-vacuum X-ray tube measurement o f X-ray spectra o f the chemical elements ' a p p l i c a t io n ç>f X-ray spectra to chemical a n a ly s is o f minerals qu a n t i ta t iv e an a ly s is by secondary e x c i t a t i o n o f X-ray spectra ap p l ic a t io n s o f X-ray f luorescence spectrometrypro to type o f the f i r s t commercial X -ray emission spectrometer development o f h igh re so lu t io n S i ( L i ) d e tec to rs

1913 N. Bohr1913 W.D. Coo lidge1913-1923 W.D. Coo lidge

1922 A . Hadding

1923 G. von Hevesy

1948

1928 R. Glocker, and H. Schre iberH. Friedman, and L .S . Birks

1960-1970

Development o f high r e so lu t io n S i ( L i ) d e tec to rs in 1960's re juvenated the i n t e r e s t in the d e t e c t io n o f c h a r a c t e r i s t i c X-rays and p oss ib le a n a l y t i c a l a p p l i c a t io n s . The bas ic l im i t a t i o n o f the energy as ide from the preamplif i e r no ise l im i t a t io n s , i s the s t a t i s t i c a l f lu c tu a t io n in the number o f ion pa irs c rea ted f o r a g iv en photon energy . Geometrica l e f f e c t s have an important in f lu en ce on the energy r e s o lu t io n . However, the f i r s t major s tep in improving energy r e so lu t io n came w ith the development o f low noise f i e l d - e f f e c t t r a n s is to rs (FET). Low-temperature ope ra t ion o f these dev ices was requ ired in order to reduce the no ise . Another s i g n i f i c a n t improvement was the in trod u c t ion o f the l i g h t - e m i t t in g diode (LED) to p rov ide low-frequency feedback to the p re a m p l i f i e r . In order to prevent contamination o f the d e tec to r su r face . f rom condensation o f im p u r i t ies , the d e t e c to r i s p ro te c ted by a pa r t sea led by a bery l l iu m window. The absorp t ion in th is th in b ery l l ium f o i l reduces the d e tec to r e f f i c i e n c y f o r lower X-ray e n e rg ie s . D e tec tor s iz e and geometry w i l l determine' e f f i c i e n c y f o r h igher X-ray e n e rg ie s . E f f i c i e n c y f o r low X-ray ene rg ie s can be fu r th e r reduced by in s e r t in g an absorber between the r a d ia to r and the d e t e c to r .

2 . FUNDAMENTALS

X-rays are produced as a r e s u l t o f the removal o f a t l e a s t one in n e r - s h e l l e l e c t r o n . S evera l processes might r e s u l t in the removal o f an e l e c t r o n from the inner s h e l l in the atom. In p r in c ip le one might d is t in gu ish between two d i f f e r e n t groups o f p rocesses. The f i r s t group cons is ts o f the s c a t t e r in g p ro cesses in which the incoming charged p a r t i c l e ( e l e c t r o n , p roton, » - p a r t i c l e , heavy ion ) h i ts the e l e c t r o n in the К- s h e l l and tran s fe rs pa r t o f i t s k in e t i c energy to the e le c t r o n . As a r e s u l t the vacancy is c rea ted which can be f i l l e d w ith the e l e c t r o n from one o f the outer s h e l l s . Obviously the vacancy can be produced a ls o in the process where X-ray or -y-ray t r a n s fe r t h e i r energy to the К- s h e l l e l e c t r o n . This t r a n s fe r o f energy can be p a r t i a l o r complete. In order to r e a l i z e these processes charged p a r t i c l e s ( e l e c t r o n , proton, a - p a r t i c l e s , heavy io n s ) produced by a v a r i e t y o f a c c e le ra to r s can be used.

The second group o f p rocesses in vo lv es the i n t e r a c t i o n - o f atomic nucleus w ith i t s e l e c t r o n cloud and as a r e s u l t o f th is in te r a c t io n a vacancy is c r e a ted in the К- s h e l l . R ad ioac t ive iso topes that decay by -y-emission may undergo in te rn a l convers ion . The y-photon is absorbed w ith in the atom o f i t s o r i g in g i v in g i t s energy to an o r b i t a l e l e c t r o n . Rad ioact ive iso topes that decay by 0-em iss ion may a ls o undergo in te rn a l convers ion . The e l e c t r o n from the nucleus may t r a n s fe r i t s erasrgy to the o r b i t a l e l e c t r o n and c rea te the vacancy in the К- s h e l l . The th i rd process which produces vacancies in the inner s h e l l s i s o r b i t a l e l e c t r o n capture.

Once the atom is e x c i t e d i t s d e - e x c i t a t i o n can occur not on ly by emission o f e lec trom agn et ic r a d ia t io n but a l s o by some o ther processes. The f luorescence y i e l d o f an atomic s h e l l i s the p r o b a b i l i t y that a vacancy in that s h e l l w i l l

D E T E C T IO N O F C H A R A C T E R I S T IC X - R A Y S 5 1

be f i l l e d through a r a d io a t i v e t r a n s i t i o n . The competing processes a r e : emiss ion o f Auger e l e c t r o n s and Coster-Kironig t r a n s i t i o n s , which are t r a n s i t io n s between the subshe lls o f the same p r in c ip a l quantum number.

When a t a r g e t i s bombarded by protons or h ea v ie r ions, e l e c t r o n s are e j e c ted from the atomic s h e l l o f the ta rg e t atoms, wh ile the incoming charged p a r t i c l e lo ses i t s energy in passing through the t a r g e t . In 1912 Chadwick [1 ] f i r s t observed and i d e n t i f i e d the c h a r a c t e r i s t i c X-rays o f s e v e r a l elements exposed to cy-particles from ra d io a c t i v e sources. E ar ly t h e o r e t i c a l and e x p e r i mental work on ch a rged -p ar t ic le - in d u ced X -ray emission has been summarized in the a r t i c l e by Merzbacher and Lewis [ 2 ] . The modern t h e o r e t i c a l concepts and a v a i l a b l e exper imenta l data on energy spectra o f e le c t ro n s e j e c t e d in ion-atom c o l l i s i o n s are summarized by Ogurtsov [3 ] . T h e o r e t i c a l c a lc u la t io n s are usually performed using the Born approximation f o r i n e l a s t i c c o l l i s i o n s .

Another theory which many authors used i s c l a s s i c a l b inary-encounter approximation as developed by G ryz insk i [ 4 ] . In the b inary-encounter approximat io n an in t e r a c t io n i s cons idered between two c l a s s i c a l p a r t i c l e s : the i n c i dent ion (o r e l e c t r o n ) and the atomic e l e c t r o n . The atomic fea tu res o f the l a t t e r are taken in to account by in troduc ing a proper v e l o c i t y d is t r ib u t i o n .

As poin ted out by Garcia [5 ] , the b inary-encounter model o f f e r s a s ca l in g law f o r the cross sec t ion s in c lud ing on ly e l e c t r o n b ind ing energy and p a r t i c l e energy . A un ive rsa l curve is obtained i f uâ ( i = К or L ) i s p l o t t e d versus Е/ X u . , where u^ is the e l e c t r o n b ind ing energy , E the proton energy , a the io n i z a t io n cross s e c t io n and \ the p ro to n -e le c t ro n mass r a t i o .

Y ie ld s o f X-rays produced by charged p a r t i c l e s have been measured fo r a v a r i e t y o f charged p a r t i c l e s a t d i f f e r e n t bombarding e n e rg ie s . The e a r l y work has been summarized by Merzbacher and Lewis [2 ] , wh ile the p a r t i a l l i s t o f r e cent work is presented by Duggan e t a l . [ 6 j . Cross 'sections f o r К- s h e l l i o n i za t io n by p, d, 3He and 4He have been tabu la ted by Rutledge and Watson [7 ] .

Protons are the most o f t e n used charged p a r t i c l e s f o r the e x c i t a t i o n o f c h a r a c t e r i s t i c X -rays . Even so, the data on proton-induced X -ray emission cross sections are s t i l l sca rce . Compilat ion o f measured cross s ec t io n s a t proton ene rg ie s most o f t e n used in X-ray product ion is presented by Va lkov ic [8]

Johansson and Johansson [9 ] have f i t t e d a l l the a v a i l a b l e data on proton X-ray product ion w ith a* f i f t h degree polynomial

In(a uf) = E .b xnL n=0 n

where X = In (10 3E/ \u^). u^ i s the i o n i z a t io n energy in eV and E i s the p ro ton energy in eV. The un it o f the io n i z a t io n cross s ec t io n i s 10” 14cma . The obta ined values f o r c o e f f i c i e n t s b. f o r К X-rays a r e : b0 = 2.0471, b^ = -0.65906 b3 = -0.47448, b3 = 0.99190, b4 = 0.46063, and b5 = 0.60853.

3. TRACE ELEMENT ANALYSIS: EXPERIMENTAL SETUP

In recen t years the d é t e c t io n o f c h a r a c t e r i s t i c X-rays f o r t race element an a ly s is has increased s i g n i f i c a n t l y , p r im a r i ly because high r e s o lu t io n S i ( L i ) d e tec to rs have made p os s ib le energy d is p e r s iv e a n a ly s i s . X-rays from X-ray tubes and ra d io a c t i v e sources have been used to e x c i t e c h a r a c t e r i s t i c X-rays in d i f f e r e n t samples. Because o f la rge cross sec t ion s f o r X-ray product ion , charged p a r t i c l e s can a ls o be used to produce c h a r a c t e r i s t i c X-rays from e l e ments p resent in on ly t race amounts in t a r g e t m a te r ia ls . F ig . 1 in d ica tes the p r in c ip l e o f X -ray emission spectroscopy .

A ra d io is o to p e X-ray f luo rescence an a lyse r con s is ts o f the f o l l o w in g bas i c components (see F ig . 2 ) : ( i ) a sca led ra d io a c t i v e source; ( i i ) a d e te c t io n system which s e l e c t s the ene rg ie s o f the e x c i t e d c h a r a c t e r i s t i c X-rays and measures th e i r i n t e n s i t i e s ; ( i i i ) an e l e c t r o n i c and read-out system whose ou tput may be r e la t e d to the l i s t o f elements p resent in the sample and th e i r concen tra t ions .

5 2 V A L K O V I C

F IG . l . The princip le o f X -ray em ission spectroscopy.

SPECIMEN

FIG.2. Schematic representation of a radioisotope X-ray fluorescence analyser.

D E T E C T IO N O F C H A R A C T E R I S T IC X - R A Y S 5 3

Very use fu l is a rev iew a r t i c l e by Rhodes [1 0 ] . He reported a compact X -ray energy spectrometer c on s is t in g o f a ra d io is o tope X-ray source, an S i ( L i ) or G e (L i ) d e t e c to r and a small computer. The e f f i c i e n t e x c i t a t i o n o f К X-rays from Na to U and L X-rays from Ag to U was ach ieved using d i f f e r e n t i n t e r changeable sources. This apparatus was reported to re so lv e К X-rays from adjacen t elements down to Na. Depending on the source and on the pa r t o f the spectrum examined, c h a r a c t e r i s t i c X-rays from up to 15 elements can be s imultaneously measured f o r e i t h e r q u a l i t a t i v e or q u a n t i t a t iv e a n a ly s is . A small computer was used to s to re spectra and to perform simple data p rocess ing .

There are s e v e r a l commercial systems on the market using ra d io a c t iv e source to e x c i t e c h a r a c t e r i s t i c X-rays in the sample. The most o f t e n used ra d io is o to p es a r e : B6Fe (t i = 2.7 yea rs , E = 5.9 keV) and l o 9 Cd ( t i = 1.3years , E = 22.2 keV and 288.0 keV ).

Sample e x c i t a t i o n by X-rays produced by an X-ray tube has been reported by many authors . By exposing the sample to bremsstrahlung ra d ia t io n from an X-ray tube i t i s p os s ib le to e x c i t e s imultaneously a l l the elements present in the t a r g e t . E lec trons em itted from the heated f i lam en t are a c c e le ra t ed towards the ta r g e t by a p o t e n t i a l d i f f e r e n c e o f up to 50 kV, and when they s t r ik e the t a r g e t they generate X-rays c h a r a c t e r i s t i c o f the elements p resent in i t as w e l l as bremsstrahlung. I f one is in t e r e s t e d in e x c i t in g a p a r t i c u la r element in the ta r g e t , the tube v o l t a g e should be ad justed so that the maximum o f th is energy d is t r ib u t i o n is ju s t above the energy o f the К absorp t ion edge o f that element, which g iv e s the g r e a t e s t e x c i t a t i o n e f f i c i e n c y . In o rder to improve s e n s i t i v i t y (by reducing background), a monoenergetic X-ray beam should be used to e x c i t e the t a r g e t . In p r in c ip le there are two ways o f ob ta in in g mono- e n e rg e t ic ra d ia t io n from an X-ray tube. One i s to pass the r a d ia t io n from the tube through su i ta b le f i l t e r s . Another way o f ob ta in in g monoenergetic X-rays is to use rays from an X -ray tube to e x c i t e a t a r g e t o f some pure metal and then make the secondary r a d ia t io n from th is e x c i t e the ta r g e t to be examined.

There are s ev e ra l commercial systems a v a i l a b l e on the market using a variety o f X-ray tubes. One should remember tha t the e x c i t a t i o n by ra d io a c t i v e sources and X-ray tube requ ires a ra th er th ick t a r g e t and that m atr ix e f f e c t s can be s i g n i f i c a n t . E x c i ta t io n by using r a d io a c t i v e sources or X -ray tubes i s , howe v e r , com plete ly n ondest ruc t ive . As a r e s u l t , i t can be a p p l ied to a v a r i e t y o f problems where the sample is not supposed to be damaged.

Charged -part ic le - induced X-ray emission has been used as an a n a ly t i c a l to o l w ith a v a r i e t y o f machines a c c e le r a t in g d i f f e r e n t ions. A lthough i t is d i f f i c u l t to compare r e su l t s from d i f f e r e n t lab o ra to r ie s , th is has o f t e n been done. These comparisons are sub jec t to many su b je c t iv e e s t im a tes . When d i f f e r ent ions are compared, t h e i r ene rg ie s have to be equ iva len t (MeV/amu). Comparisons o f d i f f e r e n t modes o f e x c i t a t i o n have been r e c e n t ly summarized by Johansson and Johansson [ 9 ] . E xc i ta t io n s by protons, a - p a r t i c l e s , heavy ions, e le c t r o n s , and X-rays were d iscussed . T h e ir d iscuss ion fa vors proton induced X -ray emission.

When a choice o f beam is made, the s c a t t e r in g chamber should be con s tru c ted. M o d i f i c a t io n o f a s c a t t e r in g chamber used in nuclear re a c t io n stud ies is a p o s s i b i l i t y . Beside X-ray d e t e c to r i t i s p os s ib le to mount charged p a r t i c l e d e tec to rs in such a chamber. This then a l low s simultaneous measurement o f back- s ca t te red protons and beam in t e n s i t y and ta r g e t th ickness m on ito r ing . In some instances the use o f s c a t t e r in g chamber may be very d i f f i c u l t (wet samples, f o r example). In cases l i k e th is the proton beam can be passed through th in f o i l s onto samples in a i r .

Seve ra l s c a t t e r in g chambers constructed f o r the a n a l y t i c a l work using ch a rged -p ar t ic le - in d u ced X-ray emission have been reported in the l i t e r a t u r e .L e t us mention the s c a t t e r in g chamber descr ibed by Va lkov ic e t a l . [1 1 ] . The ta r g e t chamber was constructed with optimum e f f i c i e n c y f o r the c o l l e c t i o n o f produced X -rays . Care was taken to preven t the S i ( L i ) d e t e c to r from see ing any h igh-Z m a te r ia ls which could produce f luo rescence by beam s c a t t e r in g from the l a s t beam s l i t or Compton s ca t te red X -rays . In the chamber the ta r g e t frames r id e up and down on an aluminum ta r g e t ladder which is m i l l e d o f f cen te r so that the t a r g e t faces are in the exact c en te r o f the chamber. In o rder to e l i -

54 V A L K O V I C

t ELEMENTAL SENSITIVITY /

4

X/

//

1 Ep = 3 MeV

\\

\' \

I I I I I - _I___ I____ » 1 1 1 ____ L— I I I I I16 2 0 24 28 32 36 4 0 4 4 4 8 52

ATOMIC NUMBERi í i i i—í i l i i i I I _________ I I I____ L

К Ca Ti V Cr Wife Co Ni Cu Zn As Se Sr Y Zr Mo Ag I

ELEMENT

FIG.3. The relative elemental efficiency o f the system at Ep = 3 MeV shown as the ratio o f the number o f К X-rays o f yttrium (dopant) and o f the given element for the same number o f atoms (From R e f [\\]).

mínate X-rays from elements below those of potassium and secondary electro n - induced bremsstrahlung, a 0 .1 cm polystyrene absorber was inserted between the target and the detector. A proton energy of 3 MeV was chosen to maximize sens i t iv ity for elements in the region of Fe and Zn (essen tia l trace elements)(see F ig. 3 ) . While increasing the energy has the e ffe c t of increasing the ch a ra c te r is tic X-ray production of a l l elements, the bremsstrahlung background is increasing a t a fa s te r ra te . Thus, se n sitiv ity is lo s t in the Fe to Zn region.

When the proton beam strik es the target in the scatterin g chamber a number of X-rays are produced. Some of them w ill h it the X-ray detector which w ill generate a pulse through the pream plifier. In most cases the pream plifiers with pulsed op tica l feedback are used. Because of the extremely long fa l l time of S i(L i) pulses, high-energy ta ilin g of X-ray peaks due to pile-up can be a serious problem in trace element an a ly sis . In order to avoid pile-up a t higher counting rate a pulse pile-up re je c to r should be used to in h ib it X-ray am plifier whenever piled up pulses are received. Pulses from the am plifier can then be fed into multichannel ADC which is interfaced to a computer. Pulse pile-up problems can be avoided by keeping the counting rate low. Each charac- t e te r is t ic X-ray lin e w ill resu lt in a peak in the spectrum. Once the data are stored in the computer or analyzer the reduction involves calcu lations of peak location s, amplitudes and subtraction of background counts.

Analysis of charged-particle-induced X-ray emission (using the acce lera to rs) is greatly sim plified because of the absence of matrix e f fe c ts . In order to obtain absolute abundances of d ifferen t elements in the target, the target m aterial is usually doped with the known concentration of an element which one does not expect to find in the targ e t. What is needed is only standards containing known ra tio s of the doping agent to the d iffe ren t elements which might be present in the ta rg e t. Such a re la tiv e measurement does not require the knowledge of detector e ffic ie n cy as a function of X-ray energy.

Target preparation is d iffe ren t for d ifferen t types of specimen e x c ita tio n s. In the case o f exc ita tio n by X-ray tube or radioactive sources, rather th ick targets are needed. Loose powders or liquids are usually analyzed using a container of some kind. F i lte r papers are becoming increasingly important in

D E T E C T IO N O F C H A R A C T E R I S T IC X - R A Y S 55

CHANNEL NUMBER

FIG.4. An X-ray spectrum obtained by bombardment o f aluminum formvar backing with S-Me V protons (from R e f [II]).

the study of a ir p o llu tants, and also f i l t e r s or membranes are used to concentra te trace elements from liq u id . Interferences due to in ternal absorption and enhancement of the fluorescent radiation and the changes in emitted intens ity due to specimen heterogeneity and f in ite p a rtic le size are d ifferen t for d ifferen t specimens. Minimization of errors due to these e ffe c ts is a central problem in excita tio n by X-ray tube or by radioactive sources.

Charged-particle-induced X-ray emission has the advantage with respect to others that the targets used are much thinner and matrix e ffe c ts can be neglected. The targets are usually prepared as deposits on some kind of backings. D ifferent backings were investigated with the aim of finding one which would produce the lea st amount of background radiation and which would support the beam w ell. Any m aterial to be deposited should be f i r s t reduced to a solution or a suspension of microscopic p a rtic les in an in ert solvent. Pure water is the best solvent for most m aterials.

A very useful technique is the one involving the preparation of Al + formvar backings [11]. By rein forcing aluminum fo ils with formvar one obtains backings which may be used in charged-particle-induced X-ray spectrometry. The formvar provides mechanical strength while the liquid or suspension is drying, while the aluminum provides resistence on the beam of charged p a r t ic le s . Such a backing is free of any interference- lin es (see Fig. 4 ) . Targets of water, blood serum and many solutions are very e a sily prepared by simply allowing a few drops to dry on such aluminum formvar backing. Another backing that can be used, for powder samples is scotch tape (3M company, S t . Paul, Minnesota).The main contaminant in th is backing is bromine (see F ig. 5 ).

Two factors determine the choice of the p a rtic le bombarding energy*, the X-ray production cross section dependence and the bremsstrahlüng produced by secondary e lectro n s. Small cross section elim inates the use of low energies.

56 V A L K O V I C

CHANNEL NUMBER

FIG. 5. X-ray spectrum resulting from 3-MeV proton bombardment o f scotch tape (3M Company).

On the other hand, bremsstrahlung of secondary electrons lim its the use of higher energies. When a l l the parameters of the system have been chosen (in cluding beam intensity , and beam energy) the reproducibility of the method for many equivalent targets under routine analysis conditions should be studied. This should be performed for every type of target.

There are several good review papers published on the subject of X-ray emission spectroscopy [9, 12, 13]. The subject of particle-induced X-ray emission spectroscopy has been discussed a t the International Conference, Lund, , Sweden, 1976. The papers presented there w ill be published as a separate issue of Nuclear Instruments and Methods (1977). X-ray emission spectroscopy is a powerful an a ly tica l method; using i t a variety o f fundamental problems involving the movements of elements may be studied.

4 . ■ ELEMENTS IN THE NATURE ¡

The problem of determinating the "Universal abundances" of elements has been discussed by many authors. Beyond the Earth, the Moon and the m eteorites, the abundance of a t lea st some elements can be obtained for the solar system, for the Sun and other s ta rs , for gaseous nebulae, including some of external galaxies, for in te rs te lla r space and for cosmic rays. Probably the best known compilation of abundances is that of Suess and Urey [14]. The predominance of hydrogen and helium in the Universe lead to the theories of development of the Universe and to the formulation of theories about element formation. I t is


CHANNEL NUMBER

FIG.6. X-ray spectra obtained by the proton irradiation o f meteoritic dust. Top: Orgueil meteorite. Bottom : Allende meteorite.

also now believed that the natural abundances of most elements have been established in .the few fin a l seconds of a s ta r 's lifetim e as i t disrupts explosively.

The "peninsula" of the known nuclei in the N-Z plane terminates because of nuclear fis s io n . As one moves along the peninsula towards heavier nuclei, the disruptive Coulomb forces increase fa s te r than the cohesive nuclear forces.

The "peninsula" of the known nuclei in the N-Z plane terminates because of nuclear fis s io n . As one moves along the peninsula towards heavier nu clei, the disruptive Coulomb forces increase fa s te r than the cohesive nuclear fo rces. "Island" of s ta b il i ty has been predicted on the basis of sh e ll model calculations around Z = 114 and N = 184. Several experimental attempts have been made to detect the spontaneous fiss io n of superheavy elements that might s t i l l be stored in natural m aterials. Evidence for superheavy nuclei has been reported by Gentry e t a l . [15] in an experiment where small inclusions of monazite in mica from Madagascar have been bombarded by a proton beam with subsequent detection of produced X-rays. E ffo rts in several other labora to ries have not been able to reproduce th e ir re su lt, and the observed peak has been associated with the (p,nV ) reaction .

The elemental composition of m eteorites has been studied for some time.Of in te rest are isotopic ra tio s as well as elemental abundances. Since often only small amounts of m aterial are av a ilab le , proton-induced X-ray emission

58 V A L K O V I C

FIG . 7. X-ray spectrum obtained by proton irradiation o f the lunar soil sample.

is a valuable tool in determining the re la tiv e elemental abundances. Fig. 6 shows the X-ray spectra obtained by proton irrad ia tion of m eteoritic dust.Peaks corresponding to lig h t elements are supressed by an absorber in front of the S i(L i) detector.

Trace elements in various rocks can give valuable clues as to th e ir mode of formation. For example, the d istrib u tion of trace elements and th eir re la tive proportions in igneous, metamorphic and sedimentary rocks has been usefu l in establish ing the genetic relationship between various rocks and between rocks and ore deposits.

The f i r s t d irect chemical analysis of the Moon was performed by <*-particle back-scattering [16]. Heavier elements can be easily measured by the X-ray emission spectroscopy. Fig. 7 shows an X-ray spectrum obtained by proton ir r a d iation of the lunar s o il sample.

In the natural system (rock-soil-aqueous solutions-organisms) so ils are an exceptionally important lin k . Trace element composition of s o ils can be easily determined by X-ray emission spectroscopy. When protons are used for e x c ita tio n , only small amounts of m aterial are needed for target preparation. Fig. 8 shows X-ray spectra obtained from the proton irrad iation of s o il samples.

5. TRACE ELEMENTS IN LIVING MATTER

One of the most important ch a ra cteris tics of liv ing c e lls is th e ir a b i l i ty to take up elements from a solution against the concentration gradient.This is most obvious for marine micro-organisms which obtain th e ir nutrients


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FIG. 8. X-ray spectra obtained by the proton irradiation o f two types o f soil samples.

d irectly from the sea water. The concentration factor is then defined as .the ra tio of element concentration in the organism and the element concentration in sea water. The organisms concentrate a l l elements present in th e ir environment. However, a l l of these elements are not e sse n tia l for l i f e . C riteria which an element must s a tis fy to be e sse n tia l for l i f e are well established [17].

The re la tio n of the uptake o f e sse n tia l elements to y ield or growth may be considered as a d efin itio n of e ss e n tia lity . For an e sse n tia l trace element there is a rather narrow range of adequacy of element concentration in the organisms. Smaller concentrations resu lt in d ifferen t abnormalities induced by d efic ien cies which are accompanied by pertinent sp e c ific biochemical changes. Higher concentrations resu lt in to x ic ity .

The bulk of liv in g matter consists of eleven elements which have Very low atomic weights (H, C, N, 0 , Na, Mg, P, S, Cl, K, and Ca). These elements have been known to be e sse n tia l for l i f e for a long time because th e ir presence is easy to d etect. The problem of e sse n tia l trace elements is much more d if f ic u lt .

60 V A L K O V I C

□ ELEMENTS WHICH FORM THE BULK OF LIVING MATTER

ESSENTIAL TRACE ELEMENTS FOR

N a lM g

К |Ca

L J WARMBLOODED ANIMALSm 1 3 H H l Ш

S c Tiщ

[ ¿ г ¡Mn| Fe Со! Ni Cui Zn G a G e A s S e B r K r

Rb S r Y Z r Nb р З Тс Ru Rh P d A g C d In Sn S b Te 1 X e

C s B a RAREEARTHS Hf Та W Re O s Ir P t A u H g Tl Pb Bi Po A t Rn

Fr R a ACTANÍIDES

FIG.9. Essential trace elements fo r warm-blooded animals.

So fa r , the elements F, S i , V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Se, Mo, Sn, and I have been recognized as the e sse n tia l trace elements for warm-blooded animals (see Fig. 9 ). The great m ajority o f the e sse n tia l trace elements serve as key components of the enzyme system or of proteins with v ita l functions. I f the metal atom is removed, the protein usually loses i t s capacity to function as an enzyme. Of fourteen elements recognized u n til now as essen tia l trace e le ments for warm-blooded animals, only fluorine and s ilico n are below 20 in a to mic number. I t is in terestin g to point out that none of the 30 elements beyond iodine have ever been shown to be of any physiological sig n ifican ce . The d istrib u tion of essen tia l trace elements within the periodic table of elements might have some significance with respect to the development of l i f e in pre- b io t ic time.

5.1 Trace elements in microorganisms

Concentration factors for some elements in microorganisms are known [18J * Microorganisms require small quantities of Mn, Fe, Zn, Co, Cu, and Mo for th e ir growth. Some diseases caused by microorganisms resu lt in modified trace- element lev els . Microorganisms must compete with host metal-binding agents for these essen tia l elements. I t is well established that in the distant past massive faunal extinctions have occurred. There is mounting evidence that a corre la tio n e x is ts between major faunal extinctions and geomagnetic polarity re v ersa ls. The validy of th is correlation in recent geological time seems to have been well established by studies of fo s s i l species of sin g le-ce lled marine micro-organisms [19, 20].

Several mechanisms linking changes in the geomagnetic fie ld with e ffe c ts on liv ing organisms have been proposed. Most of them are based on the assumption that during p o larity reversals the dipole component of the geomagnetic f ie ld probably weakens or disappears for periods of a fpw thousands of years [21] allowing a much greater flux of both solar protons and g a lactic cosmic rays to bombard the surface of the Earth. The e f fe c t of increased exposure i s , however, now believed to be probably in s ig n ifica n t. Other mechanisms include climate change [22] (which also may resu lt from intense solar proton bombardment during p o larity re v e rsa ls), and the e ffe c ts of a large reduction in the content of ozone in the atmosphere, which would increase exposure to u ltrav io le t radiation [23] . D irect magnetic fie ld e ffe c ts on growth were proposed by Hays (20] and Crain [24]. A d irect cause-and-effect link between magnetic fie ld reversals and species extinctions could explain how marine organisms easily could have been a ffected . There is a lso the p o ss ib ility that extinctions discussed here may have resulted from the simultaneous e f fe c t of several facto rs .


CHANNEL NUMBER

FIG .10. X-ray spectrum obtained from the bombardment o f a mold sample on a scotch tape backing with a З-Ме V proton beam.

In order to try to explain the geomagnetic e ffe c ts on liv ing organisms the concentration factor dependence on magnetic fie ld in tensity has been recently proposed [25J . Magnetic fie ld dependence of concentration facto r can bring l i ving organisms into the range of deficiency or to x ic ity without changing trace element a v a ila b ility in the environment. Experiments aiming to e stab lish functional relationship between concentration factor and magnetic fie ld in tensity for some elements and some liv ing organisms were reported [26]. The measurements involved the growth of single c e ll organisms in defined media and contro lled external magnetic fie ld in ten sity . Trace element analysis of harvest is performed by proton-induced X-ray emission spectroscopy.

The organism used is a respiratory d efic ien t mutant of M. b acillifo rm is which in addition has lo st the a b il i ty to grow as mycelium. Instead, i t e x ists as spherical c e lls which reproduce only by budding a t the expense of a lcoholic fermentation [27]. A defined growth medium was used. I t contains two vitamins, nine amino a c id s , . glucose as an energy source, К s a lt s , and the sa lts of five trace elements: Mg, Zn, Fe, Mn, and Cu.

Growth took place a t room temperature in round-bottom flask s containing 14 ml of medium. Each flask was inoculated so as to have 10 ̂ cells/m l. Six flasks were placed in solenoids whose magnetic fie ld s were extremely uniform over the active region of growth and s ix flasks were used as a control group. Growth (increase in c e l l number) was monitored daily by measurements of the turbid ity of the c e ll suspensions. A fter ten days, when the c e l l number was maximum,' the c e lls were harvested by f i l t r a t io n .

Fig. 10 shows a typ ical X-ray spectrum from a mold sample (Mucor) . The microorganisms were harvested by f i l t r a t io n through f i l t e r paper which was then exposed to the beam of 3 MeV protons a t 150 nA for 500 seconds. The observed spectra indicate that the following elemental concentration ra tio s can be measured for both the mold and the growth medium: Mn/Zn, Fe/Zn, Ni/Zn, and Cu/Zn. The resu lts obtained so far are inconclusive about the existence of possible magnetic f ie ld dependence of concentration facto r.

62 V A L K O V I C

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F I G .11. Characteristic X-ray spectrum from the tree target (ash on scotch tape). Beam intensity 1 0 -2 0 nA, proton energy Ep ~ 3 MeV.

5 . 2 Trace elements in plants

The elemental composition of the plants re fle c ts to some extent the composition of the s o il or other growing medium. The re la tiv e ra tio s of the e le ments in a plant are not necessarily the same as those in the s o il or even in the nutrient solution. In other words, plants are to some degree selectiv e in th e ir absorption of elements. The uptake of the elements from the s o il is determined by many factors including: the element abundance, the form of e le ment, the pH of the s o i l , the physical condition of the s o il and the genetic constitu tion of the plant sp ecies. For an essen tia l element there is a rather narrow range of the adequacy of element concentration in organisms. Smaller concentrations resu lt in d ifferen t abnormalities induced by d efic ien cies which are accompanied by pertinent: sp e c ific biochemical changes. In plants i t is possible to have, under severe deficiency conditions, a decrease in the concentration of an element which resu lts in a small increase in the growth. This phenomenon is known as the Steenbjerg e ffe c t [28 ]. The elements shown to be e sse n tia l for a l l the plants include B, Ca, Cl, Co, Cu, Fe, K, Mg, Mn, Mo,Zn, and possibly I , Na, Se, S i , and V.

Data on the uptake of the elements from s o ils by higher plants are very scarce although i t is known that certa in species have the a b il ity to accumula te uniquely high concentrations of a p articu lar element.

Tree-rings have been studied for some time. I t seems that the trees store a record of atmospheric temperature in th e ir rings. This is of in te rest to the


1 2 3 4 5 6 7 8 ? 10 II 1.2 13 14 15

DIVISIONSFIG. 12. Variations in the Rbf ßr and K/Br concentration ratios. K/Rb concentration ratios across the tree rings show no variations.

h is to r ic a l meteorology since in some areas the tree-rin g records extend back for up to eight m illen ia . The science of dendroclimatology is based on the observation that the narrow rings represent a cold (or dry) year. Since the water conduction remains lim ited to the outmost annual ring tree rings offer a lso the p o ss ib ility to study the h is to r ica l trends in the movements of the elements. Elemental concentrations in the tree rings should r e f le c t the a v a ila b il ity of the elements in question. One possible source of the additional e- lement loading is the atmospheric discharge.

In the recent measurements [29] samples from an approximately 500 year old tree were analyzed for trace elements using proton-induced X-ray emission spectroscopy. A lin ear cut across the cross section of the trunk was devided in 15 d iv ision s; the div ision No. 1 being the c lo sest to the centre of the tre e . Each sample was then cut in small pieces and transferred into a separate crucible and ashed a t 450°C. The obtained ash was smeared on the scotch tape and exposed to a 10-20 nA beam of 3 MeV protons. Fig. 11 shows the resulting X-ray spe'ctrum. The peaks associated with K, Ca, T i, Mn, Fe, Cu, Zn, As, Pb, Br, Rb, and Sr could be id en tified in a l l spectra. Fig. 12 shows the variations in the Rb/Br and K/Br concentration ra tio s . The top of the figure shows

64 V A L K O V I C

the variations in the K/Rb concentration ra tio s across the tree rings from the centre of the tree outwards. Within the error bars, the ra tio K/Rb is 1000 across the tre e . This fa c t should be explained by keeping in mind that Rb is a member of the series NH4 - К - Rb - Cs, and that the members of th is series are more sim ilar in th e ir chemical and physical properties than are the members of any other group, with the exception of the halogens. The radius of the Rb ion is 1.48 A (only about 10$ larger than the potassium iori radius); as a re s u lt , the Rb ion can be accommodated into the same structures as the К ion

The Pb/Zn concentration ra tio shows two. peaks corresponding to the d iv isions 5 and 8 '( one div ision covers approximately 30 y ears). A recent increase in the Pb/Zn ra tio (divisions 14 and 15) is probably due to the atmosphere part icu la te loading resu lting from the ind u strial a c t iv it ie s and automobile t r a f f i c . I f the two peaks in the Pb/Zn concentration ra tio d istrib u tion are due to the Pb increase,da ta would suggest past p articu late loading of the atmosphere due to the natural events (volcano a c t iv it ie s and s im ila r). The a ltern a te approach to Pb/Zn d istrib u tion data is to assume that the observed peak structure is due to Zn depletion. In th is case Zn depletion is strongly correlated with the peak in К/Ca d istrib u tio n .

Plants provide the main source of minerals to animals and to most human beings. Several in terestin g problems deserve further study:

(1) Developing a method of id e n tifica tio n of the plant source (s o il type, location) from the knowledge of p lan ts ’ trace element composition. There is a p o ss ib ility of using such a methodology in legal enforcement (drugs, for example)

(2) Determining the concentration factors for toxic elements (Hg, Cd, Pb) for plants in food chains. The dependence of concentration factors on physical and chemical properties of the s o il should be determined.

(3) Search for plants with large concentration factors for some elements of in te re s t . This may lead to plants which could be used in geoprospecting, in the "harvesting of elements" using plants as a co lle c to r of elements.

5.3 Trace elements in human blood

The serum concentrations of numerous trace elements have in recent years become an area of great in te re s t in biochemical sciences [30]. Blood sàmples contain the trace quantities of Cu, Fe, A l, Ba, Mn, Ni, Cs, Sn, Sr, Cr, Zn,Pb, Mo, Cd, and others. However, only copper, iron, strontium and zinc appear in 100$ of the specimens investigated. These elements attracted in te rest among many research groups, since i t appears that the concentrations hold much promise as a c l in ic a l te s t in several pathological conditions. Several methods have been developed to determine the concentration levels of these elements. X-ray emission spectroscopy o ffe rs a very in terestin g approach to these problems because of i t s a b il ity to detect simultaneously several trace elements. In add itio n , charged particle-induced X-ray emission has the advantage that only very small quantities of serum are needed in target preparation; good resu lts can be obtained with a drop of blood serum on an aluminum formvar backing.Fig. 13 shows a typical spectrum of proton-induced X-rays from the blood target. The in te n s itie s of lig h t elements P, S, Cl, Ca, and К are a r t i f i c ia l l y suppressed

In a comparative study of X-ray fluorescence, X-ray excita tio n with protons and atomic absorptions [31] i t was found that a l l three techniques were routinely capable of measuring elemental concentrations of blood serum a t ppm lev els with re la tiv e standard deviations less than 10 percent. As p ractica l a n a ly tica l to o ls , each possess certain inherent advantages that must be weighed in deciding which technique is the most appropriate for a given study.

C lin ica l studies of trace element concentrations in Hodgkin's d isease, non-Hodgkin's lymphoma, and acute leukemia and other diseases should be done.The sp e c ific aims of such investigations should be: ( i ) to determine the c l i n ica l usefulness of multiple trace element concentration measurements for the d efin itiv e relationships to disease a c tiv ity and e ffic ien cy of therapy; ( i i ) to explore further the preliminary observations that changes in trace element levels precede other symptoms of disease a c t iv ity ; ( i i i ) to determine trace element concentrations in normal versus diseased tissu es.


100 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1100 1 200 1300CHANNEL NUMBER

2 4 6 8 10 12 14 16 Í8 2 0ENERGY IN keV

FIG. 13. A characteristic X-ray spectrum from a blood serum target prepared as a dried droplet on an aluminum formvar backing and bombarded with 3-MeV protons (from R e f [ 11]/

Several serum trace element concentration levels are a lso modified as a resu lt of in fectious d iseases. There are several examples of in fectious d iseases that occur because of iron imbalance: patients with hyperferremias caused by such diseases as s ick le c e ll anemia-, m alaria, h ep atitis or relapsing fever frequently su ffer from a variety of secondary b acteria l in fectio n s.Serum zinc level decreases a lso upon invasion of the host by micro-organisms. Contrary to iron and zinc, serum copper levels r ise in acute and chronic in fectio n s. The levels return to normal in severely i l l patients as well as in cases in which the in fectio n disappears.

5 .4 Trace elements in tissues

Trace element concentrations in d ifferen t organs have been studied for some time. The accumulation of some metals in some organs in malignant d is eases has a ttracted special a tte n tio n . This fact coupled with the observed geographical variations in death rates from cancer suggested that environmental factors may play an important ro le . Tumors depend on e sse n tia l trace elements as does healthy tis su e . By changing the trace element concentration, by substituting one element for another, i t is a p o ss ib ility that manipulations can be developed that are detrimental to the tumor without endangering the host.

Let us mention some examples of trace element-malignant disease re la tio n ships. For example, selenium is concentrated by rapidly growing tumors, poss ib ly in an e ffo r t by the body to in h ib it the unchecked growth. This fa c t a l lows radioactive selenium to be used to diagnose brain and bone tumors because of i t s concentration action in these tissu es [32]. Human cancer death rates appear to be considerably higher in areas with low selenium concentrations [33].

6 6 V A L K O V I C

-The reported data demonstrate that c i t ie s with high selenium levels have fewer cancer death» on an age-adjusted b asis , than c i t ie s with low selenium lev els . Selenium has also been reported to in h ib it carcinogenesis in animals.

D istin ct d ifferences were found between normal bone and human osteogenic sarcoma in regard to eight trace metals [34]. In osteogenic sarcoma Fe, Zn,Cu, and Mn were increased and Ca, Mg, Co, and Ni were decreased. There were a lso d ifferences in the relationsh ip between the metals when taken in pairs and in terms of the overall picture of metals in bone.

X-ray emission spectroscopy, with i t s capacity to detect several elements simultaneously in very low concentrations, o ffers a very convenient tool for the study of trace elements and th e ir relationship to malignant d iseases. Preliminary resu lts [12] indicated d ifferences in trace metal concentrations in the spleen of the normal C3H. mouse and the spleen of the C3H mouse with ra- , diation-induced fibrosarcoma. In a recent study [35] the contents of zinc, iron, antimony, chromium, cob alt, and scandium in DNA. were investigated during the growth of transplanted sarcoma M -l. I t was found that the concentrations of most elements varied over a wide range during the process of the disease.

Since tumors need e sse n tia l trace elements for th e ir growth as does the healthy tissu e , time variations of trace element concentrations should be studied. The simultaneous measurement of the concentration of e sse n tia l trace elements in d ifferen t organs, blood serum, and hair as a function of time in terval elapsed from the beginning of the tumor growth w ill provide the information on k in etics of trace elements. This information then can be used to understand the processes involved in tumor growth, the cross correlation of element concentrations and eventually to use measured trace element levels as the signature of tumor a c t iv ity . The importance of such work is in the establishment of trace element levels and th eir in terrelationsh ips during tumor growth. This might help the development of : ( i ) new diagnostic tools for early detection of tumor growth; ( i i ) e stab lish tumor dependence on trace , elements which may lead to the a ffec tio n of tumor by the change, of trace e le ment concentration.

5.5 Trace elements in hair

Hair is a unique biologic m aterial which re fle c ts the biomedical and environmental h istory of the s u b je c t. Since i t is convenient to handle and sample, and since i t has re la tiv e ly high concentrations of metals, trace-element analysis of human hair has been applied widely. I t has been shown [36] that the diagnosis of chronic plumbism in children can be confirmed by hair lead. Poisoning by other heavy metals and environmental pollution should a lso be accompanied by elevated levels in the growing h a ir . Head hair analysis appears to< o ffe r a unique approach to the investigation of human trace element nutrition and metabolism. There is a strong p o ss ib ility that the trace element content of hair correlates with body sto res , esp ecia lly of bone. Analyses of feces and urine are of limited value as indicators of sto res , and blood has re s tric ted use because the hemostatic mechanisms operate to keep many of the components of blood constant. Human head hair is a recording filament which can re f le c t metabolic changes of many elements over a long period of time, and thus i t re f le c ts past n u tritional events. The idea of hair analysis is very invitin g , because hair is easily sampled, shipped, and analyzed.

The concentration of trace elements in human hair may vary for most of the elements from individual to individual, but some correlation due to sex, age, and color has been found. Assuming that the growth of hair is approximate ly constant and continuous during the lifetim e of the su b ject, the concentration s of trace elements should d if fe r with increasing distance from the scalp . The concentrations of: most trace elements should increase with increasing distance from the scalp i f the exposure of hair to the elements of the environment is constant. Therefore, the influence of the environment on a population group can be indicated by trace-elem ent an a ly sis . Renshaw e t a l . [37] have studied the concentration of lead along a single hair and found a nearly lin ear increase in concentration with distance from the scalp .


zЭоо

<t(Г

FIG .14. X-ray spectrum from the proton bombardment o f hair target (ash).

Because of i t s growth, hair re fle c ts previous elemental concentrations in serum and body (history of. previous biochemical and medical events in man), as well as previous environmental e f fe c ts . Several measurements of trace e le ment d istributions along.the hair length have been reported [38, 39, 40, 41 ].As an example, Fig. 14 shows the-X-ray spectrum from the proton bombardment of hair targ e t. Peaks corresponding to Ca¿ T i, Cr, Fe, Ni, Cu, Zn, Pb, Se, Br,Rb, Sr,..and Mo are present in the spectrum. The investigators agree that the variations are ch a ra c te ris tic of the sub ject and that-Zn variations along hair are n eg lig ib le .. This la t te r fact may be also due to the re la tio n of Zn with the production of melanin pigment.in h a ir . Assuming constant d istrib u tion of Zn along the. h a ir , only the ra tio s of elemental concentrations to Zn concentration need to be measured to obtain elemental concentrations along h a ir . Such re la tive measurements can be e a sily performed on single hairs using proton-induced X-ray emission spectroscopy. I t has been shown [42] that the elements whose concentrations increase monotonically along the hair can be id en tified as pollutants in the area. For widespread pollutants (such as lead) even the medium value of a group o f. subjects can be used as a measure of the exposure. Actually only elemental concentration ra tio s (re la tiv e to Zn) need to be measured. Fig. 15 shows the concentration ra tio s As/Zn# Pb/Zn, and Ni/Zn for the hair of one subject as described in r e f . [42]. I t is important to note that the shape of the dependence on the distance from the scalp is sim ilar for a l l three ra tio s . This would indicate that elements As, Pb, and Ni are contaminants in the environment of the su b ject. Note that the Pb concentrations are an order of magnitude higher than those of Ni and As.

Examining blood and urine provides immense insight into human d iseases.I t is natural that one would like to add to these examinations a routine analysis of h a ir . The d if f ic u lt ie s associated with such an idea are due to the fa ct th at, as yet, normal and accepted.standards have not been established . A d efin ition of "normal" hair for d iffe ren t population groups is d iffe ren t for

68 V A L K O V I C

DISTANCE FROM SCALP ( - 3 5 c m / d i v )

FIG. 15. Concentration ratios: As/Zn, Pb/Zn and Ni/Zn as functions o f distance from the scalp. The Pb/Zn ratio has been divided by a factor o f 10. The errors shown are the statistical ones.

various geographical areas because of the strong environmental influence. X-ray emission spectroscopy was found to be a very convenient method in the determination of these normal values [43].

Kopito and Schwachman [45] have suggested that a ltera tio n s in hair e le mental composition may serve as diagnostic aids in cy stic f ib r o s is , ce lia c disease, phenylketonuria, in toxication with heavy metals, severe nu tritio n al d e fic ien c ie s , and geophagia. Hair analysis o ffers p o s s ib ilit ie s in determining the interdependencies of the trace elements, and the possible disease id en tifica tio n by trace element pattern . Such an approach has been reported by Larsen e t a l . [44] in a r th r it ic p atien ts . A sim ilar approach can be undertaken in a number of other d iseases. For example, i t is known that trace element leve ls in hair during pregnancy deviate s ig n ifica n tly from normal values.

The e ffe c ts of ionizing radiation on hair have been studied to some extent. In man, scalp hair is the most sen sitive hair to i r r a d ia t io n .'F ir s t , only temporary dépilation occurs; with the exposure to a higher dose, permanent dépila tio n re su lts . The pigment anomalies have been observed in the hair growth a fte r the temporary d épilation : the hair is usually darker. In an e ffo r t to evaluate the radiation e ffe c ts the trace element analysis of mouse hair has been performed [46] . Hair from mice irradiated with 600 rad Oy-rays) and hair from control group have been analyzed. Fig. 16 indicates some of the changes observed.

6. CHARACTERISTIC X-RAY SPECTRA FROM FOSSIL FUEL SAMPLES

In addition to hydrocarbons, compounds of sulphur, oxygen, and nitrogen and traces of metals are present in crude o i ls . A ll these substances can cause problems à ffectin g refining processes and the quality of products. For example; nickel adversely a ffe c ts ca ta ly sts used in refinery processes to increase gasoline yield from heavy gas o i l . Vanadium has adverse e ffe c ts when pjresent in too great a concentration; sulphur in crude o i l can form corrosive compounds that may damage the vessels of a re fin ery . The d ifference in vanadium and nickel content of various crude o ils provides a way to identify the source of

D É T E C T IO N O F C H A R A C T E R I S T IC X - R A Y S 69

FIG.16. X-ray spectra from mouse head hair. Top: mouse irradiated with .600 rad (Си/Zn = 0.061; Fe/Zn = 0.115). Bottom : control (Си/Zn = 0.069; Fe/Zn = 0.132).

a crude o i l . Crude o il "fin g erp rin ts" are used already several times to ident i fy the source of o i l contamination. However, the impact of burning fo s s il fuels on the atmosphere is probably the most important aspect of trace elements present in o il and coal.

For most of metals in crude o ils some preconcentration procedure is re quired to concentrate these m etals; crude-oil samples are prepared for analysis and reduced to ash by one of these two methods: ( i ) dry ashing; and ( i i ) wet oxidation followed by ashing. The samples to be analyzed usually contain m aterials other than o i l , such as brine and sand, which have to be removed before the o il is analyzed. A dry ashing method is described by Horr e t a l .[47 ]. (This is a s lig h tly modified ASTM-46 procedure.) Morgan and Turner [48] have showed by radioactive tracer technique that no s ig n ifica n t losses in organic ash occur i f the ashing is carried out below 550°. The losses of metals in the dry ash method have been found not appreciable for crude o ils and r e s i dual stocks; however,, in charge stocks and overhead fractions obtained by vacuum d is t i l la t io n losses, of metals may be considerable. Loss of some metals during the ign ition has been recognized [49]. To elim inate such losses a number of investigators have u tilized methods of fixing the metals bre pre-su lfating the o i l before igniting i t .

Wet oxidation of o i l is sim ilar to wet ashing procedure of b iologic and other organic m aterial. A technique frequently used is to char the sample by

70 V A L K O V I C

О 200 4 0 0 600 8 00 1000

X-RAY ENERGY (channel number)

FIG. 1 7. X-ray spectrum obtained by the bombardment o f the target made by depositing a few drops o f Venezualan crude oil on filter paper.

heating i t with su lfu ric acid and then adding concentrated n i tr ic acid in one or two ml increments; a ltern a tiv e ly or ad d itionally , strong hydrogen peroxide is added dropwise d irectly into the charred digestion mixture. In general, the ch a ra c teris tics of the wet-oxidation techniques are that re la tiv e ly large acid-to-sample ra tios are needed and lim ited amounts of sample can be decomposed in a reasonable length of time. The two methods of ashing show generally close agreement for the metals contained in o i l .

Several other methods of preconcentration or separations have also been applied. The element of in te rest may sometimes be extracted e ith er a fte r or before decomposition. I t is often possible to find a suitable extractant for the determination of some sp e c ific element or compound.

High-speed burning is a lso applied p articu larly for the determination of sulphur and hydrogens. Rapid burning is done in an oxyhydrogen flame. Combustion in oxygen has numerous advantages. In the oxygen bomb method organic matter can be decomposed without introducing large amounts of other reagents or risk ing loss of v o la tile components.

A ll three modes of sample excita tio n have been used for the trace element analysis of o i ls . For example, Rhodes [50J has described a system for the analysis of metals which arriv e , as the resu lt of wear, in jet-engine o i l .T h e radioactive sources used were 238Pu and 66Fe. Targets were prepared as thin ' • film deposited on filte r-p a p e r d iscs . Standards in the form of solutions were also deposited on f i l t e r paper.

There are several commercial tube excited X-ray fluorescence analyzers on the market. Many laboratories are using X-ray fluorescence for th e ir routine a- n a ly sis . For example, Marangoni et a l . [51, 52] measured the determination of wear metals in lubricating o ils from a ir c r a f t motors. Cr, Fe, Ni, Sn, and Pb were determined with a detection lim it b etter than 1 ppm and good reproducib i l i ty a t the 5 ppm lev el. Lutrario e t a l . [53] achieved lim its of detection of 0 .3 ppm for iron, 0.2 ppm for nickel and chromium in lubricants by optimizing instrumental parameters. In another report Lutrario e t a l . [54] reported a0.5 ppm detection lim it for Cu in lubricants using a Cr target tube operated a t 20-60 kV and 20-60 nA, and a p la s tic sample holder with an aluminized poly ester film window. A procedure for the determination of vanadium in the concentration range of 0 .05 -0 .5$ in heavy d is t i l la te fuels by the X-ray fluorescence a fte r concentration by d is t i l la t io n was reported by Boyle [55].


Fe Ni Cu Zn As Br Sr

2 0 0 6 0 0 1000 C H A N N E L

FIG. 18. West Texas crude oil: X-ray spectrum from ash target.

Shale Oil Product

As Se Br Sr

FIG.19. Shale oil product: X-ray spectrum from ash target.

72 V A L K O V I C

СЛ

Z>оо


FIG.20. X-ray spectra obtained by bombarding new (top) and used lubricating oil (bottom / targets with З-Ме V protons.

Charged p a rtic le induced X-ray spectroscopy has also been used in some laboratories. Some elements, are present in o i l in high enough concentrations that they can be measured without preconcentration. For example, Fig. 17 shows an X^ray spectrum obtained by the bombardment of the target made by depositing a few drops of Venezuelan crude o il on f i l t e r paper. 3 MeV protons,I = 10 nA, were used. The spectrum is dominated by bremsstrahlung radiation due to the target backing. However, peaks associated with S, V, and Ni can be easily id en tified . Concentration ra tio s , S/Ni = 600 and V/Ni = 20, are easily determined with a measurement lastin g only a few minutes. In order to be able to measure the other elements present in crude o i l , dry ashing of o il was performed. Targets were then prepared by smearing ash on the scotch tape backing.

D E T E C T IO N O F C H A R A C T E R I S T IC X - R A Y S 7-3

West Texas crude o i l X-ray spectrum is shown in Fig. 18, while the shale o i l product X-ray spectrum is shown in Fig. 19. In both cases, Br is due to the contamination in the scotch tape.

Determination of wear metals in lubricating o i l from a ir c r a f t and other motors has been studied by many in vestig ato rs. Detection lim its b etter than 1 ppm are reported for many metals including chromium, iron, n ick el, t in , lead, and copper' Fig. 20 shows X-ray spectra obtained by the bombardment of new and used lubricating o i l targets by 3 MeV protons. Targets were prepared by putting a few drops of o il on the f i l t e r paper. New motor o i l reveals only peaks associated with zinc (which is a known additive to o i l ) . The used motor o il from the author's car (10 000 km) shows a variety of peaks; some of them can be associated with the additives of gasoline.

Concerns with a ir pollution and i t s e ffe c ts on human health and disease have resulted in demands for fuels with lower and lower sulphur content. Measurements of sulphur content accurately and quickly are v ita l to refinery operation s, to minimize quality give-away and delays in product te s tin g . Routine laboratory te s ts have been developed which require only a few minutes for the analysis and are accurate to 0 .01 -0 .10$ . However, on-line measurement of S is a b etter approach, esp ecia lly where d irect product dispatch is required.

Gamage and Topham [56] have described such an on-line system for the determination of S which was based on X-ray fluorescence technique. They have used a miniature air-cooled X-ray tube which was run a t 27 kV and 2Ô0 p,A. The apparatus consisted of flow c e l l , measurement head containing the X-fay tube with collim ator, flow c e ll window and leakage detection u n it, power supplies.The X-ray beam strik es the middle of the flow c e ll and the sample flows continuously (20 lit/m in). The window used was 8 p,m Kapton film supported by photoetched Ni grid. X-rays were detected by gas proportional counter. Authors concluded that in such geometry there are no e ffe c ts from 100 ppm Ni and 200 ppm Na. Vanadium in terferes as a resu lt of the resolution of gas f i l le d proportiona l counter: 200 ppm V gave e ffe c t of 100 ppm S. A system for continuously monitoring low lead levels in gasoline has been developed by Exxon Research and Engineering and Princeton Gamma Tech. The system measures lead concentration in the range of 0.003 to 0.03 kg-m“3 every five minutes by X-ray fluorescence as the gasoline passes through a p ipeline. The continuous monitoring gives a more accurate average lead content for gasoline than does conventional sampling from a storage tank. In addition, the system can be used to pinpôint sources of lead contamination in unleaded gasoline.

X-ray fluorescence analysis proved to be rapid, and reasonably accurate for determining the concentration of about 20 elements in whole coal. Although major elements in coal, carbon, hydrogen, oxygen and nitrogen cannot be analyzed by X-ray fluorescence, most other elements a t levels greater than a few ppm are readily determined. Results of analysis of whole coal samples by X-ray fluorescence agreed well with values determined by several other independent methods [57]. Trace elements determined by the X-ray fluorescence method are limited to those occurring in whole coals a t a few ppm at least. The l i s t of these elements can be a long one.

Because of the speed and sim plicity of the method, X-ray fluorescence is highly adaptable to large-scale surveys of coal resources. As an example,Fig. 21 shows the X-ray spectrum obtained by the bombardment of a coal target by 3 MeV protons. The target was prepared by affixing some coal dust to scotch tape. The spectrum shown has been collected in a few minutes of irrad iation with the beam in ten sity of 10 nA..

7. ENVIRONMENTAL POLLUTION

The d istrib u tion of elements in Nature, th e ir concentration and th eir movements can be seriously affected by the a c t iv it ie s of Man. Environmental pollution resu lts from Man's introduction of d ifferen t substances into the marine, atmospheric or s o il environment. Sometimes th is can resu lt in deleterious e ffe c ts such as harm to liv ing resources and hazards to human health

74 V À L K O V I C


FIG.21. X-ray spectrum obtained by the bombardment o f a coal target by 3-MeV protons.

7.1 A ir polluti on

Many e ffe c ts of a ir pollution on plant and animal l i f e have been reported. Because most metals can be extremely toxic to plants even in low concentration, airborne pollution may be causing s ig n ifican t changes in the genetic structure of the plant population exposed to them. There is evidence that a great increase in p articu lar loading of the atmosphere has occurred during the past few years. I t has been suggested that th is increase is due to human a c t iv i t ie s , and that i t is the probable cause of the observed decline in the average temperature of the planet. P artic les in the size range of 0 .1 -1 .0 p,m have the greatest influence on the absorption and scattering of solar radiation in the atmosphere. Therefore concentrations of these p a rtic les in the atmosphere are of special importance to the planetary heat budget.

Transportation sources account for over 60$ by mass of the to ta l pollution. Exhausts from automobile engines produce most of the atmospheric po llution .The exhausts include smoke, carbon monoxide, oxides of nitrogen, hydrocarbons, lead, and other things. These pollu tants, once in the atmosphere, participate in d ifferen t photochemical reactions. The elemental composition of atmospheric aerosols has been determined in several areas around the world. Obviously the composition is influenced mainly by the local source d istrib u tion and meteoro logical conditions. Several in stitu tio n s are activ e ly monitoring a ir pollution using X-ray emission spectroscopy. The samples are collected on a f ib e r glass f i l t e r paper or on a cellu lose membrane f i l t e r . The cellu lose membrane f i l t e r is much more convenient for X-ray emission analysis because of the lower lev el of contamination. For the summary of recent work see references [9] and [30]. Lead is present in rather large amounts in the motor-engine exhausts. A ltogether, about 50$ of the lead in gasoline is emitted into the atmosphere as a fine sub-micron partícula te aerosol. There is a clear evidence of lead pollution caused by the motor vehicles close to the busy roads. In addition, more and more evidence is being accumulated about the world-wide spread of pollutants originated in the urban areas. I t has been shown that the atmospheric lead, mainly from the combustion of the leaded gasolines in the in ternal combustion engines, has increased on a t lea st a hemispheric scale [58]. This has suggested i t s use as a tracer for the large-scale atmospheric movement of anthropogenic p articu late m aterial. As an i llu s tra t io n , Fig. 22 shows an X-ray spectrum from the aerosol sample collected on the f i l t e r paper. The sample was


i------- 1------- 1--------1--------1--------г

P b S e B r

R I J E K A - K R I Z A N I C E V A U L .

S r

400 600 800CHANNEL NUMBER

FIG.22. X-ray spectra from an aerosol sample collected on the filter paper (top), and from the filter paper (bottom).

bombarded with 3 MeV protons, I « 10 iA, for a few minutes. C ahill and collaborators [59, 60] a t the Crocker Nuclear Laboratory (University of C alifornia a t Davis) have developed an extensive network of aerosol monitoring stations through C alifornia and are using th e ir fu lly automated p a r t ic le - in duced X-ray emission (cyclotron: cr-p articles) .

Host studies of suspended p articu la te pollutants in urban a ir are limited to estim ating the quantity of to ta l p articu late matter (p,g of p articu late matter per m3 of a ir ) and i ts elemental composition. In order to assess the inhala tio n health hazard, a knowledge of p a rtic le -s iz e d istrib u tion is needed. P artic le size is one of the very important factors in determining the degree of respiratory penetration, the extent o f v is ib i l i ty reduction, the nature of p a rt ic le -p a r tic le in teractio n and the mechanisms of a wide range of atmospheric phenomena.

7.2 X-rav emission spectroscopy of water

Elemental composition of the water is of great importance to l i f e . Water has to provide the e sse n tia l elements for the animals liv ing in i t s environment and through the food chain to the man. Drinking water is a s ig n ifican t source of e sse n tia l elements to man and animals liv ing on the s o i l . In addition, i t has to be free of elements which can have d eteriorating e ffe c ts on health.

Let us mention a few examples of trace-elem ent concentrations in water and th e ir re la tio n to human health . Many investigations have found a correlation between cardiovascular deaths and water composition. Total death rates were reported [61, 62] to be inversely correlated to hardness of water (to ta l c a lcium and magnesium concentrations). This association may-be due to the fact

76 V A L K O V I C

ENERGY keV

FIG.23. X-ray spectrum o f a seawater sample bombarded with 3-MeV protons. Yttrium was used as the dopant.

that so ft waters lack the b en efic ia l elements present (Cr, Cn, F, Mn, S i , V, . and Zn seem to exert b en efic ia l e ffe c ts ) in hard waters, or extract harmful elements (Cd, Co, Pb) from pipes. Certain associations were found also between the chemical composition of rocks and s o ils and cardiovascular death ra tes .

There are other numerous examples indicating the importance of the knowledge of trace element composition of water. As a resu lt many techniques for the analysis of water samples have been developed. The applications of X-ray emission spectroscopy to multi-elemental analysis of d ifferen t water samples w ill be discussed here.

Water targets are very easily prepared by simply allowing a few drops to dry on an aluminum formvar backing [11] . Such a simple target preparation a l lows the detection of elements present in the seawater with concentrations > 0 .1 y,g/ml. With proton-induced X-ray emission spectroscopy, concentrations of K, Ca, Br, and Sr can be determined in clean seawater. However, in many cases the concentration of metals can be s ig n ifican tly higher. For example, Fig. 23 (from r e f . [63]) shows the X-ray spectrum of a seawater sample bombarded with'3 MeV protons a t 200 nA for 600 s . Yttrium was used as a dopant. Iron and copper concentrations are measured to be Fe : 0.17 ^g/ml; Cu: 0.27 ^g/ml (normal values are Fe : 0.0034 p,g/ml; Cu : 0.003 ^g/ml).

The generation of water samples which are tru ly representative of any aquatic environment is a problem concerning i t s e l f with sampling methodology. Once a sample has been taken i t should have no p o ss ib ility of transporting trace metals e ith er to or from the sampling container walls or suspended mate r ia l within the aqueous soultion . Care taken a t the sampling stage is wasted • i f the representative nature of the sample is a ltered during handling and/or . storage. The tran sfer of trace metals a t low concentrations from solution to


FIG. 24. X-ray spectrum from seawater target; 0.1 liter o f seawater was prepared on filter paper (see text).

container walls is a problem fam iliar to a l l water s c ie n t is ts . There are severa l means by which to deal with th is problem. Naturally, the optimum method is to prepare and analyze the sample as soon as possible. When information is needed on elements which occur in small concentrations (< 0.1 ppm) some preconcentration procedure is required.

Trace elements are iso lated under certa in d efin ite experimental conditions as complexes such as dithiozonates, quinolates and dithiocarbam ates, which can e ith er be precipitated in an aqueous medium, because of th e ir poor so lu b ility in water, or can be extracted by an organic solvent [64].

For the analysis of dissolved trace metals in water one often uses a method which involves formation of insoluble metal chelates via coordination with dithiocarbamate (ammonium pyrrolidine dithiocarbamate or diethyl dithiocarbamate), f i l t r a t io n through a membrane f i l t e r , and the analysis of the p recip ita te . The complexes formed are highly insoluble in water and can therefore be trapped on a membrane f i l t e r which serves as a target m atrix. (The technique has been applied to analysis of water from various d ifferen t types of m aterial systems as well as to urine samples.) Simultaneous analysis of most of the tran sitio n elements is p ossib le, but a lk a li and a lkalin e-earth metals are excluded. Following elements can be separated with dithiocarbamate: V, Cr, Mn,Fe, COj Ni, Cu, Zn, Ga, As, Mo, Rh, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, T l,Pb, B i,.and U [64]. The dithiocarbamate p recip itatio n technique of target preparation enjoys several advantages over procedures based upon evaporation, among them uniformity of d istrib u tion of elements on the ta rg e t, sim plicity which reduces both handling time and contamination, and high e ffic ie n cy within a wide pH range.

Fig. 24 shows an X-ray spectrum from a seawater target prepared on m illipore f i l t e r paper from 0.1 l i t e r of seawater. The target was bombarded with only 1-2 nA beam of 3 MeV protons. This technique of se lectiv e complexation by dithiocarbamate chelates affords one a very nice way of analyzing seawater.

78 V A L K O V lf c

The chelates are very selectiv e against the sodium, potassium, and other cations found in percent quantities in seawater. Hence the trace metals can be e x tra c ted, and th e ir concentration measured even when present only in 10 3 ppm range.

8 . CONCLUSIONS

Research in te rest in several d iscip lin es has begun to focus on the complex problem of movements of elements in nature, and the relationship between the environment and liv ing organisms. A number of fascinating problems deserve more a tten tio n . The detection of ch a ra c teris tic X-rays has led to new analyt ic a l techniques capable of simultaneous detection of a large number of e le ments. The excita tio n by the radioactive sources and X-ray tube allows that such work be done even in small laboratories and without extensive funding.A number of systems are already commercially av a ilab le . Both "fundamental" and "applied" problems related to the trace element composition should be studied. In addition, the development of trace element analysis by charged- p a rtic le induced X-ray emission spectroscopy as an an aly tic technique suggests a nuclear accelerato r laboratory as an important fa c i l i ty for the study of trace elements. Large cross sections for X-ray emission along with low background radiations allow the usage of thin sample targets in which matrix e f fe cts are n eg lig ib le . At the same time, thin samples can be of considerable importance where only small quantities of the sample are av a ilab le . Available computer f a c i l i t i e s of a typ ical miclear physics laboratory allow fa s t c o lle c tion , processing, and analysis of data. With standardized target-making procedures, short data co llectio n and analysis periods, and simultaneous sen sitiv ity to a large number of elements, charged-particle-induced X-ray emission spectroscopy compares favorably with other trace element analysis techniques.

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Springer-Verlag, B erlin (1958).[3] OGURTSOV, G.N., Rev. Mod. Phys. 44 (1972) ! . .[4] GRYZINSKI, M., Phys. Rev. .138 (1965) A305.[5] GàRCIA, J .D ., Phys. Rev. Al (1970) 280.[6] DUGGAN, J .L . , BECK, W .L . , ALBRECHT, L ., MÜNZ, L ., SPAULDING, J .D .,

Adv. X-Ray Analysis Л5 (1972) 407.[7] RUTLEDGE, C.H., WATSON, R .L ., Atomic Data and Nuclear Data Tables 1¿

(1973) 195.[8] VALKOVIC, V ., Nuclear Microana-lysis, Garland Publishing In c ., New York

(1977).[9] JOHANSSON, S .A .E ., JOHANSSON, T .B ., Nucl. Instrum. Me th . 137. (1976) 473.

[10] RHODES, J .R . , Energy Dispersion X-Ray Analysis (RUSS, J .C . , E d .), ASTM publication 485 (1971).

[11] VALKOVld, V ., e t a l . , Nucl. Instrum.. Meth. U 4 (1974) 573.[12] VALKOVld, V ., Contemp. Phys. 14 (1973) 415.[13] F0LKMÍVN, F ., J . Phys. E 8 (1975) 429.[14] SUESS, H .E., UREY, H.C., Rev. Mod. Phys. 28 (1956) 53.[15] GENTRY, R.V., CAHILL, Т А . , FLETCHER, N.R., KAUFMANN, H.C., MEDSKER, L .R .,

NELSON, J .N ., FLOCCHINI, R.G., Phys. Rev. L ett. 37 (1976) 11.[ 1 6 ] TURKEVICH, A .L ., FRANZGROTE, E . J . PATTERSON, J .H ., Science .158 (1967) 635.[17] UNDERWOOD, E . J . , Trace Elements in Human and Animal N utrition, 3rd ed .,

Academic Press (1971).[18] BOJEN, H .J.M ., Trace Elements in Biochemistry, Academic Press (1966).[19] KEATING, B ., HELSLEY, С .E . , PESSAGNO, E A . , Geology 3 (1975) 73.[20] HAYS, J.D , B ull. Geol. Soc. Am. 82 (1971) 2433.[21] HARRISON, C . G A . , S0MAYAJULU, B .L .K ., Nature 212 (1966) 1193.


[22] HARRISON, C .G A ., PROSPERO, J.M ., Nature 250 (1974) 563.[23] REID, G.C., ISAKSEN, I . S A ., HOLZER, I .E . , CRUTZEN, P . J . , Nature 259

(1976) 177. ,[24] CRAIN, I . K., B u ll. Geol. Soc. Am. 82 (1971) 2603.[25] VALKOVIC, V ., Origins of L ife (1977), to be published.[26] BIEGERT, E .K ., CRAIG, M., VALKOVIC, V ., STORCK, R ., Proc. 4th Conf. on

Applications of Small A cce lerators, Denton, Texas, 1976.[27] STORCK, R ., MORRILL, R .C ., Biochem. Genetics 5 (1971) 467.[28] STEENBJERG, F . , Plant So il 3 (1951) 97.[29] VALKOVld, V ., RENDIC, D., BIEGERT, E .K ., ANDRADE, E ., to be published.[30] VALKOVIC, V ., Trace Element A nalysis, Taylor and Francis, London (1975). -[31] WHEELER, R.M., e t a l . , Medical Physics 1 (1974) 68.[32] CAVALIERI, R .R ., SCOTT, K.G., J . Amer. Med. Assoc. 206 (1968) 591-595.[33] A LIA WAY, W.H., KUBOTA, J . , LOSSEE, F ., ROTH, M., Arch. Environ. Health

16 (1968) 342.[34j JANES, J.M ., MCCALL, J .T . , ELVEBACK, L.M., Mayo Clin. Proc. 47 (1972)

476-478.[35] ANDROMIKASHVILI, E .L ., e t a l . , Cancer Res. 34 (1974) 271-274.[36] KOPITO, L ., BRILEY, A.M., SCHWACHMAN, H .J ., JAMA 209 (1969) 243.[37] RENSHAW, G.C., POUNDS, C A ., PEARSON, E .F ., Nature 238 (1972) 162.[38] VALKOVld, V ., e t a l . , Nature 24J3 (1973) 543.[39] RENSHAW, G. C ., POUNDS, C A ., PEARSON, E .F ., Nature 238 (1966) 59.[40] EADS, E A ., IAMBDIN, С .E . , Environ. R es., 6 (1973) 247.[41] OBRUSNIK, I . , e t a l . , Forens. S e i. 12 (1972) 426.[42] VALKOVld, V ., RENDld, D., PHILLIPS, G.C., Environmental Science and

Technology (1975) 1150.[43] RENDIC, D., e t a l . , J . Invest. Dermatol. bb_ (1976) 371.[44] IARSEN, W.B., LORD, R .S ., MILLER, J . J . , J . Am. Osteopath. Assoc. 74

(1974) 131-136.[45] KOPITO, L .E ., SHWACHMAN, H., The F irs t Human Hair Symposium (BRCWN, A.C.,

E d .), Medcom Press (1974).[46] VALKOVIC, V ., ОТТЕ, V ., ANDRADE, E ., BIEGERT, E .K ., to be published.[47] HORR, C A ., MYERS, A .T ., DUNTON, P . J . , U.S. Geological Survey B ulletin

1100-A (Washington: US Government Printing O ffice) 1961.[48] MORGAN., L .O ., TURNER, S .E . , Anal. Chem. 23 (1951) 978.[49] MILNER, O .I . , e t a l . , Anal. Chem. 24 (1952) 1728.[50] RHODES, J .R . , e t a l . , Environ. S e i. Tech. 6 (1972) 922.[51]. MARANGONI, O., LUTRARIO, P ., TRONCA, A ., Met. I t a l . 64 (1972) 381.[52] MARANGONI, О., e t a l . , Met. I t a l . 78 (1973) 6199.[53] LUTRARIO, P ., TRONCA, A ., CAPRIOTTI, R ., Rass. Chim. 24 (1972) 397.[54] LUTRARIO P ., e t a l . , Rass. Chim. 78 (1973) 1617.[55] BOYLE, J . F . , Amer. Soc. Test. Mater.'., Spec. Tech. Pub. 531 (1973).[56] GAMAGE, C .F ., ТОРНАМ, W.H., Advances in X-Ray Analysis Y¡_ (1974) 542.[57] KUHN, J .K . , HARFST, W .F., SHIMP, N .F., Trace Elements in Fuel, (BABU, S .P .,

E d .), Advances in Chemistry Series 141 (1975).[58] MUROZUMI, M., CHCW, T . J . , PETERSON, C.C., Geochim. Cosmochim. Acta 33

(1969) 1247.[59] CAHILL, Т А ., University of C alifornia a t Davis Report (1973), UCD-CNL-162.[60] FLOCCHINI, R.G., e t a l . , Advances in X-Ray Analysis 18 (1975) 579.[61] SCHROEDER, H A ., J . Chronic. Dis. 12 (1960) 586; ibid 8 (1965) 647.[62] MORRIS, J .N ., CRAWFORD, M.D., HEADY, J A . , Lancet 1 (1961) 860.[63] ALEXANDER, M.E., BIEGERT, E .K ., JONES, J .K . , THURSTON, R .S ., VALKOVld, V.,

WHEELER, R.M., WINGATE, C A ., ZABEL, T ., In t. J . Appl. Radiat. Isotopes25 (1974) 229.

[64] PINTA, M., Detection and Determination of Trace Elements (English transla tio n ), Jerusalem (1966).

N E U T R O N A B S O R P T I O N P H Y S I C S IN

T H E D E V E L O P M E N T A N D P R A C T I C E

O F A C T I V A T I O N A N A L Y S I S

S.S. NARGOLWALLANuclear Applications Research Laboratory,SCIN TREX Ltd,Concord, Ontario,Canada

Abstract

N E U T R O N A B S O R P T I O N P H Y S I C S I N T H E D E V E L O P M E N T A N D P R A C T I C E O F

A C T I V A T I O N A N A L Y S I S .

T h e r e v i e w o u t l i n e s t h e c o n t r i b u t i o n o f n e u t r o n a b s o r p t i o n p h y s i c s t o w a r d t h e d e v e l o p

m e n t a n d p r a c t i c e o f n e u t r o n a c t i v a t i o n a n a ly s is t e c h n i q u e s . I n s o f a r as t h e y c o n c e r n t h is

d e v e l o p m e n t , f u n d a m e n t a l n u c l e a r p a r a m e t e r s o f n e u t r o n e n e r g y , a b s o r p t i o n c r o s s -s e c t i o n a n d

i n d u c e d n u c l e a r i n t e r a c t io n s a r e b r i e f l y d is c u s s e d . T h e b a s ic a n a l y t i c a l t o o l s , s u c h as t h e

n e u t r o n s o u r c e , s a m p le i r r a d i a t i o n a n d r a d i a t i o n m e a s u r e m e n t i n s t r u m e n t a t i o n , a re d e s c r i b e d .

I n t h i s r e s p e c t , a s p e c ia l e m p h a s is is p la c e d o n t h e u t i l i z a t i o n o f e s s e n t ia l ly m o n o e n e r g e t i c fa s t

n e u t r o n s o b t a i n e d f r o m n e u t r o n g e n e r a t o r s f o r a n a l y t i c a l a p p l i c a t i o n s . T h e r o l e o f a c t i v a t i o n

t e c h n i q u e s , i n b a s ic c r o s s -s e c t i o n s t u d ie s a n d i n a n a l y t i c a l c h e m i s t r y , is d is c u s s e d w i t h a v i e w

t o e s t a b l is h in g t h e t e c h n i q u e as a f u n d a m e n t a l t o o l i n n u c l e a r p h y s i c s r e s e a r c h a n d m a t e r ia ls

c h a r a c t e r i z a t i o n , r e s p e c t i v e l y . T h e i m p o r t a n c e o f n e u t r o n a c t i v a t i o n a n a ly s is is d e m o n s t r a t e d

b y a c r i t i c a l c o m p a r i s o n o f i t s c a p a b i l i t i e s w i t h t h o s e o f f e r e d b y c o m p e t i n g c h e m i c a l a n d o t h e r

i n s t r u m e n t a l a n a l y t i c a l m e t h o d s . T h i s c o m p a r i s o n , i n e f f e c t , p r o v i d e s a n i n s i g h t i n t o t h e m a n y

a re a s w h e r e t h e c o n t r i b u t i o n o f n e u t r o n a b s o r p t i o n p h y s i c s h a s s e r v e d i n t h e d e v e l o p m e n t o f a

h ig h ly s en s i t iv e t o o l f o r t r a c e an a ly sis . T h e i m p a c t o f n e u t r o n a c t i v a t i o n a n a ly s is in b i o lo g ic a l

a n d b io m e d i c a l s t u d ie s , m a t e r i a l s c ie n c e a n d i n d u s t r i a l a p p l i c a t i o n s , e n v i r o n m e n t a l a n d

e c o l o g i c a l a s s e s s m e n t s , g e o - a n d c o s m o -c h e m i c a l i n v e s t i g a t i o n s , m i n e r a l a n d e n e r g y r e s o u r c e

e v a l u a t io n s , a n d i n a r c h a e o l o g ic a l a n d f o r e n s i c s c ie n c e s , is e v i d e n c e d i n t h e a b o v e a re a s o f

a p p l i c a t i o n . S o m e c o m m e n t s r e g a r d i n g t h e a p p l i c a t i o n o f n e u t r o n a c t i v a t i o n a n l y s is t o f u t u r e

c h a ll e n g e s in t h e r e a l m o f g e n e r a l m a t e r i a ls c h a r a c t e r i z a t i o n a re o f f e r e d .

1. INTRODUCTION

Neutrons in te ract with elemental nuclei over a wide range o f incident neutron energies. Because o f th e ir lack of e le c tr ic a l charge, neutrons o f very low energies w ill e a sily penetrate the e le c tr ic a l f ie ld surrounding nuclei and produce in teractio n s. Neutron sources o f su ffic ie n t in te n sitie s are available for the study o f nuclear reactions over an energy range from about 10~ to 10 eV. The nuclear reactor o ffe rs an opportunity to conduct such kinematic reaction studies through an energy range o f 10 to 107 eV. The reaction types induced depend upon the incident neutron energies u t iliz e d . I t is therefore convenient' to c la ss ify incident neutrons in terms o f th e ir energy groups. These

8 1

82 N A R G O L W A L L A

c la ss if ic a tio n s can be-broadly related to the possible types o f neutron induced reaction s, and to some extent to the various types o f neutron sources.and experimental methods applied to neutron absorption stud ies. ■ '

The most commonly used c la ss if ic a tio n s are : thermal neutrons - the mostprobable velocity o f the d istrib u tion a t 293°K is 2200 m.s- , corresponding to an energy o f about 0.025 eV; intermediate-energy neutrons - th is range extends from thermal to about 10 keV and exh ibits sharp resonances for neutron absorption for most elemental n u clei; fa s t neutrons - th is energy group is defined to include neutron energies from 10 keV to approximately 20 MeV. The upper lim it also coincides with the cap ab ility o f useful neutron sources; and r e la t i - v is t ic neutrons - th is range e sse n tia lly includes neutrons above 20. MeV and exh ibit r e la t iv is t ic e f fe c ts .

In considering the probability o f neutron absorption in teractions of an aly tica l iirçportance due regard must be given to the energy c la ss if ic a tio n s outlined above. The in teractio n probability or cross section ( a)- combined with the neutron in ten sity or flux ( Ф) form the foundations upon which the technique o f neutron activation analysis is based. Therefore, a l l factors a ffectin g the energy o f the incident neutrons w ill control the probability of neutron absorption, and the nature and extent o f the probable activation processes. The importance o f th is fa c t is brought out in th is review in terms o f the excita tio n functions fo r neutron absorption for each c la s s if ic a tio n of neutron energy. Selected references, containing cross section tabulations, cross section vs. energy dependence and other relevant data, as annotated in th is review, provide the working tools for the development and p ractice of neutron activation analysis. .

The importance o f activation a n a ly s is , in general, was recognized almost immediately upon the discovery o f a r t i f i c ia l rad io activ ity . However, i t s genes is is linked with the c la s s ic studies o f Hevesy and Levi in 1936, [ l] and Seaborg and Livingood in 1938 '[2] . During the f i r s t ten years o f i t s development, less than two dozen published works could be found in open lite ra tu re . Nevertheless, the exponential growth during the past two decades has resulted in the inclusion o f almost 10,000 publications in the current acquisition, f i l e s . This fa n ta stic growth can be a ttrib u ted , to a large measure, to the ever increasing acceptance o f the basic technology as a tool to probe the composition o f matter. In acknowledgement o f the technique's broad c a p a b ilit ie s , a considerable e ffo r t has been expended toward the development o f neutron sources, radiochemical an aly tica l techniques, sampling and nuclear instrumentation d esig n .. Today, neutron activation analysis rig h tfu lly belongs in the rapidly expanding analytical arsenal o f measurement techniques. Rather than present i t s e l f as a competitive an aly tica l to o l, i t s usefulness is more evidenced in i t s role as a complementary or comparative technique directed towards the pursuit o f achieving b e tter accuracy in the overall chemical measurement process.

With the a v a ila b ility o f portable medium in ten sity neutron sources such as rad ioiso topic, fissio n and from small acce lera to rs , considerable emphasis is currently being placed to e stab lish neutron activation as a basic technique for s c ie n t i f ic and technological applications. As such, i t s use in the f ie ld of b io log ica l and biomedical stu d ies, environmental and ecological assessments, material science and indu strial applications, geo-and cosmo-chemical analyses, evaluation o f mineral and energy resources, and in archaeological and forensic sciences, can be evidenced through a review o f the vast l ite ra tu re in existence.

In th is review an attempt is made to trace the development o f neutron activ ation analysis from fundamental nuclear concepts. Certain basic parameters associated with neutron absorption physics and governing the development o f activation techniques are considered.. These are the incident neutron energy, the probability or cross section o f the absorption process and the energy/ abundance ch a ra c teris tics o f the in teractio n decay products. A special discussion o f the an aly tica l application o f fa s t neutrons in research, and in some o f the important areas described above, is presented. The contribution o f neutron absorption physics is in d irectly assessed by a presentation o f a comparison of neutron activation analysis with other non-nuclear techniques for both trace-and macrochemical characterization o f matter. Although not a panacea for a ll

N E U T R O N A B S O R P T IO N P H Y S IC S 83

an aly tica l problems, neutron activation analysis w ill continue to o ffe r a viable means for the study o f tomorrow's challenges associated with the characterization of m aterials. '

2. • NEUTRON ABSORPTION PHYSICS

2.1 General ConsiderationsIn contrast to the generally predictable behaviour o f gamma-ray attenuation,

the physical processes governing neutron absorption are quite complex. This fact has lead to a general diversion from the pure analytica l approach to the use o f empirical methods for the determination o f neutron absorption cross sections.

Upon in teractio n with atomic n u cle i, neutrons can be absorbed and/or scattered with d ifferin g p ro b a b ilit ie s . The scatterin g events, be they e la s t ic or in e la s t ic , iso trop ic or assymmetric, also resu lt in degradation o f the incident neutron energy. Therefore the re la tiv e proportion o f the absorbed to the scattered component o f the incident neutron in ten sity in i t s passage through a given target medium is varying. The problem is further compounded by the consideration o f the cross section versus energy behaviour o f neutrons.This dependence is often an e r ra tic function punctuated by sharp excursions or resonance absorption peaks. In view o f these d if f ic u lt ie s , neutron absorption kinematics are b e tte r evaluated and understood by monitoring the rad ioactiv ity o f expected decay products formed by single or multiple in teractio n s. Such methods are dependent upon the a v a ila b ility o f source neutrons possessing well defined energies. From an examination o f l ite ra tu re , i t is evident that studies related to the u t iliz a t io n o f neutrons with energies less than 1 keV and greater than 100 keV have been abundantly documented. Cross section values in the above mentioned range are su ffic ie n tly re lia b le For the purpose o f calcula tin g in teractio n y ie ld s . Unfortunately, in the neutron energy range o f 1 - 100 keV, the cross section information is fa r from sa tis fa c to ry ; p rincip ally due to the experimental d if f ic u lty o f making accurate cross section measurements.

In view o f the energy dependence o f neutron absorption cross sectio n s, a rather arb itrary c la s s if ic a t io n o f neutron energies is found to be useful and is tabulated (Table I ) . ■ From the neutron activation analysis viewpoint, only those c la ss if ic a tio n s defined as thermal, resonance and fa s t neutrons need be considered. The following discussion o f absorption cross sections is therefore re s tr ic te d to the above mentioned an a ly tica lly useful neutron energies.

2.2 Neutron absorption cross sectionsThe probability o f a given neutron absorption process independent o f a l l

other events can be denoted in terms o f a cross section for that sp e c ific in te ra ctio n . In i t s sim plest form, the concept o f a nuclear cross section may be visualized as the cro ss-sectio n al area presented by a target nucleus to an incident neutron. The cross section versus energy dependence, i . e . the e x c ita tion function, exh ib its d iffe rin g d istributiore depending upon the neutron energy group being considered. The absorption cross section for the three e a r lie r mentioned neutron energy groups are b r ie fly discussed in the following subsectio n s.

2 .2 .1 Thermal neutron cross sectionsA study o f the excita tio n functions in the thermal region show irreg u la

r i t i e s with an almost complete absence o f sharp resonance peaks, due to the very lim ited energy range ty p ified by the thermal energy region. However, as the neutron wavelength 1.8 Я) is comparable to the interatom ic distance О 1 X), neutron wave sca tterin g from nuclei occur. Consequently, complex d iffra c tio n e ffe c ts are evidenced and resu lt in sharp changes in cross section values for the elements. Perhaps the most useful bibliography documenting sources for microscopic neutron cross section data are the CINDA [з] and CINDU


TABLE I. CLASSIFICATION OF NEUTRON ENERGIES

Term Description Neutron energy range

Cold Neutrons in equilibrium with a cold medium

Energy a t 25°K corresponds to 0.005 eV

Thermal Neutrons in equilibrium with a medium at 293°K

Most probable energy at 293° is 0.0253 eV. The Maxwelliai d istribu tion at 293 К extend: to about 0.1 eV

Epithermal Neutron energies in excess o f thermal

0.2 - 1.0 eV

Resonance Neutrons o f energies cor- resg^gding to resonances in U, In, Au e tc .

1 - 300 eV

Intermediate - 300 eV - 10 keV

Fast Neutron energies capable o f inducing threshold reactions

0.5 -20 MeV

U ltrafast R e la tiv is t ic neutrons Greater than 20 MeV

[4] se r ie s . Compilation o f early cross section values are published by the Brookhaven National Laboratory H and in a more recent update [б] issued by the same laboratory. The la t te r publication includes data on thermal neutron cross sections, resonance parameters and an extensive bibliography.

The great importance o f neutron absorption systematics in the thermal energy range is best exemplified by the large number o f possible interactions with atomic n u cle i. Therefore cross section studies for the en tire Maxwellian d istrib u tion o f thermal neutrons have been carefu lly carried out using neutron velocity se le c to rs . Such experimental resu lts have been documented i n ,user oriented tabulations. In general, the accuracies assigned to such data in recent compilations are about an order o f magnitude b e tter than those for data documented in the la te 1950's . Certain absorption cross sections are currently known to a v e j^ h ig h degree o f certain ty , e .g . the thermal activation cross section for Au is given [б] as 9 8 .8± 0 .3 bam s,while cross sections in the rare earth region are known only to a re la tiv e accuracy o f about ±20%. The sometimes large errors in cross section values arise from numerous sources,.By fa r the most s ig n ifica n t error is introduced when converting the measured counts o f a decay product into an absolute d isintegration ra te . Another s ig n ifican t source o f error resu lts from the d if f ic u lty o f measuring the e ffe c tiv e neutron flux o f a s p e c ific energy impinging upon a sample o f the element under consideration. Perturbation o f the neutron energy and in ten sity by the sample i t s e l f can introduce s ig n ifica n t error in the cross section determination. Recently, vast e ffo rts have been expended in standardization o f cross section measurement techniques and neutron flux evaluations. A d irect resu lt o f such e ffo r ts is re flected in the good accuracy lim its assigned to values recently documented [6] .


2 .2 .2 Resonance neutron cross section3, Except for some in teraction s with certa in elements, e .g . He(n,p)lH;

Li(n,a)^H; ̂ B(n, a )''b i ; 14N(n,p)12c; 1^0(n,a)l^C; 235u(n, fiss io n ) ; thermalneutron reactions are a l l o f the (n,y) type. Resonance neutrons also induce (n,y) reaction s. In th is neutron energy range,and for isotopes with atomic numbers usually in excess o f 30, certain elements exhibit sharp, high in ten sity cross section peaks. In p rin cip le , therefore, by the use o f subs ta n tia lly monochromatic beams from neutron spectrometers, a high degree of se lectiv e neutron absorption is p o ssib le . The u tiliz a tio n o f the resonance neutron absorption phenomenon for an a ly tica l purposes is however lim ited to a neutron energy upper lim it o f about 20 eV because the resonance neutron intens ity in a reactor neutron spectrum fa l ls o f f as 1. . In the compendium [б] , radiative neutron capture resonance in tegral E2 cross sections are tabulated.In general, the tabulation contains resu lts o f computations based upon the Breit-Wigner shape for resonance cross sections occurring close to the cadmium cu to ff energy o f about 0.5 eV. For resonances su ffic ie n tly removed in energy from the above cu to ff , the sum o f s in g le -lev el Breit-Wigner contributions are used to develop a sin gle expression for the calcu lation o f the resonance in te g ra l. In compiling th is tabulation, the authors have taken great care in assessing the discrepancies between computed and measured values.

The resonance absorption e ffe c t is p articu larly useful in increasing the s e le c tiv ity for element determination, since in most cases the resonance peaks do not overlap. In the rare earth region and in the region o f heavy elements, resonance in tegral cross sections can vary from about 300 - 30,000 barns. From a knowledge o f the to ta l peak resonance in tegrals and the neutron flux as a function o f energy, the expected induced rad ioactiv ity for a given resonance (n,y) reaction can be ca lcu lated . The emphasis on the rare earth isotopes is obvious,when consideration is given to the fact that from the an aly tica l viewpoint, determination o f individual rare earths is d if f ic u lt . Therefore, by u t i liz in g the concept o f resonance neutron absorption, i t is possible to illu s tra te[7] that substantial enhancement in the induced rad ioactiv ity o f a given rare earth element present in a matrix o f another can be achieved. Such enhancements make i t possible to apply the resonance neutron activation technique for the solution o f some d if f ic u lt an aly tica l problems.

2 .2 .3 Fast neutron cross sectionThe discussion o f fa s t neutron cross sections is in ten tio n ally addressed

to se le c t fa s t energy groups which are readily available to the activation analyst. The principal sources o f such energy groups are the fissio n neutron spectrum from reactors or from spontaneous fiss io n isotopes, and d iscrete monoenergetic groups available for low-voltage p ositive ion accelerators such as neutron generators and small cyclotrons, and from certa in nuclear reactions.

2 .2 .3 .1 Fission neutron cross sectionNeutrons produced by fiss io n o f 235ц can t>e described by a semi-empirical

expression as proposed by Leachmann [8] .f(E) = 0.7725E^ exp(-0.775E) ........... 1

where f(E) is the fiss io n neutron flux at energy, E, in the units o f n.cm'"2s- ^> This expression is however only good for the fiss io n neutron spectrum <9 MeV.The u tiliz a t io n o f the reactor fa s t neutron spectrum poses considerable problems in analysis due to the in terferin g reactions induced by the preponderant thermal neutron component. Published work [9] describing the ap p licab ility o f such a generalized neutron group is worthy o f study. Excitation functions relevant to reactor fa s t neutrons can be developed by a study o f the induced threshold type in te ra ctio n s . For th is purpose a knowledge or a measurement of the average neutron absorption cross section fo r the fiss io n flux is necessary.To f a c i l i t a te such studies,concepts such as "e ffe c tiv e threshold energy" and "e ffe c tiv e absorption cross sections" have been introduced. Examples o f some experimental and calculated excita tion functions for fiss io n neutron absorption

8 6 N A R G O L W A L L A

are given by Liskien and Paulsen [lôj . Jung e t a l . [ ll] describe simple experimental procedures for the determination o f e ffe c tiv e threshold energies and cross sectio n s. Cross section information fo r reactor fa s t neutrons is also available [l2] .

2 .2 .3 .2 Accelerator produced neutron cross sectionIn the following discussion, the information is heavily drattn from some

basic d issertation s [13-15] , sections o f which elaborate on neutron absorption cross sections for neutrons o f d iscrete monoenergetic groups. S p e c if ica lly , the neutron energies o f in te re s t are the 2.5 MeV and 14 MeV obtained by acce le rating a beam of deutercms on to a deuterium and tritium ta rg e ts , respectively . These low-voltage acce lera to rs , o r neutron generators, have been successfu lly employed for the analysis o f lig h t elements with a high degree o f se n sitiv ity and s p e c if ic ity . The emphasis here w ill be placed on the u tiliz a tio n o f 14-MeV neutrons, although for certa in sp e c ific applications, 2.5-MeV neutrons have also been used for an alysis. The principal neutron absorption processes o f in te re s t using 14-MeV neutrons are:

(n ,у ) : AZ + In — >(A+l)z + Y(n ,n ’ Y) Az + — >AZ + In + Y(n,p) : Az + n —— ?ACZ-i) .+ p(n,a) : Az + n ^ (A 3)(-Z-2) +

(n,2n) : Az + n — >(A-1)Z + 2nIn order to provide some insight into the p ro b ab ilities o f occurrence

o f these reactions, a q u alita tiv e description o f the in teraction energetics involved, is presented.

For incident neutron energies much less than the nuclear binding energy, the reaction promoted w ill strongly depend on the magnitude o f the binding energy. For instance, for targ et mass numbers A<20, the neutron binding energy exh ibits large and periodic fluctuations from nucleus to nucleus. For mass numbers between 20 and 130, the neutron binding energy shows, on the average, a very slow increase from about 8 to 8.5 MeV, followed by a slow decrease to about 7.5 MeV for the heaviest elements. However, even th is slow trend is interrupted by a few instances o f anomalous v aria tio n s. Examples o f such anomalous behaviour are evidenced by nuclei with odd mass and neutron numbers whose binding energies are about 1 to 2 MeV greater than those for even nuclei adjacent to them.

The (n.y) reaction is in a l l cases exoergic, and the absorption cross section in almost a l l instances decreases with increasing neutron energy. In contrast to the large v ariation s, often by as much as 10*° bams in the isotopic absorption cross sections for thermal neutron absorption, 14-MeV (n,y) absorption p ro b ab ilities are o f the order o f a few m illibam s and are usually neglected.In general, for neutron energies from 1 to 15 MeV, the (n,y) cross section increases as the th ird power o f the mass number A up to a mass number A=100 when a saturation value is reached. For 2.5-MeV neutrons, certain nuclei exhib it re la tiv e ly high (n,y) absorption p ro b ab ilities which can be useful for certa in analytica l applications. An assessment o f documented [l4] 14-MeV (n,y) cross sections indicate rather large errors associated with the cross section values; in some instances o f the order o f ±30%. These erro rs, to a large measure, re su lt from chemical separation processes employed for the separation of product n u cle i, and from d if f ic u lt ie s in making absolute counting measurements. In general however, the (n,y) cross sections are o f the order o f 5 mb. I f the cross section is plotted against the neutron number as shown in Fig. 1, i t can be readily seen that evidence o f lower values in the region o f neutron magic numbers is indeed s lig h t . For 2.5-MeV neutrons, cross section values as high as 100 mb have been measured and documented. An excellen t compilation by Csikai e t a l. [13] is summarized in Fig. 2. This figure i l lu s tr a te s the (n,y) cross section d istribu tions for 1 -, 3- and 14-MeV neutrons as a function o f the target neutron number. Cross section maxima o f ''-lOO mb, "'■ЗО mb and ^15 mb for 1 -, 3- and 14-MeV neutrons, resp ectiv ely , can be observed.


FJG .l. Radiative capture cross sections fo r 14.5-MeV neutrons.

The neutron sca tterin g reactions leading to isomeric sta tes and designated as (n ,n 'y )are s lig h tly endoergic. Only limited cross section information is currently av ailab le. Among these reaction s, only a few produce metastable sta tes with h a lf-liv e s s u ffic ie n tly long to be useful for an a ly tica l purposes. For neutron energies above 10 MeV, the (n,2n) reaction o ffe rs strong competition to the (n ,n 'y) in teractio n . From a lim ited l is t in g [ 14] o f some useful (n .n 'y) reactions, i t can be observed that for 14-MeV neutrons, the cross section values l ie in the range from about 100 to 600 mb. However, the errors are o f the order o f ±10 to ±20%. Most o f the (n .n 'y) reactions are also induced by 2.5-MeV neutrons due to the existence o f low energy lev els o f target n u clei.

Insofar as the threshold reactions are concerned, the cross section values for (n ,p ), (n,a) and (n,2n) threshold reactions range from about 5 to 500 mb. Some examples o f excita tion functions are illu s tra te d in Figs. 3-5 . Research studies have indicated that systematic trends observed in the (n ,p ),(n,ci) and(n,2n) cross sections bear great sig n ifican ce , esp ecia lly for the estimation o f unknown cross section values. In th is review the measurement techniques employed fpr the determination o f the above mentioned cross sections are not discussed. Su ffice i t to note that a host o f techniques have been u tilize d and described in the l ite ra tu re [9 ,16 ,17] . A series o f symposia [ I 8- 21] covering a broad range o f neutron cross section studies and appropriate technologies are worthy o f study. The subject matter includes both the measurement o f cross sections and th e ir c r i t i c a l evaluations, and application o f neutron absorption processes for the study o f nuclear decay schemes. An examination o f such co llected works leaves the user or activation analyst with the d is tin c t impression th a t, in general, poor communication e x is ts between the pure p h ysicist who is most q u alified and experienced in cross section measurement technology, and the an aly tica l nuclear chemist who invariably demands

FIG. 2. Radiative capture cross sections fo r fast neutrons plotted against target neutron number at 1, 3 and 14 MeV.

N E U T R O N A B S O R P T IO N P H Y S IC S

FIG.3. Excitation function for (n,p) reaction: 60M (n.p)60Co.

FIG.4. Excitation function for (n,a) reaction: 63Си(n,a)60Co.


FIG.5. Excitation function fo r (n,2n) reaction: 14N (n,2n)13N.

highly accurate nuclear d a ta .. Cox [19] in h is c la s s ic summary o f fa s t neutron cross sections emphasizes the fa ct that the user is prim arily in terested in obtaining re lia b le cross section information for his research, while the cross section measurer is e sse n tia lly interested in physics. Expanding on th is observation, Cox re ite ra te s that experimental physicists would do the data user and data evaluator a great deal o f service by including a l l pertinent d eta ils of the experimental procedures so that intercomparisons between data from d ifferen t sources could be ob jectiv e ly analyzed.

3. NEUTRON SOURCES

Neutron sources used in activation analysis can be c la s s if ie d into four main categories: (1) nuclear reactors, (2) rad ioisotopic, (3) fis s io n , and (4) accelerator neutron sources. Depending upon the type and irrad iation assemblies used( fa s t , resonance or thermal neutrons can be produced. The choice o f anappropriate source depends p rincip ally on the application being pursued. C riteriagoverning neutron source selectio n are : (1) co st, (2) physical dimensions and shielding requirements, (3) source handling' f a c i l i t i e s , (4) source strength, and(S) available space.

3 .1 Nuclear reactorI t is weljjjcnown that when a thermal neutron is absorbed in a f i s s i l e

nucleus such as U, the binding energy added to the ta jg g t nucleus is su ffic ie n tto separate the nucleus into two fissio n fragments e .g . U + *n -----^148La + ®^Br.Since 1942, when the f i r s t graphite-uranium p ile was constructed, numerous reactors have been b u ilt using a variety o f geometrical configurations, fuel elements and moderators. The reacto r, therefore, is c la ss if ie d in terms o f the fuel and moderator used, and i t s sp e c ific function.

From e a r lie r discussions i t can be gathered that (n,y) in teractioncross sections are much larger than those for threshold reactions with a giventarget isotope. Since the former in teractio n is quite p r o l i f ic , i t is obvious that any reactor providing access to a high thermal neutTon flux can be used for activation analysis. To minimize the production o f in terferin g threshold in teractio n s, a highly pure thermal neutron spectrum is d esirab le . Reactors used for radioisotope production are generally equipped with a "thermal column" constructed from graphite blocks and su b stan tia lly removed in distance from the core i t s e l f . I f however, the activation analyst is in terested in using threshold reactions as means for material characterization , then a pure fissio n neutron spectrum is d esirable . Often, th is condition is reasonably sa tis f ie d by carrying


out sample irrad ia tion s inside a hollow fuel element where moderation o f fissio n neutrons is a t a minimum. Special reactor assemblies such as those described by Lukens e t a l . [22] , and Yule and Guinn [23] can be pulsed by rapid removal and rein sertion o f the reactor control rod. Neutron fluxes o f the order o f 10^ n. cm"^s_l can be obtained during the "pulsed" time in terval o f a few m illiseconds. This procedure resu lts in a s ig n ifica n t enhancement o f the in duced a c tiv ity o f short-lived nuclides.

In view o f the operational aspects o f nuclear reacto rs, samples for neutron .irrad iation have to be transported to and from the irrad ia tio n s ite by remote methods. The sample tran sfer container, or " ra b b it" , is care fu lly se le c ted to be e sse n tia lly in sen sitiv e to neutron absorption and radiation damage.The container is transported by pneumatic or hydraulic power inside a "rabb it" tube whose terminal is located in some desired neutron environment inside the reactor containment v esse l. In order to ensure that the sample inside the "ra b b it" is exposed to a reasonably homogeneous neutron flu x , a judicious choice o f the terminal emplacement is necessary. The sample must, a t the same time, have the ben efit o f a high neutron flux for maximum s e n sitiv ity o f analysis. Sample tran sfer times o f the order o f 1 to 3 seconds are ty p ica l. After ir r a d iation , the "ra b b it" is exited into a small shielded enclosure for "cooling" o f f prior to handling. In the design o f such sample tran sfer f a c i l i t i e s due attention is given to the e ffe c t o f in-core gamma heating and rad io ly sis of liquid samples. Perhaps the most d if f ic u lt irrad ia tion parameter to evaluate is the expected temperature r is e in the sample. The sample temperature is dependent upon such conditions as perturbation o f neutron flux by the sample, attenuation o f gamma-rays and th e ir buildup in the m aterials surrounding the sample, and the sample matrix i t s e l f . D etails, essen tia l to the user, have been summarized [24,25] .

3.2 Radioisotopic sourceCertain natu rally occurring and a r t i f i c ia l l y produced low-mass elements

have very low neutron binding energies and su itab le absorption cross sections for neutron production. The most commonly used rad ioisotopic neutron sources are o f the ( a,n) and ( y,n) type. The source consists o f an alpha-or gamma- em itting radioisotope intim ately mixed with a target element capable o f in te r action . The target element used in O .n) sources is beryllium . Principal а-em itters used include^^ Ra, 210po> 227дС) ’ °pu, 239pu> 2 4 1 ^ anc¡ 242rm_

The (y,n) sources u t i l iz e such high-energy у-em itters as 124Sb, Y,Na and l^ L a in conjunction with such targets as beryllium and deuterium oxide.

Some radioisotopic neutron sources are fabricated to permit the separation o f the radioisotope and the ta rg e t, thus o fferin g a means for turning o f f the neutron supply. From a detailed survey of (ct,n) and (y,n) radioisotopic neutron sources, relevant data useful to the user is tabulated Table I I . Some o f the obvious advantages in the u tiliz a t io n o f such sources are ; mechanical s ta b il i ty , compact size and co st. The neutron output, though decaying with the pertinent h a l f - l i f e , is predictable from day to day; and in most cases a constant over reasonable irrad ia tio n periods. The major disadvantage i s , of course, the re la tiv e ly low-neutron output when compared to the nuclear reactor cap ab ility . Furthermore, d if f ic u lt ie s are envisaged due to the in a b ility for turning o f f neutron production without some s a c r if ic e o f the optimum source strength possible fo r a given source design. Typical neutron energy d istr ib u tions available from such sources are shown in F igs. 6 -9 . The reader is also directed to a comprehensive characterization o f radioisotopic neutron sources as described by Ansell and Hall [26] .

3 .3 Isotop ic spontaneous fissio n sourceSome transuranic elements d isin tegrate to a s ig n ifica n t degree by spon

taneous fiss io n and are therefore potential sources o f energetic neutrons. In Table I I I is presented a l is t in g o f spontaneous fiss io n sources and th e ir emission ch a ra c te r is t ic s . Most o f the sources lis te d are produced in minute quant i t i e s from the irrad ia tio n o f fuel elements in extremely high-flux reactors

T A B L E И. A L P H A -N E U T R O N A N D G A M M A -N E U T R O N R A D IO IS O T O P E S O U R C E S

Type SourceMain

ReactionQ

(MeV)

AveNeutron

Energy(MeV)

Neutron Yield per sec per curie

(nps/curie)

H a lf- life o f Alpha em itting nuclide

242 Cm-Be 9 12 Be(a,n) С 5.65 4 MeV 4 x 106 163 d244 Cm-Be - 5.65 4 MeV 3 x 106 18.1 у241Am-Ве mixture 9 12 Be(a,n) С 5.65 4 MeV 2.2 x 106 433 у

О.Ю 226_ „ . „ Ra-Be mixture 9 12 Be(a,n) ZC 5.65 3.6 MeV 1.5 x 107 1 620 у210_ „ . „ Ро-Be mixture239 Pu-Ве mixture

Be(a,n) С9„ , , 12„ueici^) L

5.655.65

4 .3 MeV 4 .5 MeV

2.5 x 106 2 x 106

138 d 24 360 y

227Ac-Be 9 12 Befa,n) С 5.65 4 MeV 2.4 x 107 21.8 y228Th-Be - 5.65 4 MeV 2 .8 x 107 1.9 y238Pu-Be - 5.65 4 MeV 2 .8 x 106 89 y

24Na-Be 9Be(y,n)8Be -1 .67 0.2 1.4 x 105 15 h

(Y ,n)*

24Na-D 0 '88Y-Be88y- d. o

124Sb-Be

2H (y ,n )1H 9В е(т,п )8Ве2H (Y .n )^9 8 Be(y,n) Be

-2 .23-1 .67-2 .23-1 .67

0.80.160.30.02

2 .9 x 105 1 x 1053 x 1031.9 x 10S

15 h 108 d 108 d 60 d

- 140. D La-Be ^Ве(у,п)8Ве -2 .23 0.6 2 x 103 40.2 h140. n n La-D20 2H (Y ,n)XH -2 .23 0.15 7 x 103 40.2 h

* Neutron y ields are those far a 1 g. beryllium or heavy water targ et at 1 cm from the gamma source o f one curie


NEUTRON ENERGY, MeV

FIG. 6. Neutron energy distribution o f 239 Ри-Be (a,nj source.

for an extended period o f time. Successive neutron absorption events produce beta-unstable products. After several beta decays, the unstable nuclide can decay by spontaneous fiss io n and alpha decay, to generate high fluxes o f neutrons whose energy d istribu tions are comparable to that obtained from the thermal neutron fiss io n o f 235ц. Among these sources, 252çf ¿S) ^y f ar> enjoying wide-spread u tiliz a t io n for an a ly tica l applications. I t is o f rugged construction, small in s iz e , provides a stab le flux О 2 .3 x lO^2 n .s~ *. g"*) for a reasonably long period o f time (h a l f - l i f e 2.65 years). The source is generally encapsulated in s ta in le s s -s te e l , platinum-rhodium a llo y , zircaloy or certain types o f ceramics. Currently the source is manufactured and marketed by the Energy Research Development Administration (ERDA) o f the United States at a cost o f about $10 per microgram plus encapsulation co sts . For a 100 microgram source, the encapsulation costs are o f the order o f $1000-$2000.I f a high source strength is needed without adding to tKe 252çf weight, a uranium blanket [27] is used to boost the neutron output. Boosting facto rs approaching a facto r o f ^40 have been achieved. Concise information regarding the purchase,encapsulation and research applications relevant to th is source can be found inperiod icals released by ERDA.

3.4 A ccelerator neutron sourceDuring the la s t twenty years accelerato r produced neutrons have played

a s ig n ifica n t ro le both in terms o f experimental neutron physics and neutronactivation an alysis. The th eo retica l ca p a b ilit ie s [28] o f such devices suggests the u tiliz a t io n o f a wide variety o f neutron producing in teractio n s. However the choice is often d ictated by the cost o f irrad ia tio n s. These accelerators can be used to induce useful neutron producing in teraction s as summarized in Table IV. The neutron yields as a function o f incident p a rtic le boiAbarding energy for some o f the reactions lis te d in Table IV are shown in Figs. 10 and11. Neutron yield information given herein has been abstracted from a guide

9 4 N A R G Ó L W A L L A

NEUTRON ENERGY, MeV

FIG. 7. Neutron energy distribution o f 241 Ат-Be (a,n) source.

NEUTRON ENERGY, MeV

FIG. 8. Neutron energy distribution o f 226Ra-Be (a,n) source.


NEU TRO N ENERGY, keV

FIG.9. Neutron energy distribution o f i24Sb-Be(y,n) source.

T A B L E I I I . S P O N T A N E O U S F I S S I O N N E U T R O N S O U R C E S

Alpha Decay Fission Fissions NeutronsNuclide Tij(years) V-% (years) per io ta 's per Fission

238ц 4 .5 x io 9 . 6 .5 x 1015 5 .6 x ÎO '1

236_ 2.85 3.5 x 109 8 .1 x ÎO '4 ' 1.89 ± 0.20Pu

238n 86.4 4 .9 x 1010 2 .3 x 10*3 2.04 ± 0.13Pu

240_ 6580 1.3 x 1011 5 .4 x 1 0 '2 2.09 ± 0.11Pu

242n 3 .8 x 10S 7.1 x 1010 5 .3 2.32 ± 0.16Pu

244d 7 .6 x 107 2.5 x 1010 3.0 x 10‘ 3 'Pu • . 1

242„ ‘ 0.45 7.2 x '106 6 .2 x 10~2 2.33 ± 0 .1 1Cm

244_ 17.6 1.3 x 107 1.3 • 2.61 ± 0.13Cm

252c f 2 .6 85 3.3 x 104 .' 3.51 ± 0.16

254 c f Long ,,0.17 Large . - :


T A B L E IV. N E U T R O N P R O D U C T IO N B Y A C C E L E R A T O R S

Reaction

Threshold energy o f incident radiation

(MeV)Q value

(MeV)

Emitted Neutron Energy at Threshold Bombarding Energy

(MeV)

Principal Accélérato

Used

2H(d,n)3He - + 3.266 2.448 NG

3H(p,n)3He 1.019 - 0.764 0.0639 VG, С

V d ,n ) 4He - +17.S86 14.064 NG

9 12 Be(a,n) С - + 5.708 5.266 VG, C, N

12C(d,n)13N 0.328 - 0.281 0.0034 VG, С

13C (a ,n )160 - + 2.201 2.07 VG, С

^Li(p,n)^Be 1.882 - 1.646 0.0299 VG, С

9 10 Be(d,n) В - + 3.79 1-6 VG, С

7Li(d,n)®Be - + 15.0 - VG, С

9 8 Be(y,n) Be 1.66 - Polyenergetic LA

V y . n ) ^ 2.2 - Polyenergetic LA

*NG = Neutron generator; VG = Van der Graaff; С = Cyclotron LA = Linear accelerato r

[29] on neutron production using small accelerators and a comprehensive tre a tis e £14] on the subject o f neutron generators. Since th is review emphasizes the u tiliz a tio n o f small low-voltage accelerato rs or neutron generators, the following descriptions pertain to th is p articu lar type o f accelerator only. Readers in terested in the larger and more expensive p a rtic le accelerators are referred to references [29] and [30].

Small neutron generators have been widely used for activation analysis.The commercial instruments are e sse n tia lly scaled-down charged-particle acce le rators producing moderate fluxes o f 14- and 3-Mev neutrons by exoergic reactions such as^H(d.n) He and 2 H ( d ,n ) 3H e ,resp ectiv ely . Since a l l p ositive-ion accelerators b asica lly sim ilar in design, they can be represented by the diagram shown in Fig. 12. Prin cipally a neutron generator is composed o f an ion source, an accelerating tube and a su itab le ta rg e t. Positive ions generated in the ion source are accelerated through a potential d ifference o f about 60-200 к V in the accelerator tube which is maintained continuously at low pressures o f about 1 0 '4 to 10"6 mm Hg. Upon strik in g the ta rg e t, the accelerated ions enter into in teractions to generate neutrons which are e sse n tia lly iso trop ic in emission.


FIG. 10. Neutron yields (d,nj reactions.

Average fluxes o f about 5x10 n.cm“2s ' (14 MeV) and 5x10 n.cm~2s (3 MeV) at about 1 cm from the ta rg e t, are available for activation analysis. The high energy neutrons can be moderated in a paraffin block placed adjacent and, around the target to y ield small fluxes o f thermal neutrons. The application-and mechanical aspects o f siich low-voltage accelerators are comprehensively documented [13-15J . Use o f small sealéd-tube type o f high energy neutron generators has been demonstrated by th e ir application in the geological f ie ld . Such tubes are capable o f pulsed operations for the purpose o f performing special analyses for uranium in mineral deposits. An example o f á sealed-tube generator is shown in Fig. 13.

The application o f neutron generators o ffe rs the following advantages:(1) the neutron beam can be turned o f f by simply d e-exciting the acceleration high voltage, and (2) threshold reactions often provide a unique solution to certain nuclear in terferen ce problems, not possible with thermal neutron a c t i vation. On the other hand, the low-voltage accelerators present some problems in terms o f the d if f ic u lty o f accurate flux determination o f the primary neutron emission from the ta rg e t, and mechanical d if f ic u lt ie s associated with the lim ited l i f e o f ta rg e ts .

N A R G O L W A L L A

Bombarding energy, MeV

F IG .ll. Experimental neutron yields under normal laboratory conditions.

ExtractionProbe

R.FPower

Supply

1Ж1

Ion Source Base

/

Щ J —-,— f ' Gap Drift I

CanaL

Ion Source Bottle

GapLens

DriftTube

Solenoid CoilAccelerating Electrodes

VacuumPump

D.C. High Voltage

FIG. 12. Schematic of a neutron generator.


SampleTarget Shield Irradiation

FIG.13. Sealed-tube neutron generator.

U l)

Counting castle - '

FIG. 14. Schematic o f a neutron generator activation analysis facility.

When using neutron generators, two paramount problems must be recognized i f reproducible resu lts o f analysis are to be expected. F ir s t ly , the inhomogeneity o f the accelerated p a rtic le beam and the d istrib u tion o f target element nuclei resu lt in the production o f a neutron flux which is fa r from homogeneous. Secondly, the degradation o f the neutron flux and i t s normal gradient through a sample introduce a corresponding gradient in the induced rad io activ ity . As a general solution to the two problems, great e ffo rts [31-39] have been made to design irrad ia tio n f a c i l i t i e s which permit the sample to be subjected to a homogeneous exposure to neutrons.

Because o f the lim ited target l i f e i t is obvious that neutron generators are most suited for short irrad ia tio n s, therefore th e ir application is directed

1 0 0 N A R G O L W A L L A

to the analysis o f short-lived decay products. This fa ct automatically implies the inclusion o f a very fa s t automatic and programmable sample tran sfer system as an in tegral part o f an activation fa c i l i t y u tiliz in g a neutron generator.A typ ical fa c i l i ty is schem atically shown in Fig. 14. The large amount o f concrete shielding necessary for radiation safety may be observed.

In order to correct for d ifferences in the neutron flux impinging upon a sample from irrad iation to irra d ia tio n , neutron flux monitoring is e sse n tia l. Many methods have been used and the reader is referred to detailed descriptions given in a tre a tis e by Nargolwalla and Przybylowicz [14], and a d issertation by Iddings [40] .

4. NEUTRON ACTIVATION ANALYSIS

As mentioned in the Introduction, neutron activation analysis has gained wide acceptance in recent years as a very sen sitiv e analytica l tool for the characterization o f m aterials. In the early days, radioisotopic neutron sources were used; these, however, were low in neutron output and consequently o f lim ited use. With the a v a ila b ility o f nuclear reactors and the development of sm all, re la tiv e ly inexpensive neutron generators, the growth o f activation analys is was considerably accelerated . Today the p ractitio n er has available several monographs [13-15, 41-54] to provide adequate background in a l l aspects o f th is technique. The popularity o f the method has resulted in the establishment of a ser ie s o f conferences [55-59] named "Modem Trends in Activation Analysis" to provide a common ground for the exchange o f ideas and comparison of developments. Comprehensive bibliographies [14, 44, 60-65] , including almost 10,000 studies o f a very broad range o f applications, provide ready references for the un initiated and also serve as a general compendium o f information for the veteran.

4 .1 Principle o f the technique.The fundamentals o f neutron activation analysis have been adequately

described in texts [ l4 , 41-47', 50] and general review s[l3, 15, 48, 49, 51-S4],In general, the method involves the irrad iation of a sample with a flux of neutrons. Upon neutron absorption, the irradiated nuclei are converted into various radionuclides or stab le isotopes. The radionuclides decay with the emission o f ch a ra cteris tic radiation at a rate that is well defined and through a sp e c ific decay scheme [66-7o] unique to the p articu lar decaying nuclide.The decay ch a ra cteris tics and the decay rate permit the unambiguous id e n tif ic a tion o f the radionuclides in the irradiated sample.

The rad ioactiv ity generated from the irrad iation o f a target nucleus for a sp e c ific nuclear reaction can be expressed as a function o f the operational parameters o f the activation technique in the following manner:

. -0 .6 9 3 ti , r -0 .6 9 3 td ,A - DCNacf | l̂-exp | ——---- JJ exp ^------ ---- J J ................. (2)

where,

A = the measured count rate at a time t^, counts per second.D = the branching ra tio for the emission being measured.С = the detector e ffic ie n cy o f the counter.N = the number o f target nuclei exposed to the incident flux.0 = the neutron absorption cross section o f the p articu lar reaction with

the target isotope at a given incident neutron energy, cm̂Ф = the neutron flux o f constant in ten sity and energy, n.cnT^ s '*t i = irrad iation timet j = decay time from the termination o f the irrad iation to the s ta r t o f

counting.Tjj = the h a l f - l i f e o f the product nucleus being measured.The units of time should o f course be the same throughout Eq. 2.

N E U T R O N A B S O R P T IO N P H Y S IC S 1 0 1

In Eq. 2, factors such as ф, С and a are not generally evaluated because th e ir determination is both d if f ic u lt and inaccurate. To improve accuracy the more p ractica l comparative approach is used. This method involves the simultaneous irrad iation o f a standard and the sample. Thus, the amount o f a given element in an unknown sample is given by,

Element (in unknown) = Element (in standard) x — ............. (3)

where,Au = Measured count ra te o f the element in the unknown sample at time t^,

counts per second.As = Measured count rate o f the element in the known sample at time t ¿ , counts

per second.Although almost a ll analyses are performed using the comparative approach,

Eq.2,or some modification o f i t , is very useful in establish ing optimum conditions for a given analysis, and for the compilation o f se n s itiv ity tabulations [53, 72-76]. In regard to reported detection lim its or s e n s it iv it ie s given in the tabulations, the reader is cautioned to use these figures o f merit only as a very approximate indication o f expected s e n s it iv it ie s . Uncontrolled fa cto rs , such as the energy distribution o f the incident neutron flux and the u ser's choice o f irrad ia tio n , decay and counting conditions preclude a ready comparison o f expected ra d io a ctiv itie s with those published in the tabulations.

Activation analysis d if fe rs from a l l other non-nuclear chemical analysis methods in that i t is based on a nuclear in teractio n rather than on an in teraction with the o rb ita l electrons surrounding a nucleus. This immediately implies the independence o f the activation technique from any chemical or physical s ta te o f the target element. The technique is sen sitiv e only to the nuclear properties such as neutron absorption cross sectio n s, h a lf-liv e s o f the product isotopes and th e ir energy emission decay scheme, and is unrelated to the chemical properties o f the target elements. Therefore, elements that are chemically s im ila r, such as rare earths and the tran sitio n element groups, and thus d if f ic u lt to d iffe re n tia te by normal chemical methods, can often be d iscriminated by activation analysis.

To fu lly appreciate the sign ifican ce o f neutron activation as a technique for compositional an aly sis, one must compare i t s operational parameters and ca p a b ilitie s with those o f other techniques designed to perform sim ilar functions.A chart showing such a comparison is given in Table V. Note that the neutron a c t i vation technique has been subdivided according to the type o f neutron source used. The s e n s itiv ity aspects o f the lis te d techniques have been d eliberately excluded as much controversy can arise from the selectio n o f basic analytica l parameters used in the ca lcu lation o f detectable lim its . Su ffice i t to mention that a l l o f the techniques l is te d , with the exception o f 14-MeV neutron activ a tion and x-ray fluorescence, are capable o f reaching detection lim its in the sub-ppm range. Since in th is review, a special consideration is given to the u t iliz a t io n o f 14-MeV neutrons from small acce lera to rs , i t is necessary to include Table VI which gives the detection lim its for pure elements under acceptable laboratory conditions. For the in terp retation o f the detectable lim its , certa in an aly tica l conditions need to be q u alified . A 14-MeV neutron flux o f 109 п.спГ2 s~l impinging upon a 10 gram sanple o f the pure element is assumed. The counting arrangement o f a 3-in right c ircu la r cy lin d rica l sodium iodide (Nal (ТЯ)) s c in t i l la t io n detector and a 2тг counting geometry, is considered. The detection lim it is calculated by: to ta liz in g counts in the gamma-ray photopeak region; computing the background counts under the photopeak by a single base-lin e subtraction technique; computing the square root o f th is background; and expressing the lim it o f d e te c ta b ility as the amount o f target which w ill . generate counts in the photopeak which are 6 times the square root o f the background counts.

TABLE V, COMPARISON OF TECHNIQUES FOR COMPOSITIONAL ANALYSIS

Qualitative analysis Quantitative analysis Time of analysis Multicomponent analysis Instrumentation costTechnique Property measured Evaluation Property measured Evaluation Specificity /Long: > 30 min\

yShort: < 30 min)capability /High > $ 30 000\

\Low < $ 15 OOOy

14-MeV neutron activation

Gamma energy and decay rate of product nucleus

Excellent Gamma energy and amount of induced activity

Excellent in the trace- element region

Excellent Short Good to excellent Medium to high

Classical chemical (gravimetric, volumetric combustion etc.)

Volume or weight

Excellent Long Low

Mass spectrometry Mass/charge of ionized particles

Excellent No. of particles specific mass/ charge

Excellent (gases, liquids) Good (solids)

Excellent Long (solidsShort (gas and liquids)

Excellent Medium (gases and liquids) High (solids)

X-ray spectroscopy Wavelength of emitted X-ray

Excellent Intensity of X-ray emission at specific wavelength

Good insemi-microconcentrations

Excellent Short Good Medium

Atomic absorption spectrophotometry

Not applicable for qualitative survey

Absorption of specific line emission

Excellent Excellent Short Poor Low

Infraredabsorption

Wavelengthdistribution

Good Absorption at specific wavelength

Good Fair Short. Fair Medium

Ultravioletabsorption

Wavelengthdistribution

Poor Absorption at specific wavelength

Excellent Fair Short Fair Low-medium

NOTE: Only 14-MeV neutron activation and X-ray spectroscopy offer non-destruction analyses; however, the latter technique provides analytical information about the compositional nature of the sample surface.

102 N

AR

GO

LWA

LLA

TA BLE VI. DETECTION LIMITS FO R 14-MeV NEUTRON ACTIVATION AN ALYSIS Note: s = seconds; m = minutes; h = hours; d = days.

E l e m e n t

N u c l e a r r e a c t i o n

p r o d u c t H a l f - l i f e

G a m m a - r a y e n e r g y

a n a l y z e d

( M e V )

^ a c t T d e c ^ c n t

D e t e c t i o n l i m i t

( m g )

R e l a t i v e s t a n d a r d

d e v i a t i o n

A l u m i n i u m 27M g 9 .5 m 0 . 8 4 5 m 1 m 5 m 0 .3 + 6%

A n t i m o n y n o S b 1 5 .9 m 0 .5 1 5 m 1 m 5 m 0 . 4 + 6 %

A r s e n i c 75G e m 4 9 s 0 . 1 4 3 m 1 5 s 3 m 1 0 + 5 %

B a r i u m i3 7 B a m 2 .6 m 0 . 6 6 2 5 m 1 m 5 m 0 .2 ± 4 %

B o r o n n B e 1 3 .6 s 2 .1 2 5 0 s 1 5 's 5 0 ; s 0 .0 1 ± 1 0 %

B r o m i n e 78 B r 6 .5 m 0 .5 1 5 m 1 m 5 m 0 .1 + 6 %

C a d m i u mm c d m

4 9 m 0 .2 5 5 m 1 m 5 m 0 .8 + 6 %

C e r i u m 139C e m 5 5 s 0 . 7 4 3 m 1 m 3 m 0 .3 + 6 %

C e s i u m 132C s 6 . 5 8 d 0 . 6 7 5 m 1. m 5 m 2 0 , + 7 %

C h l o r i n e ^ C l " 1 3 2 m 0 .5 1 5 m 1 m 5 m 4 5 + 6 %

C h r o m i u m52 у

3 .7 7 m 1 .4 3 5 m 1 m 5 m 0 . 4 5 + 6 %

C o b a l t “ M n 2 . 5 8 h 0 . 8 4 5 5 m 1 m 5 m 1 0 + 7 %

C o p p e r 62C u 9 .9 m 0 .5 1 5 m 1 m 5 m 0 ,1 + 6 %

D y s p r o s i u m 16SD y m 7 5 . 4 s 0 . 1 0 8 4 m 8 0 s 4 m 0 .5 ±

00

F l u o r i n e 1 8 p 1 1 0 m 0 .5 1 5 m 1 m 5 m 2 '+ 5%

NEU

TRO

N

ABSO

RPTIO

N

PH

YSIC

S

TABLE VI. (Cont.)

E l e m e n t




a n a l y z e d

( M e V )

T a c t T d e c T e n t


( m g )


d e v i a t i o n

E r b i u m I 6 7 E r m2 .5 s 0 . 2 0 8 8 s 3 s 8 s. 8 ± 1 3 %

E u r o p i u m 150P m 2 .7 h 0 .3 3 5 m 1 m 5 m 4 0 ± 7 %

G a d o l i n i u m 161 G d 3 .7 m 0 .3 6 5 m 1 m 5 m 3 0 ± 6 %

G a l l i u m 68G a 68 m 0 .5 1 5 m 1 m 5 m 0 . 4 + 6 %

G e r m a n i u m 74G a 8 m 0 . 6 0 5 m 1 m 5 m 1 2 ± 6 %

G o l d 1 9 7 A u m 7 . 2 s 0 . 2 7 9 2 5 s 8 s 2 5 s 1 .2 ± 1 2 %

H a f n i u m 1 9 s 0 . 2 1 7 6 0 s 2 0 s 6 0 s 2 ± 6 %

I n d i u m П б щ Ш 5 4 m 1 .2 7 5 m 1 m 5 m 0 .8

00

+1

I r o n 56M n 2 . 5 8 h 0 .8 4 5 5 m 1 m 5 m 3 + 7 %

I r i d i u m 191 I r m 4 . 9 s 0 . 1 2 9 1 5 s 5 s 1 5 s 4 0 ± 1 2 %

L e a d203 in

6 .1 s 0 .8 3 2 0 s 6 s 2 0 s 4 5 ± 1 6 %

M a g n e s i u m M N a 1 5 h 1 .3 7 5 m 1 m 5 m 8 ± 7 %

M a n g a n e s e 52V 3 . 7 7 m 1 .4 3 5 m 1 m 5 m 1 ± 1 8 %

M e r c u r y 199H g m 4 4 m 0 .3 7 5 m 1 m 5 m 8 1+ 00

M o l y b d e n u m 91 M o ' 1 5 .5 m 0 .5 1 5 m 1 m 5 m 2 ± . 6 %

N e o d y m i u m 139C e m 5 5 s 0 . 7 6 3 m 1 m 3 m 0 .7 ± 7 %

141 N d m 6 4 s 0 . 7 4

104 N

AR

GO

LWA

LLA

TABLE VI. (Cont.)

E l e m e n t




a n a l y z e d

( M e V )

^ a c t T d e c T e n t


( m g )


d e v i a t i o n

N i o b i u m9 0 y m

3 .2 h 0 . 2 0 5 m 1 m 5 m 4 0 ± 7 %

N i c k e l 57N i 3 6 h 0 .5 1 5 m 1 m 5 m 1 .0 ± 6 %

N i t r o g e n 13N 9 . 9 6 m 0 .5 1 5 m 1 m 5 m 1 .0 ± 6 %

O x y g e n 16N 7 .2 s 6 . 1 3 2 5 s 8 s 2 5 s 0 .0 1 ± 1 0 %

P a l l a d i u m1 0 9 p d m

4 .8 m 0 . 1 8 5 m 1 m 5 m 0 .8 ± 5 %

107R u 4 . 2 m 0 . 1 9

P r a s e o d y m i u m 1 4 0 f r 3 . 4 m 0 .5 1 5 m 1 m 5 m 0 .1 ± 6 %

P h o s p h o r u s 28 A l 2 .3 m 1 .7 8 5 m 1 m 5 m 0 .1 ± 1 0 %

P o t a s s i u m 38K 7 .7 m 0 .5 1 5 m 1 m 5 m 7 .0 ± 6 %

R u b i d i u m 8 6 R b m 1 m 0 . 5 6 5 m 1 m 5 m 0 .5 ± 7 %

R u t h e n i u m « » Т е 1 4 m 0 .3 1 + 0 . 3 4 5 m 1 m 5 m 3 ± 8 %

104T c 1 8 m

95 R u 9 9 m

S a m a r i u m 141 N d m + 143S m m 6 4 s + 1 m 0 .7 5 5 m 1 m 5 m 5 ± 7 %

S e l e n i u m 79 S e m + 8 I S e m 3 .9 m + 5 7 m 0 . 1 0 5 m 1 m 5 m 0 .5 ± 7 %

S i l i c o n 28A 1 2 . 3 0 m 1 .7 8 5 m 1 m 5 m 0 .2 ± 6 %

оon

NEU

TRO

N

ABSO

RPTIO

N

PH

YSIC

S

TABLE VI (cont.)

E l e m e n t




a n a l y z e d

( M e V )

T act T dec T e n t


( m g )


d e v i a t i o n

S i l v e r 106 A g 2 4 m 0 .5 1 5 m 1 m 5 m 0 .2 ± 6%

S o d i u m 23 N e 3 8 s 0 . 4 4 2 m 4 0 s 2 m 1 ± 12%

S t r o n t i u m 87S r m 2 .8 h 0 .3 9 5 m 1 m 5 m 2 ± 6%

T i n 123S n 4 0 m 0 .1 5 3 5 m 1 m 5 m 1 .5 1 +

T i t a n i u m 49S e 5 7 .5 m 1 .7 6 5 m 1 m 5 m 1 5 ± 1 0 %

V a n a d i u m 51T i 5 .8 m 0 .3 2 5 m 1 m 5 m 0 .3 ± 6 %

Y t t r i u m

00 3

1 6 s 0 .9 1 5 0 s 2 0 s 5 0 s 0 .6 ± 6 %

Z i n c 63Z n 3 8 m 0 .5 1 5 m 1 m 5 m 2 . 0 - ± 6 %

Z i r c o n i u m

00 \£> N и 3

4 . 2 m 0 .5 9 5 m 1 m 5 m 0 .5 ± 6 %

106 N

AR

GO

LWA

LLA


Upon neutron absorption most o f the isotopes in a sample become radioa ctiv e , The gamma rays emitted during subsequent decays are therefore representa tiv e o f a l l decay species generated in the sample. The method o f measurement should therefore be able to detect and discrim inate between the radioisotopes.Two principal approaches can be followed, and are b r ie fly described below.

4 .2 .1 Radiochemical separation

The trad itio n al method o f detecting a p articu lar radioelement is radiochemical separation. In almost a l l o f these procedures, the element of in te re s t is iso lated by chemical means and i t s rad ioactiv ity measured. The chemical separation process is however time consuming except when only one or two elements are analyzed. The method is also destructive and the sample is consumed in the procedure. However, the exceptional discrim ination factors possible from separation procedures permit extremely high s e n s it iv it ie s to be obtained. Considerable e ffo rts have been expended in the development and pract ic e o f radiochemistry. Fast and automatic radiochemical separation procedures and apparatus are currently used in conjunction with activation analysis techniques, and are well documented in texts [44, 46] .

4 .2 .2 Instrumental gamma-ray spectrometryThe counting o f radionuclides i s invariably done by gamma-ray spectrometry

in a non-destructive manner. The gamma rays are measured by placing the i r r a diated sample over a sodium iodide (Nal(Tí.)) s c in t i l la t io n detector or a solid sta te detector o f lith ium -d rifted germanium or in tr in s ic germanium. The germanium system provides exce llen t resolution but with a considerably reduced e ffic ie n cy when compared to an equivalent size sodium iodide (Nal (Tí.)) s c in t i l la to r . The output from the detector system is fed into pulse processing and sorting instrumentation ca lled multichannel analyzers which have the capability of presenting graphical, o sc illo sco p ic and printed gamma-ray spectrom etric data.Modem gamma-ray spectrometry systems are equipped to perform with a considerable degree o f automation. The instrumentation can be computer coupled with a high degree o f f le x ib i l i ty in terms o f software usage. Such systems have the capacity o f raw data storage, location o f sp e c ific gamma-ray photopeaks, id e n tifica tio n o f these peaks by comparison with lib rary spectra , introduction o f relevant an aly tica l parameters such as neutron flux normalization fa c to rs , detector e ff ic ie n c ie s e t c . , and fin a lly to perform required computations and produce an aly tica l answers. Nuclear instrumentation requirements for the activation analyst are given in recommended [77-80] tex ts and monographs. Gaimna-ray spectra catalogues useful in neutron activation analysis with neutron generators [76, 8l] and reactor neutrons [82, 83] are mandatory additions to the p ra c titio n e r 's reference f i le s .

I t is opportune at th is juncture to emphasize some o f the advantages and demerits o f neutron activation analysis. The judgements to follow are primarily re lated to the use o f neutron generators fo r activation an aly sis; however, some o f the statements are applicable to the f ie ld as a whole. The choice of any an aly tica l method is governed by four principal fa cto rs : s e le c tiv ity ,accuracy, p recision , and s e n s it iv ity . The selectio n w ill depend on the stress given each o f these factors based on the ob jectives o f the analysis and the nature o f the sample m atrix. Furthermore, speed and economic considerations sometimes control the fin a l choice o f a method. Analytical chemists are contin u ally seeking to develop new methods which w ill complement ex istin g methods o f analysis and thereby provide a fu ll spectrum o f techniques and approaches for the characterization o f a l l types o f m aterials. The ro le o f experimental nuclear physics has played a s ig n ifica n t part in the development o f the neutron generator and made i t possible for most an aly tica l laboratories to se t up i t s own activation analysis f a c i l i t y . From the e a r lie r mentioned cross section discussion i t is obvious that the neutron generator o ffe rs some unique approaches to the analysis o f certa in elements. In p a rticu la r , the lig h t elements such as N, O, F, Al and Si can be rapidly determined by 14-MeV neutron activation

4 . 2 A n a l y t i c a l c o n s i d e r a t i o n s


100 % = 3 6 0 PUBLICATIONS

ELEMENTS DETERMINED = 41

,C u B? P/ U4 JU -I-----=Ks

10 20 30 4 0 50 6 0 70 80 90 ATOMIC NUMBER

FIG. IS. Frequency plot o f element determinations in the literature.

>-Hz<

•I-a :LUОz=>

FIG. 16. Improvement with time in calculated uncertainties o f reported analyses fo r oxygen and silicon by 14-MeV neutron activation.

techniques. A recognition o f th is fa ct has resulted in concentrated e ffo rts for the analysis o f elements in the low mass number region. A survey o f l i t e rature carried out in 1970 [S i] revealed a d istrib u tion o f studies as shown in Fig. 15. This survey indicated that fifty -od d elements had been successfully determined in the concentration range between 10 ppm to 50%. Almost 50% o f the an aly tica l e ffo r t was however devoted to the determination o f oxygen and s il ic o n . The potential for automation, esp ecia lly in indu strial process control, has made th is technique quite a t tr a c tiv e . The technique is also highly se le c tiv e , owing to several parameters that are under control o f the analyst. For instance, nuclear properties such as h a l f - l i f e , reaction cross sectio n , and decay scheme permit the analyst to d iffe re n tia te between elements in a matrix optimally through a judicious selectio n o f irrad ia tio n , decay and counting times. Through


a choice o f neutron targets and moderating media, neutron energies o f thermal,3 -, or 14-MeV can be obtained. Workers in terested in multiple neutron energy u tiliz a tio n are referred to the development o f Cosgrove [84j who has described a neutron producing system involving a " tu rre t" type target holder which permits, a rapid interchange o f neutron producing ta rg e ts .

Perhaps the most s ig n ifica n t advances made with th is technique have been in improved accuracy and p recision . In sp ite o f the anisotropic and time-varying nature o f the neutron output from neutron generators, extensive research and development has found great success in combating th is inherent source of analy t ic a l im precision. A graphic representation o f th is improvement is illu s tra te d in Fig. 16. The selectio n o f the elements was based on th e ir frequency of determination, and th e ir respective q uantities are based on the average quant i t i e s determined in the studies examined. To a large measure, the improvements indicated are a d irect resu lt o f the recognition o f possible sources o f error followed by in-depth studies o f system atic error correctio n s.

The existence and evaluation o f system atic error have been thoroughly discussed in a comprehensive tre a tis e [l4] , and w ill not be elaborated on herein. However i t is in stru ctiv e to l i s t some o f the principal sources of systematic e rro r: 1) nuclear constants, 2) in terferin g nuclear reactions,3) nuclear reaction re co il e f fe c t , 4) sample blank, 5) neutron flux normalization, 6) sample attenuation, and 7) instrum ental. The reader is s p e c if ica lly referred to published works |_13, 31, 85-90] dealing s p e c if ic a lly with error analysis.

There are, o f course, certa in lim itations to the technique when i t is compared to others in the an aly tica l "arsen a l". The principal drawback appears to be the lack o f s u ffic ie n t flux to enable analyses in the low ppm range.From an operational standpoint, the lim ited target l i f e for neutron production is perhaps the greatest disadvantage. However, considerable research continues in th is area o f targ et development. Hopefully, both o f the above mentioned disadvantages w ill b e .e ith e r eliminated or reduced su b stan tia lly in the near future. Cost o f a neutron generator fa c i l i t y can be comparable to equivalent capability non-nuclear instrum entation. But th is cost is by no means inconsequential. The to ta l cost o f a system including measurement equipment and b io logical shielding can be as high as $50,000-$75,000. However, generator f a c i l i t i e s for so called "one element" analyses, can be assembled for h a lf the co st.

In summary i t can be mentioned that many elements can be determined at the milligram level rapidly and nondestructively. Neutron generators are comm ercially available and have placed th is technique within reach o f every w ell- equipped an aly tica l laboratory. Although lim ited in se n sitiv ity and neutron- targ et l i f e , continuing research w ill undoubtedly re su lt in s ig n ifica n t improvements in the future.

5. APPLICATION OF NEUTRON ACTIVATION ANALYSIS

In turning to the scope and applications o f neutron activation analysis i t is necessary to assert that a l l applications in a sense are related to the investigation o f the nature o f the environment and man's in teractio n with i t . Although th is statement can be applied to many other branches o f knowledge, the principal areas o f application have been c la ss if ie d under nomenclatures readily understood in the language o f modem day technology. The application o f reactor and acce lerato r produced neutrons in these areas w ill be discussed with a view to establish ing th is technique among others performing equivalent task s. In doing so, the ro le o f neutron absorption.physics in the development and p ractice o f activation analysis is inherently emphasized.

The selectio n o f p articu lar studies pertaining to each area o f application from the thousands described in l ite ra tu re is obviously beyond the scope o f th is review. The reader is urged to consult extensive annotated b ib lio g raphies [14 ,4 4 ,5 2 ,5 4 ,6 0 -6 4 ] and conference reports [55-59] , previously referenced, for such reported applications. In th is review the stre ss is applied to the

1 1 0 N A R G O L W A L L A

various an aly tica l a c t iv it ie s which can be c la ss if ie d under an area o f in te re s t .As such an attempt is made to itemize the most s ig n ifican t areas o f application in the following subsections.

5 .1_____ B iological and biomedicalAnalytical problems related to the b io logical environment are also ex

perienced in the cosmic and geological surroundings. This is to be expectedsince both the biosphere and the geosphere represent e sse n tia lly the same raw m aterials i . e . liv in g matter i s nothing e lse but a rearrangement o f atoms cons titu tin g the geosphere. Consequently, the wide variations in elemental compositio n s is evidenced in both o f the above environments. However, in the b io lo gical world, se lectiv e concentration o f elements such as carbon, calcium, oxygen and sulphur at the expense o f other elements such as helium and s il ic o n , occurs. The importance o f activation analysis for b io logical studies does not l ie completely in the determination o f macro constitu ents, but in the examination o f elements in the ppm and ppb range for which l i t t l e i f any data e x is ts .In any given situ a tio n , the activation analyst must decide upon a sp e c ific technique which w ill allow the best possible discrim ination for the elements of in te re s t . Through a choice of bombarding p a rtic le s and the u tiliz a tio n o f such nuclear physics parameters as thresholds and resonances, i t is en tire ly possible to apply th is technique to almost a l l elements. The application of activation analysis can be discussed in terms o f various aspects o f the b io lo g ica l and biomedical f ie ld s . For the study o f the in ta c t human body, the in vivo method o f analysis o ffers an in terestin g aspect o f future research. The analyst, o f course, w ill be sen sitive to the fa c t that a typ ical human specimen cannot be exposed to high radiation dosages without severe damage. Therefore in th is sp e c ific area, the analyst must exercise great care and be highly s k ille d . One o f the main areas, i . e . the estimation o f to ta l body sodium can be studied by the irrad iation o f the sub ject with 14-MèV neutrons. Although fa s t neutrons are highly penetrating,the absorbed radiation dose would be excessive compared to an equivalent flux o f thermal neutrons. Unfortunately thermal neutrons are e ffe c tiv e ly absorbed in the human tissu es and homogeneous irra d ia tions are therefore d if f ic u lt to achieve. After irrad ia tion with 14-MeV neutrons, the subject is transferred to a whole body counter and the most conspicuous ra d io a c tiv itie s Na and 38C1 are measured. Cyclotron-produced high energy neutrons can also be used to achieve the same purpose. Suggestions by Palmer e t a l . [91] indicate that the elements sodium, calcium and chlorine can be determined by thermal neutron activ a tio n ; nitrogen and phosphorus by fa s t neutrons using the reactions^^N(n,2n) 1% and ^^P(n,a)2®Al ; hydrogen by а measurement o f the 2.23 MeV capture gamma rays from the lH(n,y) H in teractio n ; Except for the element carbon which requires the use o f photon activation , the technique o f neutron absorption can be e ffe c tiv e ly used for the in vivo analyses o f b io lo g ica lly important elements. Perhaps the greatest d iff ic u lty in the application o f neutron activation is in the a v a ila b ility o f a suitable standard. A reasonable accuracy is gained by u tiliz in g a p la s tic phantom f i l le d with an appropriate solution o f sodium; however for elements such as calcium which is heterogeneously distributed in the human body, the above technique o f standardization does not work very w ell.

One o f the best examples o f in vivo analysis o f lim ited areas o f the human body is the determination o f iodine in the thyroid. The general method includes the in jectio n o f the long-lived isotope o f iodine (^2®I,Tij = 1.6x10^ years). Before in je ctio n th is isotope is made s lig h tly radioactive by the

130j in teractio n . The isotope ISOj has a convenient h a l f - l i f e o f about 12 hours. The gamma emission from meaSured by external counting to estimate the amount in the thyroid gland. Once deposited,the 129I is used as a flux monitor for the subsequent in vivo neutron irrad ia tio n . The indigenous thyroidal 127l is converted to the 25 minute decay isotope 2̂®I. Since the element iodine is concentrated in the thyroid gland while in terferin g elements such as sodium and chlorine are rapidly removed by c ircu la tio n , the determination o f iodine concentration is considerably fa c i li ta te d .

N E U T R O N A B S O R P T IO N P H Y S IC S 1 1 1

Although in vivo neutron activation analysis has made some strid es in the measurement o f the macro elements in the human body, i t s application for trace element determination in the human system is faced with severe d if f ic u lt ie s .For the nutrition- o f man only a lim ited number o f trace elemts are considered to be e ss e n tia l. These include zinc, copper, iodine manganese, molybdenum, cobalt and selenium. Although almost seventy additional elements are found in the human system, th e ir e ffe c t on the biochemical processes is not yet understood with any degree o f confidence. Progress in th is regard is however being made and the function o f such elements as chromium, vanadium and zirconium is being studied.

The b io log ica l sig n ificance o f trace elements in various tissu es has provided the activation analyst with a means for careful sampling and subsequent trace analysis. With the threat o f radiation exposure removed, novel lines o f investigation have been carried out. For example, i t was found that the accumulation o f such elements as tin and cadmium bear some correlation with the advancing age o f the sample donor. However the ingestion o f these elements from the environment as a function o f time has not been discounted as a possible source o f accumulation. The increased concentration o f copper and chromium in newbome is another example on which hypotheses can be construed.

In general activation analysis has served in the discovery o f traceelements which require highly sen sitive analytica l to o ls . The significance o f d iscrete trace elements in terms o f the biochemical processes is d if f ic u lt to assess, mainly because o f the rather wide concentration lim its present in the human system. S ta t is t ic a l approaches have been followed to e stab lish the demarcation lim its below which elemental d eficien cies can be contemplated. The b io logical and biomedical fie ld s o ffe r challenges to the analyst unsurpassed byany other f ie ld o f in vestigation . The relevance o f an aly tica l observations, bethey from neutron activation or from other an aly tica l methods, w ill not be obvious u n til the b io log ical and physiological sign ificance o f trace element d istributions are fu lly understood.

5.2_____ Material science and ind u strial applicationsIn the area o f m aterials science neutron activation has enjoyed i t s

greatest success. In a l l ind u strial manufacturing operations m aterials quality is a basic sp e c ifica tio n , and most o f the world's an aly tica l chemists are engaged in industry with the almost sole ob jective o f m aterials characterization . Because o f the keen competition offered by other non-nuclear techniques, neutron activation analysis has had to bear i t s sharpest critic ism s and in-depth assessments. Nevertheless the nuclear technique has been exploited to advance the s ta te o f the a r t o f the semiconductor industry in the examination o f u ltra pure m aterials. To th is end, the activation technique has been pushed to the very lim it o f i t s s e n s itiv ity . In industry, activation analysis has been applied in the analytica l laboratory, and for process control on an on-line b a s is . In many cases th is approach has rendered great economic b enefits and has been considered to be most expedient. The non-destructive aspect o f neutron activation has often been the determining facto r for i t s se le c tio n . A prime example illu s tra t in g a l l o f the above advantages is evidenced in the analysis o f oxygen in metals, p articu larly in s te e l . The popularity enjoyed by neutron generators was due, to a large measure, in i t s a b il ity to.determine ppm amounts o f oxygen in metals on a rapid and non-destructive b a s is . Each analysis could be performed at a cost o f a few cents and with a high through-put. The 14-MeV neutron in teraction

0(n,p)^^N resu lts in the emission o f very high energy (6 .1 , 7.1 MeV)gamma rays, so that the measurement process is e sse n tia lly in te rfe re n ce -fre e . Much o f the research done to improve th is technique was in itia te d by th is great need in the metals industry. Today, the 14-MeV neutron activation technique for oxygen in metals is firmly established and o ffe rs advantages in co st, rapidity and r e l ia b i l i ty .

The estim ation o f nitrogen in foodstuffs for the purpose o f determining the protein content is another example o f 14-MeV neutron activation analysis. Despite some d if f ic u lt ie s from matrix in terferen ces, the 14N(n,2n)l3N is u tilize d for protein estimation o f grain on a routine b asis .

\


Standard reference m aterials, so c r i t i c a l for instrumentation and c a l i bration control, have also been subjected to neutron activation analysis. In general such m aterials must be analyzed by two or more unrelated methods o f analysis p rior to c e r t i f ic a t io n . Neutron activation analysis has made sig n ifican t contribution to analytica l data issued by standards disbursing agencies such as the National Bureau o f Standards o f the United S ta te s . Since the ultimate goal o f an an aly tica l measurement is to obtain a re su lt o f known accuracy, the importance o f standard m aterials in the m aterials science cannot be overly emphasized. In the area o f standard m aterials for industry, the neutron generator has made sig n ifican t contribution. The example o f oxygen in various types of iron and s te e ls has already been c ited . The determination o f the metal components such as co b alt, s ilv e r , magnesium and s ilico n in organometallic compounds used to determine engine wear is another example where 14-MeV neutron activation analysis o ffers s ig n ifica n t advantages.

On-line activation analyses such as: nitrogen in grain products, oxygen in co a l, flu o rite in fluorspar, sodium and phosphorous in detergents, s ilico n in iron ore or in coal, aluminum in coal, copper in copper ore and barium in barytes, are only a few examples in which the small accelerator generating high energy neutrons has been successfu lly exploited. In most o f the above instances,the ingenuity o f the activation analyst has resulted in accurate analysis bythe elim ination o f interferences and sources o f system atic error.

5.3_____ Geo-and cosmo-chemical analysesThe geochemist is prim arily concerned with the formation, d istrib u tion ,

and chemical and physical in teractions o f minerals o f rocks and ores. In order to te s t h is theories and hypotheses for the above geochemical properties i t is e sse n tia l that compositional knowledge o f the geological sample be known with good accuracy. Therefore an aly tica l information fo r te r r e s tr ia l rocks, minerals, ores, s o i ls , water and the atmosphere for the geochemist; and the composition o f m eteorites, te k tite s and lunar material for the cosmochemist is o f fundamenta l importance. In many instances the compositional knowledge is used to determine the age o f the m aterial.

In general the elemental abundances in the geo- and cosmo-sphere are grouped into three c la s s if ic a t io n s : major (>1%), minor (0.01 - 1%) and trace (<0.01%) levels o f concentration. In the major c la s s , elements such as 0, Mg,Al, S i , Ca and Fe are found. Elements H, C, P, S , Cl, V, Cr, Mn, S r, Zr and Ba form the minor constituent group, while the noble gases, L i, Be, B, N, Sc and Cu, and a l l elements beyond Cu constitu te the trace group o f elements. E lements such as Na, K, T i, F, Co, Ni can belong to one or more o f the groupsdefined above.

A review o f elemental abundances published by Suess and Urey in 1956[92^ indicated serious errors in the e a r lie r reported abundance data for meteor i t e s . Since a large amount o f data were highly suspect an immediate need arose for an an aly tica l method capable o f analysis o f small samples with high sen sit iv i ty . Neutron activation was therefore immediately introduced to accurately estab lish a new generation o f abundance data. In fa c t activation analysis was one o f the two principal methods used in the analysis o f lunar m aterials re trieved by the Apollo 11 and 12 missions. The popularity o f th is method isshown by the existence o f over 600 published works in the f ie ld o f geo- and cosmo-chemistry.

The 14-MeV neutron activation method has been applied to the determination o f Si and 0 on a non-destructive b a sis . As is well known, the c la ss ic a lmethods for these elements are rather tedious. E ffo rts in these analyses were directed towards the achievement o f high degrees o f precision and accuracies. Using 2 - 3 gram samples o f rock, average fractio n al deviation o f ±0.54% for 0 and 0.44% for Si have been reported for U.S.G.S. standard rocks [9 3 j .In these standard rocks, the oxygen content varies from about 41 - 49% and the s ilic o n composition between 18 - 31%. The 14-MeV neutron activation method has also been applied to other elements such as T i, Nb, Ce, Pr, Cu and У in ores and minerals. The application o f activation analysis to geochemistry and cosmochemical studies is described in several proceedings Q>5-59, 94J .


Instrumental thermal neutron activation analysis has been widely used for multielement determinations. Combined with high resolution Ge(Li) detector systems, th is method o f analysis provides the geochemist with a very powerful tool to probe the composition o f the geosphere. When combined with adequate radiochemical separation techniques, the general activation method has been applied to the analysis o f 45-50 elements in standard rock samples. A few o f the trace elements can be determined in the ppb range o f concentrations.

Some o f the areas in which neutron absorption processes have played a sig n ifica n t part include: m eteoritic studies with 14-MeV neutron activ atio n ; geological age determinations by a measurement o f К/Ar and I/Xe ra t io s ; determination o f the nickel content o f te k t i te s ; and abundances o f over 30 elements in lunar samples. Perhaps neutron activation analysis faces one o f i t s greatest challenges in the areas o f geo- and cosmo-chemistry.

5 .4_____ Mineral and energy resources

The application o f neutron activation analysis in the mineral and' resource fie ld s has been well documented in the bibliographies referenced previously. A selectio n o f such applications is also given in in ternational symposia £96,97] devoted to nuclear techniques and mineral resources.However, the m ajority o f th is research has been carried out in highly contro lle d laboratory environments. Despite the fa ct that many o f 'th e se studies purport d irect applications to the mineral and resource industry, very few have been tested under typ ical f ie ld conditions. Furthermore, only a handful are actu ally being u tiliz e d for mineral exploration and mine development.I t is in stru ctiv e to l i s t the principal causes for the d if f ic u lt ie s experienced by activation analysts in th e ir attempts to " s e l l " th e ir individual developments to the mineral resource industry:

1). The displacement o f ex istin g chemical techniques by neutron activationor by other nuclear techniques has been met with great reluctance by the industry. . -

2) A perusal o f activation l ite ra tu re reveals numerous instances where the activation analyst has fa ile d to recognize the sp e c ific needs o f a p a r t icular exploration or prospecting technique. In a l l fa irness the analyst has had to face variable requirements from d ifferen t developers o f the same mineral.

3) Neutron activation analysis time as visualized in the laboratory often takes much longer when applied to f ie ld conditions. In mining c ir c le s ,

■ the time facto r is the major determinant as to whether or not a given method is u t iliz e d .

4) The activation analyst is often guilty o f overlooking the fact that labora- • tory nuclear instrumentation may not function in the h o s tile environment

synonymous o f f ie ld conditions.5) The high degree o f sophistication b u ilt into neutron activation methods

often acts as a deterrent for i t s f ie ld acceptance.I t is generally found that those nuclear methods being practised on a routine basis were developed with a fu ll recognition o f the.above fa cto rs . Consequently, th e ir application in the mineral resource industry has led to substantial economic b en e fits .

Although the petroleum industry has been using nuclear techniques for well logging o f o i l bearing formations since the 1930's , the mineral industry has been rath er tardy in th e ir acceptance o f such techniques. In the l i t e r a tu re, i t is d if f ic u lt to search for such applications for sp e c ific elements since the methods themselves are generally c la ss if ie d according to the nuclear princip les used. However, some exce llen t reviews on the general application o f nuclear excita tio n techniques are available £э7-Юо] .

Among nuclear techniques, the ones involving neutron absorption processes are considered to be the most promising. Studies by E is le r e t a l . [ lO lJ , Hoyer and Locke [ю Л , Landström e t a l . £ ю з] , Moxham e t al / [l04 ] , and Nargolwalla e t a l . £ l® í * are examples o f the researchers recognizing the needs o f industry. Neutron absorption methods used in the mineral resource industry


have depended heavily on the (n,y) reaction induced by rad ioisotopic, spontaneous fiss io n and accelerator-produced neutron sources. The' detection systems used include the sodium iodide s c in t i l la t o r and the Ge(Li) solid s ta te assembly.To date there are several case h is to r ie s developed from the use o f the sodium iodide detector. In sp ite o f the ingenious e ffo rts Q 06, 107j to adapt the Ge(Li) system for downhole applications, an adequate production logging case h istory is yet to be reported.

Fast neutron-induced reactions have been extensively used for mineral exploration. With the rapid advancement in e lectro n ic technology, i t is poss ib le to construct small sealed-tube type accelerators which can be introduced into boreholes o f only 3 inches in diameter. In one o f the early applications o f the neutron generator fo r surface an alysis, i t was found that 3-MeV neutrons were more p ra ctica l than 14-MeV neutrons. Elements amenable to th is type of application include; Na, F, 0 , S i , B, P. Ag, Br, Mg, Cl, Cu, Rh, S and Fe.Both Dibbs [l08] and E is le r et a l . LlOlJhave studied the ap p licab ility o f the neutron generator for the determination o f copper. In general, the elements capable o f being analyzed by fa s t neutron activation without s ig n ifican t in terferen ce from other elements are oxygen, s ilico n and aluminum.

The a v a ila b ility o f has considerably accelerated the growthof applications u t iliz in g thermal neutron activation techniques. The in tro duction o f thermal neutron activation for both surface and downhole applications in the resource industries w ill go a long way towards a general acceptance of nuclear techniques by the mining community.

5 .5_____ ArchaeologyThe non-destructive a ttr ib u te o f neutron activation analysis is perhaps

best demonstrated in i t s application in the fie ld s of art and archaeology. I t i s , o f course, o f paramount importance that any technique u tilized must leave the ob ject in an unaltered s ta te . In many cases, the art ob ject can be analyzedwithout sampling. I t is sometimes possible to obtain minute samples from thesurface o f the a r t o b ject for c e r t if ic a t io n purposes. Neutron activation can then be applied to the specimen, and is one o f the few methods with the s e n s i t i v ity o f determining a large number o f trace elements. The trace element abundances are used to id en tify m aterially or chronologically the composition or age o f the a r t i fa c t , resp ectiv ely . The origin o f an ob ject sometimes can be assessed by the id e n tifica tio n o f elements present in the ppb range. The composition o f a rt o b jects can not only be determined by activation techniques, but th e ir structure can also be examined. This examination is done by neutron activation autoradiography to reveal subsurface structure o f o i l paintings.

Fast neutrons have been used to id en tify gold and s ilv e r in ancientcoins, s ilv e r in lead roof t i l e s o f h is to r ic e d ifices and the l ik e . The a b il ityto determine low atomic weight elements has made the neutron generator a popular choice for investigating th is so cia l area. Studies o f ancient pottery dominated the number o f applications devoted to a rt and archaeology. Pottery symbolic of Grecian, Roman, Mesoamerican, B r itis h , Eastern Mediterranean and Western Asia cultures have been subjected to neutron activation analysis. Coins and metal o b jects have also been analyzed. S ta in -g lass windows and pigment from paintings have also been authenticated by using comparative methods.

Entire o i l paintings have been irrad iated by neutron fluxes o f the order o f 10 n.an~^s"l. Following irra d ia tio n , a series o f autoradiographs are obtained by placing x-ray films in contact with the painting. Experts examining such autoradiographs have been able to determine the d istribu tion o f individual pigments .

The f ie ld o f archaeology o ffe rs the analyst an excitin g area o f research, and thereby demonstrates the important ro le o f neutron absorption physics in the so c ia l sciences.

5 .6_____ Forensic sciencesIn forensic sciences the application o f neutron activation analysis as

an id e n tifica tio n tool has been lim ited . Only in a few instances can i t be


said that the nuclear technique yields comparable information to those from "fin g erp rin t" methods. In a l l o thers, i t serves as a means to confirm or deny the p o ss ib ility o f common orig in o f two samples and generally as a ch aracterization tool for m aterials evaluation.

This is one f ie ld o f application where the activation analyst must e x e rc ise , great caution in h is expression o f data and conclusions because o f the obvious social, and legal ram ifications. To.deduce comparisons or d iss im ila ritie s from a research study o f a lim ited number o f samples pertinent to a given type o f evidence material is a dangerous path to follow; and has often led to d iscred it o f the b asic technique. At times the proponents o f th is technique are a l l too quick to make hurried judgements based on th e ir an aly tica l re s u lts .

Trace analysis has become an in tegral part o f a modem day forensic laboratory. Neutron activ ation analysis must compete with others such as atomic absorption and x-ray fluorescence. The a ttractio n for neutron activation analysis is due to i t s survey cap ab ility . Routine and simultaneous determinations o f many elements on a non-destructive basis have a ttracted major e ffo rts in forensic laboratories around the world. As a re s u lt , forensic activation analysis is now routinely used for the characterization o f evidence m aterials in certa in types o f criminal cases. The forensic s c ie n tis t must exercise good judgement in the adoption o f the given technique or techniques depending upon the p articu lar circumstance o f the in vestigation . Speed o f analysis is invariably a key fa c to r . An excellen t discussion on the merits and demerits o f forensic activation analysis is given by Krishnan £lOi)J . From the forensic standpoint, a b r ie f discussion o f some types o f evidence m aterials to which neutron activation analysis has been applied follow s.

A common evidence m aterial, namely h a ir , has received the most a tten tion . Thousands o f trace element d istributions in human h air have been obtained through neutron activ ation . From these re s u lts , concentration patterns and frequency d istributions for up to th ir ty odd elements are av ailab le . Forensic activ a tion has been p articu larly suited for these analyses, esp ecia lly when the ev idence material is very small and elemental s e n s it iv it ie s down to the ppm and ppb range are desired. However, uncontrolled parameters such as human d ie t, environment, growth phase and others which have a marked influence on elemental d istrib u tion , have precluded the u tiliz a t io n o f such evidence for positive id e n tifica tio n . In the absence o f other types o f evidence however, neutron activation analysis o f h a ir does o ffe r a means for possible id e n tifica tio n .

The examination o f metal fragments taken from the scene o f some criminal actions can also be done by forensic activation analysis. The purpose of such examination may be to id en tify whether or not the fragment i t s e l f is that from a shattered b u lle t . In such circumstances i t is only necessary ta analyze for elements present in b u lle t a llo y s , and attempt to e stab lish the common origin through trace element an aly sis. Therefore the concentration patterns o f e le ments such as antimony, arsen ic, copper and s ilv e r are studied.

Other examples o f forensic activation analysis include the examination o f glass chips, paint fragments, so il samples, certa in types o f fib res and to x ic m aterials. The analysis o f gunshot residue is one examination in which neutron activation has made a s ig n ifica n t contribution. Systematic research has yielded valuable information on the e ffe c t o f parameters such as washing o f hands and elapsed time a fte r the incid ent.The f ie ld o f forensic science o ffe rs the neutron activation analyst a wide f ie ld o f p ra c tice . Evidence samples include b io lo g ica l, biochemical, inorganic and organic m aterials. In th is area o f application the activation analysis technique, serves as a good complement to other non-nuclear methods o f analysis.

S .7_____ Environmental and ecologicalDuring the la s t decade or so communities throughout the world have be

come increasingly conscious o f the ever increasing d eterioration o f the environment. Much o f th is d eterioration has been attributed to industry. Consequently, governments have taken steps to formulate laws and regulations that are s t r ic t ly administered and offenders are dealt with e ffe c tiv e ly . The application o f


a n a l y t i c a l m e tho d s f o r p o l l u t i o n c o n t r o l a n d e n v ir o n m e n t a l r e s e a r c h fo rm s th e b a s i s o f many s u c h ju d g e m e n t s . The q u a l i t y o f o u r w a t e r , b io e n v ir o n m e n t an d a tm o sp h e re m u s t , a t a l l c o s t , b e p r o t e c t e d . I n r e c o g n i t i o n o f t h i s f a c t , th e I n t e r n a t i o n a l A to m ic E n e r g y A g e n c y c o n v e n e d t h e i r f i r s t c o n f e r e n c e [ 1 1 0 ] t o d e a l w i t h n u c l e a r t e c h n iq u e s i n e n v ir o n m e n t a l p o l l u t i o n . S h o r t l y a f t e r w a r d s th e U . S . E n e r g y R e s e a r c h an d D e v e lo p m e n t A d m in i s t r a t i o n s p o n s o r e d a m a jo r s y m p o s iu m [ l l l j w h ic h b r o u g h t t o g e t h e r a c o l l e c t i o n o f e n v ir o n m e n t a l an d e c o l o g i c a l s t u d i e s i n w h ic h th e a p p l i c a t i o n o f n e u t r o n a c t i v a t i o n t e c h n iq u e s w as

s i g n i f i c a n t l y e v id e n t .The e le m e n t m e rc u ry am ong a l l o t h e r s h a s b e e n t h e s u b j e c t o f much

d e b a te an d a n a l y s i s . P e r h a p s t h i s h a s b ee n i n p a r t due t o i t s p r o p e r t y o f b e i n g m e t h y la t e d i n n a t u r e , t h e r e b y b e i n g t r a n s f o r m e d fro m a l e s s t o x i c fo rm t o one o f h i g h e r lo n g - t e r m t o x i c i t y w i t h g e n e t i c e f f e c t s . I n a d d i t i o n t o m e rc u ry s e v e r a l o t h e r e le m e n t s s u c h a s a r s e n i c , cadm ium , s e le n iu m , a n t im o n y , ch rom ium an d in d iu m h a v e b e e n r i g o r o u s l y s t u d ie d i n a h o s t o f e n v ir o n m e n t a l m a t r i c e s .T he p re p o n d e r a n c e o f s u c h s t u d i e s h a s p r e c lu d e d th e s e l e c t i o n o f a n y on e o r m ore f o r i n c l u s i o n i n t h i s r e v ie w . S u f f i c e i t t o s t a t e t h a t t h i s a r e a o f i n v e s t i g a t i o n p o s e s c o n s i d e r a b le d i f f i c u l t i e s f o r t h e a c t i v a t i o n a n a l y s t ; a n d t h e r e f o r e o f f e r s th e g r e a t e s t c h a l l e n g e s . I f i t i s a t a l l p o s s i b l e t o p r e d i c t th e f u t u r e a p p l i c a t i o n o f n e u t r o n a b s o r p t i o n t e c h n iq u e s f o r th e s o l u t i o n o f p r o b le m s i n t h e e n v ir o n m e n t a l s c i e n c e s , i t c a n b e s a i d t h a t t h e a c t i v a t i o n a n a l y s t w i l l p l a y a v e r y im p o r t a n t r o l e .

6 . CO NCLU SIO N - P R ESEN T AND FUTURE

T he c u r r e n t s t a t u s o f n e u t r o n a b s o r p t io n t e c h n iq u e s c a n be su m m a r iz e d a s f o l l o w s . I n g e n e r a l n e u t r o n a c t i v a t i o n o f f e r s a r a p id m ethod f o r c o m p o s it i o n a l a n a l y s i s . The t e c h n iq u e ca n b e e a s i l y a d a p te d f o r a u t o m a t ic o p e r a t io n an d d a t a p r o c e s s i n g , a n d ca n b e d e v e lo p e d i n t o a r e a s o n a b le s u r v e y m ethod f o r m u l t i - e le m e n t a n a l y s e s . I t i s g e n e r a l l y r e c o g n iz e d t h a t no m a t t e r how s e n s i t i v e t h e m ethod m ig h t a p p e a r t o b e on t h e b a s i s o f t h e o r e t i c a l y i e l d c a l c u l a t i o n s , t h e p r a c t i c a l s e n s i t i v i t y l i m i t , i n g e n e r a l , i s som ew here b e tw e e n 0 . 0 5 t o 0 . 5

ppm. I n t h i s r e s p e c t t h e m ethod i s q u i t e c o m p a ra b le t o som e o f t h e n o n - n u c l e a r m e th o d s , s u c h a s e m i s s i o n an d x - r a y s p e c t r o m e t r y . I n o r d e r t o o b t a i n a s i g n i f i c a n t im p ro ve m e n t i n s e n s i t i v i t y , i t i s n e c e s s a r y t o r e s o r t t o som e fo rm o f d i s c r i m i n a t i o n b y p e r f o r m in g r a d io c h e m ic a l s e p a r a t i o n s . I n m any c a s e s s u c h a p r o c e d u r e can r e s u l t i n an im p ro ve m e n t b y an o r d e r o f m a g n itu d e o r m ore .

I n c o n s i d e r i n g n e u t r o n a c t i v a t i o n a n a l y s i s am ong t h e h o s t o f o t h e r m e a su rem en t t e c h n iq u e s , ' i t i s p o s s i b l e t o s t a t e t h a t when co m b in e d w i t h

r e l i a b l e an d r e p r o d u c ib le r a d io c h e m ic a l m e th o d s , i t can y i e l d a c c u r a t e r e s u l t s w i t h h i g h s e n s i t i v i t i e s . W it h p r o p e r sa m p le h a n d l i n g i n a "C le a n ro o m " e n v ir o n m e n t t h i s m ethod o f a n a l y s i s can c o n c e iv a b l y b e s a i d t o h a v e n o sa m p le " b l a n k " p ro b le m s , a n d a n a l y s e s down t o th e 1 t o 10 ppb l e v e l a p p e a r f e a s i b l e .

I n s o f a r a s th e f u t u r e i s c o n c e rn e d , i t can be e m p h a s iz e d t h a t t h i s t e c h n iq u e m ust now a t t a c k t h e w o r ld o f p r a c t i c a l s a m p le s . I n th e p a s t , th e d e v e lo p m e n t o f m any a t e c h n iq u e w as b a se d on th e l a b o r a t o r y s a m p le . The c o n - c l u s i o n s o f s u c h s t u d i e s i n v a r i a b l y i n c l u d e d e x t r a p o l a t i o n s i l l u s t r a t i n g u l t r a lo w s e n s i t i v i t i e s t h a t w o u ld h a v e b e e n i n t h e g r a s p o f th e a n a l y s t h a d th e n e u t r o n f l u x a v a i l a b l e b e e n a t h o u s a n d t im e s h i g h e r . I t a p p e a r s t h a t , som ew h e re i n s u c h e x t r a p o la t io n ; ; , t h e i n v e s t i g a t o r l o s e s c o n t a c t w i t h t h e r e a l w o r ld ,

i . e . th e p r a c t i c a l s a m p le . T he c o n t in u o u s b a t t l e w i t h th e p r a c t i c a l sa m p le and o b t a i n i n g o f r e s u l t s w i t h m e a n in g f u l e r r o r e s t im a t e s i s p e r h a p s th e g r e a t e s t c h a l l e n g e t h i s t e c h n iq u e f a c e s i n th e f u t u r e . S u c c e s s i n s u c h an a r e a w i l l n o d o u b t le a d t o a r e d e t e r m in a t io n o f fu n d a m e n ta l n u c l e a r c o n s t a n t s s u c h a s th e c r o s s s e c t i o n w i t h an ad d ed d e g re e o f a c c u r a c y . T he g o a l f o r h i g h e r s e n s i t i v i t i e s w i t h m inim um e r r o r i s n o t o n l y n e c e s s a r y b u t m a n d a to ry t o a c h ie v e , i f a b e t t e r u n d e r s t a n d in g o f t h e e n v ir o n m e n t i s t o be g a in e d . Too o f t e n h a s t h i s m ethod b e e n u se d a s a l a s t r e s o r t . Y e t i t o f f e r s some u n iq u e a d v a n t a g e s t o th e


a n a l y s t . The f u l l e x p l o i t a t i o n o f i t s a t t r i b u t e s i n th e f u t u r e w i l l u n d o u b t e d ly le a d t o a b e t t e r u n d e r s t a n d in g o f th e many p ro b le m s in h e r e n t i n m a t e r i a l s c h a r a c t e r i z a t i o n .

R E F E R E N C E S

£ l ] H EV E SY , G . , L E V I , H . , The a c t i o n o f n e u t r o n s on th e r a r e e a r t he le m e n t s , D e t . K g l . D a n sk e V id e n s k a b e r n e s S e l s k a b . M a t h e m a t is k - F y s i s k e M e d d e le l s e r , 1 4 ( 5 ) , ( 1 9 3 6 ) , 3 - 3 4 .

[ 2 j SEABO RG , G . T . , L IV IN G O O D , J . J . , A r t i f i c i a l r a d i o a c t i v i t y a s a t e s tf o r m in u te t r a c e s o f e le m e n t s , J .A m .C h e m .S o c . , 6 0 , ( 1 9 3 8 ) , 1 7 8 4 - 1 7 8 6 .

[ 3J C IN D A 6 8 , An in d e x t o th e l i t e r a t u r e on n e u t r o n m ic r o s c o p ic d a ta ,USAEC D i v i s i o n o f T e c h n ic a l E x t e n s io n , U SSR N u c le a r D a ta In f o r m a t io n C e n t r e , EN EA N e u t r o n D a ta C o m p i la t io n C e n t r e , IA E A N u c le a r D a ta U n i t , ( 1 9 6 7 ) , ( 1 9 6 8 ) , ( 1 9 6 9 ) (a n d S u p p le m e n t s ) .

([4] C IN D U , IA E A N u c le a r d a ta u n i t , V ie n n a .

£ 5] BROOKHAVEN N AT IO N A L LABORATORY, "N e u t r o n C r o s s S e c t i o n s " , Rep.B N L -3 2 5 , 2 n d e d . , S u p p l . 1 (1 9 6 0 ) an d S u p p l . 2 , f i v e v o lu m e s ( 1 9 6 4 - 1 9 6 6 ) .

Q ö ] MUGHABGHAB, S . F . , GARBER, D . I . , "N e u t r o n C r o s s S e c t i o n s " , Rep.B N L -3 2 5 , 3 r d e d . , V o l 1, B ro o k h a v e n N a t i o n a l L a b o r a t o r y , ( 1 9 7 3 ) .

£ 7^ DEVOE, J . R . , e d . , " R a d io c h e m ic a l A n a l y s i s " , N a t i o n a l B u re a u o fS t a n d a r d s , T e c h n ic a l N o te 4 0 4 , ( 1 9 6 6 ) , 4 7 - 5 1 .

[ 8 j LEACHMANN, R . B . , P r o c . I n t . C o n f . on P e a c e f u l u s e s o f A to m ic E n e r g y ,G e ne va , 2 , ( 1 9 5 6 ) , 1 93 .

[93 HUGHES, D . J . , " P i l e N e u t r o n R e s e a r c h " , A d d i s o n - W e s le y , C a m b r id g e ,M a s s a c h u s e t t s , ( 1 9 5 3 ) .

Q io j L I S K IE N , H . , PA U LSEN , A . , " C o m p i l a t io n o f C r o s s - S e c t i o n s f o r someN e u t ro n In d u c e d T h r e s h o ld R e a c t i o n s " , E U R -1 1 9 e , E u ra to m , B r u s s e l s , ( 1 9 6 3 ) .

[ i l ] JUNG, R . C . , E P S T E IN , H .M . , C H A ST A IN , J . , " A s im p le e x p e r im e n t a lm ethod f o r d e t e r m in in g e f f e c t i v e t h r e s h o ld e n e r g i e s a n d c r o s s s e c t i o n s " , Rep. B M I- 1 4 8 6 , B a t e l l e M e m o r ia l I n s t i t u t e , ( 1 9 6 0 ) .

Q12] ROY, J . C . , HAWTON, J . J . , " T a b le s o f e s t im a t e d c r o s s s e c t i o n s f o r( n , p ) , ( n , a ) a n d ( n , 2 n ) r e a c t i o n s i n a f i s s i o n n e u t r o n s p e c t r u m " , C R C -1 0 0 3 , A E C L , ( 1 9 6 0 ) .

[ 1 3 ] C S I K A I , J . , BUCKZKO, M . , BODY, Z . , DEMENY, A . , " N u c l e a r d a t a f o r n e u t r o n a c t i v a t i o n a n a l y s i s " , A t . E n e r g y R e v . , V o l . V I I ( 4 ) , IA E A , V ie n n a , ( 1 9 6 9 ) .

[ 1 4 ] NARGOLW ALLA, S . S . , P R Z Y B Y L0 W IC Z , E . P . , " A c t i v a t i o n A n a l y s i s w i t h N e u t ro n G e n e r a t o r s " , V o l . 3 9 , Chem. A n a l y s i s S e r i e s , J o h n W i le y C,S o n s , New Y o r k , ( 1 9 7 3 ) . ’

£ 15] C S I K A I , J . , "U s e o f s m a l l n e u t r o n g e n e r a t o r s i n s c ie n c e andt e c h n o lo g y " , A t . E n e r g y R e v . , V o l . I I ( 3 ) , IA E A , V ie n n a , ( 1 9 7 3 ) .


M AR IO N , J . B . , FOWLER, J . L . , e d s ., " F a s t N e u t ro n P h y s i c s " ,P a r t I I , I n t e r s c i e n c e P u b l i s h e r s I n c . , New Y o r k , ( 1 9 6 0 ) .

SEG R E , E . , e d . , " E x p e r im e n t a l N u c le a r P h y s i c s , V o l s . I - I I I , W i le y New Y o r k , ( 1 9 5 3 ) .

P r o c e e d in g s o f C o n f . on N e u t ro n C r o s s S e c t io n T e c h n o lo g y ,U SA EC , R e p t . C o n f - 6 6 0 3 0 3 , ( 1 9 6 6 ) .

I b i d . . N a t i o n a l B u re a u o f S t a n d a r d s , S p e c i a l P u b l i c a t i o n 2 9 9 ,V o l s . I 5 I I , ( 1 9 6 8 ) .

Ib id . , U S A E C , R e p t . C o n f - 7 1 0 3 0 2 , V o l s . I § I I , ( 1 9 7 1 ) .

I b i d , . N a t io n a l B u re a u o f S t a n d a r d s , S p e c i a l P u b l i c a t i o n 4 2 5 ,V o l s . I & I I , ( 1 9 7 5 ) .

LUKENS J r . , H . R . , Y U LE , H . P . , G U IN N , V . P . , N u c l. I n s t r . M e th o d s , 33 , 2 7 3 ( 1 9 6 5 ) .

Y U LE , H . P . , GU IN N , V . P . , P r o c . I n t . C o n f. on R a d io c h e m . M e th od o f A n a l y s i s . IA E A , V ie n n a , V o l . I I ( 1 9 6 4 ) 1 1 1 .

S C H R E IB E R , R . E . , A L L IO , R . J . , N u c le o n i c s , 2 2 ^ 8 ) , ( 1 9 6 4 ) 12 0 .

TAYLO R, C . , W EST, R . , W H IT IN G , M . , P r o c . I n t . C o n f . on P r o d u c t i o n an d U se s o f S h o r t - l i v e d R a d io i s o t o p e s from R e a c t o r s , IA E A , V ie n n a V o l . I , ( 1 9 6 3 ) 6 7 .

A N S E L L , K .H . , H A L L , E . G . , " R e c e n t d e v e lo p m e n ts i n ( a , n ) s o u r c e s " ,

i n P r o c . o f C o n f . on N e u t ro n S o u r c e s an d A p p l i c a t i o n s , A m e r ic a n N u c l . S o c . , N a t i o n a l T o p ic a l M e e t in g , A u g u s t a , G e o r g ia , U SAEC , R ep . C 0 N F -7 1 0 4 0 2 , V o l . I I , ( 1 9 7 1 ) 9 0 -9 9 .

CALIFORNIUM 2 5 2 PROGRESS, C a l i f o r n i u m - 2 5 2 News, No. 1 7 , ( 1 9 7 4 ) 3 - 6

B U R R IL L , E . A . , MACGREGOR, M .H . , N u c le o n i c s , 18 ( 1 2 ) , ( 1 9 6 0 ) , 6 4 .

B U R R IL L , E . A . , "N e u t r o n P r o d u c t i o n an d P r o t e c t i o n " , H ig h V o l t a g e E n g in e e r i n g C o r p o r a t i o n , B u r l i n g t o n , M a s s a c h u s e t t s , ( 1 9 6 3 ) .

F L E IS C H E R , A . A . , "T h e P r o d u c t i o n o f F a s t N e u t r o n s b y S m a l l C y c l o t r o n s " , T he C y c l o t r o n C o r p o r a t i o n , TCC Rep. 2 0 0 3 , B e r k e le y ,

C a l i f o r n i a , ( 1 9 6 8 ) .

A N D ER S, O .U . , B R ID E N , D .W ., A n a l . Chem ., 36 , (1 9 6 4 ) 2 8 7 .

MOTT, W .E . , ORANGE, J . M . , A n a l . Chem .; 1_, ( 1 9 6 5 ) 1 3 3 8 .

WOOD, D . E . , J E S S E N , P . L . , JO N ES , R . E . , P i t t s b u r g h C o n f . on A n a l y t i c a l C h e m is t r y a n d A p p l ie d S p e c t r o s c o p y , P i t t s b u r g h , P e n n s y l v a n ia , ( 1 9 6 6 ) .

DYER , F . F . , BATE , L . C . , S T R A IN , J . E . , A n a l . Chem ., 39 ( 1 9 6 7 ) , 1907 .

LUNDGREN, F . A . , NARGOLW ALLA, S . S . , A n a l . Chem ., 40., ( 1 9 6 8 ) ,6 7 2 .

NARGOLW ALLA, S . S . , P h .D . T h e s i s , U n i v . o f T o r o n t p , T o r o n t o , C a n a d a , ( 1 9 6 5 ) .


WOOD, D . E . , "D e v e lo p m e n t o f F a s t N e u t r o n A c t i v a t i o n A n a l y s i s f o r L i q u i d Loop S y s t e m s " , Kaman N u c le a r C o r p o r a t i o n , C o lo r a d o S p r i n g s , C o lo r a d o , R ep . K N - 6 7 - 4 5 8 ( R ) , ( 1 9 6 7 ) .

A L - S H A H R IS T A N I, H . , P h .D . T h e s i s , U n iv . o f T o r o n t o , T o r o n t o , C a n a d a , ( 1 9 6 9 ) .

P R IE S T , H . F . , BU RNS, F .C . ', P R IE S T , G . L . , A n a l . Chem ., 4 2 ,( 1 9 7 0 ) , 4 9 9 .

ID D IN G S , F . A . , A n a l . C h im . A c t a , 3 1 , ( 1 9 6 4 ) , 2 0 6 .

KOCH, R . C . , " A c t i v a t i o n A n a l y s i s H a n d b o o k ", A c a d e m ic P r e s s ,New Y o r k , ( 1 9 6 0 ) .

LYON, J r . , W .C ., e d . , "G u id e t o A c t i v a t i o n A n a l y s i s " , Van N o s t r a n d , P r i n c e t o n , New J e r s e y , ( 1 9 6 4 ) .

L EN IH A N , J . M . A . , THOMSON, S . J . , e d s . , " A c t i v a t i o n A n a l y s i s ,

P r i n c i p l e s a n d A p p l i c a t i o n s " , A ca d é m ie P r e s s , New Y o r k , ( 1 9 6 5 ) .

DE SO ET E , D . , G IJ B E L S , R . , H O STE, J . , "N e u t r o n A c t i v a t i o n A n a l y s i s " , C h e m ic a l A n a l y s i s S e r ie s - , V o l . 3 4 , J o h n W i le y §S o n s , New Y o rk ( 1 9 7 2 ) .

TAYLO R, D . , "N e u t r o n I r r a d i a t i o n an d A c t i v a t i o n A n a l y s i s " , G e o rge N ew nes, L o n d o n , ( 1 9 6 4 ) .

BOWEN, H . J . M . , G IB B O N S , D . , " R a d i o a c t i v a t i o n A n a l y s i s , O x f o rd U n i v e r s i t y P r e s s , L o n d o n , ( 1 9 6 3 ) .

H O STE, J . , OP DE BEEC K , J . , G IJ B E L S , R . , ADAMS, F . , VAN DEN W IN K EL, P . , DE SO E T E , D . , " I n s t r u m e n t a l a n d R a d io c h e m ic a l A c t i v a t i o n A n a l y s i s " , CRC P r e s s , C le v e la n d , O h io , ( 1 9 7 1 ) .

LEN IH A N , J . M . A . , THOMSON, S . J . , e d s . , "A d v a n c e s i n A c t i v a t i o n A n a l y s i s " , V o l . 1, A ca d e m ic P r e s s , New Y o r k , ( 1 9 6 9 ) .

L EN IH A N , J . M . A . , THOMSON, S . J . , G U IN N , V . P . , e d s . , "A d v a n c e s i n A c t i v a t i o n A n a l y s i s " , V o l . 2 , A c a d e m ic P r e s s , New Y o rk ( 1 9 7 2 ) .

KRUGER, P . , " P r i n c i p l e s o f A c t i v a t i o n A n a l y s i s " , J o h n W i le y §

S o n s , New Y o r k , ( 1 9 7 1 ) .

NARGOLW ALLA, S . S . , " A p p l i c a t i o n o f N e u t ro n G e n e r a t o r s t o A c t i v a t i o n A n a l y s i s " , i n P r o c . o f t h e S e c o n d O ak R id g e C o n f . on tl\e U se o f S m a l l A c c e l e r a t o r s f o r T e a c h in g 5 R e s e a r c h , U SAEC, R e p .C o n f-r 7 0 0 3 2 2 , ( 1 9 7 0 ) , 1 8 5 -2 0 4 .

COLEMAN, R . F . , P IE R C E , T . B . , " A c t i v a t i o n A n a l y s i s , A R e v ie w " ,

T he A n a l y s t , 92 ( 1 0 9 0 ) , ( 1 9 6 7 ) , 1 - 1 9 .

D IB B S , H . P . , " A c t i v a t i o n A n a l y s i s w i t h a N e u t ro n G e n e r a t o r " , D e p t , o f M in e s a n d T e c h n ic a l S u r v e y s , M in e s B r a n c h , O tta w a ,

R e s . R ep . R 1 5 5 , ( 1 9 6 5 ) .

ADAM S, F . , H O STE, J . , " N o n - d e s t r u c t i v e A c t i v a t i o n A n a l y s i s " ,

A t . E n e r g y R e v . , V o l . I V ( 2 ) , IA E A , V ie n n a , ( 1 9 6 6 ) .


p r o c , ; 1961 I n t e r n a t i o n a l C o n f . on "M o d e rn T re n d s i n A c t i v a

t i o n A n a l y s i s " , C o l l e g e S t a t i o n , T e x a s , ( 1 9 6 1 ) .

P r o c . , 1965 I n t e r n a t i o n a l C o n f . on "M o d e m T re n d s i n A c t i v a t i o n A n a l y s i s " , C o l l e g e S t a t i o n , T e x a s , ( 1 9 6 5 ) .

P r o c . , 1 9 6 8 I n t e r n a t i o n a l C o n f . on "M o d e m T re n d s i n A c t i v a t i o n

A n a l y s i s " , N a t io n a l B u re a u o f S t a n d a r d s , S p e c i a l P u b l i c a t i o n 3 1 2 , V o l . I Ç I I , ( 1 9 6 9 ) .

P r o c . , 1972 I n t e r n a t i o n a l C o n f . on "M o d e m T re n d s i n A c t i v a

t i o n A n a l y s i s " , P a r i s , J . R a d i o a n a l y t i c a l C h e m is t r y , V o l . 18 ( 1 8 2 ) , V o l . 1 9 ( 1 5 2 ) , ( 1 9 7 4 ) .

P r o c . , 1976 I n t e r n a t i o n a l C o n f . on "M o d e m T re n d s i n A c t i v a

t i o n A n a l y s i s " , M u n i c h , V o l . I § I I , R e p r o - M a y e r - o f f s e t , M u n ic h , F e d e r a l R e p u b l ic o f G e rm any, ( 1 9 7 6 ) .

BOCK-WERTHMANN, W ., AED In f o r m a t io n S e r v i c e , S e r i e s С: B i b l i o g r a p h i e s , S e c t i o n 14 : " A c t i v a t i o n A n a l y s i s " A E D - C - 1 4 - 1 , H a h n - M e i t n e r - I n s t i t u t f ü r K e r n f o r s c h u n g , B e r l i n , ( 1 9 6 1 ) .

I b i d . , A E D - C - 1 4 -0 2 , ( 1 9 6 3 ) . '

I b i d . , A E D - C - 1 4 -0 3 , ( 1 9 6 4 ) .

LU TZ, G . J . , B O R E N I, R . J . , MADDOCK, R . S . , W ING, j . , e d s . , " A c t i v a t i o n A n a l y s i s : A B i b l i o g r a p h y t h r o u g h 1 9 7 1 " , N a t io n a l B u re a u o f S t a n d a r d s , T e c h n ic a l N o te 4 6 7 , ( 1 9 7 1 ) .

LU TZ, G . J . , e d . , " 1 4 -M e V N e u t ro n G e n e r a t o r s i n A c t i v a t i o n A n a l y s i s : A B i b l i o g r a p h y " , N a t io n a l B u re a u o f S t a n d a r d s , T e c h n i c a l N o te 5 3 3 , ( 1 9 7 0 ) .

VAN G R IE K E N , R . , H O ST E, J . , " A n n o t a t e d B i b l i o g r a p h y on 14 MeV N e u t r o n A c t i v a t i o n A n a l y s i s " , E u r i s o t o p ', O f f i c e in f o r m a t i o n b o o k le t 6 5 , S e r i e s : B i b l i o g r a p h i e s - 8 , ( 1 9 7 2 ) .

LED ER ER , C .M . , H OLLANDER, J . M . , ■ PERLM AN, I . , " T a b le o f I s o t o p e s " , J o h n W i l e y 5 S o n s , New Y o r k , ( 1 9 6 7 ) ;

A L IE V , A . I . , D R Y N K IN , V . l . , L E IP U N SK A Y A , D . I . , K A S A T L IN , V . A . , "H a n d b o o k o f N u c le a r D a ta fo r- N e u t r o n A c t i v a t i o n A n a l y s i s " , T r a n s l a t i o n fro m R u s s i a n , I s r a e l P ro g ra m f o r S c i e n t i f i c T r a n s

l a t i o n , K e t e r P r e s s , J e r u s a le m , ( 1 9 7 0 ) .

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C h e m is t r y , ( 1 9 7 1 ) 1 2 7 -1 8 8 .

I b i d . , I I , J . R a d i o a n a l y t i c a l C h e m is t r y , 8 , ( 1 9 7 1 ) , 3 7 3 -4 7 9 .

I b i d . , I l l , J . R a d i o a n a l y t i c a l C h e m is t r y , 9^ ( 1 9 7 1 ) , 1 0 1 -1 8 9 .

SO YKA , W ., ERDTMANN, G . , " D i e y - L i n i e n d e r R a d io n u k l i d e " , KFA Z e n t r a l i n s t i t u t f ü r A n a l y t i s c h e C he m ie , J u l : 1 0 0 3 -A C , J u l i c h , B u n d e s r e p u b l ik , D e u t s c h la n d , V o l s . 1, 2 , 3 , ( 1 9 7 4 ) .

BAUMGARTNER, F . , " T a b le s o f N e u t r o n A c t i v a t i o n C o n s t a n t s " , t r a n s l a t e d from K e r n t e c h n ik , Ъ_, ( 1 9 6 1 ) 3 5 6 .


G IR A R D I, F . , G U Z Z I, G ., PA U LY , J . , " D a t a H and b oo k f o r S e n s i t i v i t y C a l c u l a t i o n s i n N e u t ro n A c t i v a t i o n A n a l y s i s " , E u ra to m Rep .1 8 9 8 . e , ( 1 9 6 5 ) .

NARGOLW ALLA, S . S . , N IE W O D N IC Z A N S K I, J . , SUDDUETH, J . E . , "E x p e r im e n t a l S e n s i t i v i t i e s f o r 3 -M e V N e u t r o n A c t i v a t i o n A n a l y s i s " , J . R a d i o a n a l y t i c a l C h e m is t r y , ( 1 9 7 0 ) , 4 0 3 .

KENNA, B . T . , CONRAD, F . J . , " T a b u l a t i o n o f C r o s s S e c t i o n s , Q - v a lu e s a n d S e n s i t i v i t i e s f o r N u c le a r R e a c t i o n s o f N u c l i d e s w i t h 14 MeV N e u t r o n s " , R ep . S C - R R - 6 6 - 2 2 9 , S a n d ia L a b o r a t o r y , A lb u q u e rq u e , New M e x ic o , ( 1 9 6 6 ) .

C U YPER S, M . , C U YP ER S, J . , "G am m a-R ay S p e c t r a and S e n s i t i v i t i e s

f o r 14 -M e V N e u t ro n A c t i v a t i o n A n a l y s i s " , T e x a s A § M, C o l le g e S t a t i o n , T e x a s , ( 1 9 6 6 ) .

FR IE D L A N D E R , G . , KENNEDY, J .W . , M IL L E R , J . M . , " N u c l e a r and R a d io c h e m i s t r y " , J o h n W i l e y 5 S o n s , New Y o r k , ( 1 9 6 4 ) .

KRUGERS, J . , e d . , " I n s t r u m e n t a t i o n i n A p p l ie d N u c le a r C h e m i s t r y " , P le n u m P r e s s , New Y o r k , ( 1 9 7 3 ) .

P r o c . o f C o n f . o n " N u c l e a r E l e c t r o n i c s " , V o l s I - I I I , IA E A ,V ie n n a , ( 1 9 6 2 ) .

BROWN, W .C . , H IG IN BO TH A M , W .A . , M IL L E R , G . L . , C H A SE, R . L . , . e d s . ,

" P r o c . o f C o n f . on S e m ic o n d u c t o r N u c l e a r - p a r t i c l e D e t e c t o r s and C i r c u i t s " , N a t i o n a l Academ y o f S c i e n c e s , P u b l . 1 5 9 3 , W a s h in g t o n , D . C . , ( 1 9 6 9 ) .

NARGOLW ALLA, S . S . , N IE W O D N IC Z A N S K I, J . , SUDDUETH, J . E . ,"G am m a-ra y S p e c t r a an d E x p e r im e n t a l S e n s i t i v i t i e s f o r 3 -M eV N e u t ro n A c t i v a t i o n A n a l y s i s " , N a t i o n a l B u re a u o f S t a n d a r d s , u n p u b l i s h e d i n t e r n a l r e p o r t , ( 1 9 6 9 ) .

HEATH, R . L . , " S c i n t i l l a t i o n S p e c t r o m e t r y Gam m a-ray S p e c t ru m C a t a l o g " , V o l s . I , I I , 2nd e d n . , U SA E C , R ep . ID O -1 6 8 8 0 , ( 1 9 6 4 ) .

HEATH, R . L . , "G am m a-ra y S p e c t ru m C a t a l o g G e ( L i ) an d S i ( L i ) S p e c t r o m e t r y " , A N C R -1 0 0 0 -2 , U C -3 4 C , 3 r d e d n . , A e r o j e t N u c le a r C o . , Id a h o N u c le a r E n g in e e r i n g L a b . , Id a h o F a l l s , Id a h o , ( 1 9 7 5 ) .

COSGROVE, J . F . , " R o u t i n e d e t e r m in a t io n o f m a jo r c o m p o n e n ts b y a c t i v a t i o n a n a l y s i s " , i n P r o c . 1968 I n t . C o n f . M od. T re n d s i n A c t i v a t i o n A n a l y s i s , N a t i o n a l B u re a u o f S t a n d a r d s , S p e c i a l P u b l . 3 1 2 , V o l . 1 , ( 1 9 6 9 ) , 4 5 7 .

C U R R IE , L . A . , N u c l . I n s t r . M e th o d s , 1 0 0 , (1 9 7 2 ) 3 8 7 .

R IC C I , E . , DYER , F . F . , N u c le o n i c s , 2Z_ ( 6 ) , ( 1 9 6 4 ) , 4 5 .

MATHUR, S . C . , OLDHAM, G . , N u c le a r E n e r g y , ( S e p -O c t 1 9 6 7 ) , 1 3 6 -1 4 1 .

NARGOLW ALLA, S . S . , C RAM BES, M .R . , D E V 0 E , J . R . , A n a l . Chem .,

4 0 , ( 1 9 6 8 ) 6 6 6 .

G IJ B E L S , R . , " A c t i v a t i o n A n a l y s i s w i t h N e u t r o n G e n e r a t o r s " , i n In s t r u m e n t a l an d R a d io c h e m ic a l A c t i v a t i o n A n a l y s i s , Adam s, F . ,Op de B e e c k , J . , Van den W in k e l , P . , G i j b e l s , R . , D e S o e te , D . , H o s t e , J . , e d s . , CRC P r e s s , C le v e la n d , O h io , ( 1 9 7 1 ) .


WOOD, D . E . , " P r o b le m s i n P r e c i s i o n A c t i v a t i o n A n a l y s i s w i t h F a s t N e u t r o n s " , i n P r o c . o f C o n f . on S m a l l A c c e l e r a t o r s f o r T e a c h in g an d R e s e a r c h , O ak R id g e A s s o c i a t e d U n i v e r s i t i e s , Oak R id g e ,

T e n n e s s e e , U SAEC, Flep. C o n f - 6 8 0 4 1 1 , ( 1 9 6 8 ) .

PALM ER , H . E . , N ELP , W .B ., MURANO, R . , R IC H , C . , P h y s . Med. B i o l . , 13, ( 1 9 6 8 ) , 2 6 9 .

S U E S S , H . E . , UREY, H . C . , R e v . M od. P h y s . , 2 8 , ( 1 9 5 6 ) , 5 3 .

V IN C E N T , H . Z . , VOLBORTH, A . ; N u c l . A p p l . , 3 , ( 1 9 6 7 ) , 7 5 3 .

P r o c . o f th e N a to A d v a n c e d S t u d y I n s t i t u t e on " A c t i v a t i o n A n a l y s i s i n G e o c h e m is t r y a n d C o s m o c h e m is t r y " , S t e i n n e s , E . , B r u n f e i t , A .O . , e d s . , K j e l l e r , N o rw ay , ( 1 9 7 0 ) .

P r o c . o f a S ym p o siu m on " N u c l e a r T e c h n iq u e s and M in e r a l R e s o u r c e s " , B u e n o s A i r e s , IA E A , V ie n n a , ( 1 9 6 9 ) .

P r o c . o f an I n t e r n a t i o n a l S ym p o siu m on " N u c le a r T e c h n iq u e s i n E x p l o r a t i o n , E x t r a c t i o n an d P r o c e s s i n g o f M in e r a l R e s o u r c e s " ,IA E A , V ie n n a , ( 1 9 7 7 ) , i n p r e s s .

B E R Z IN , A . K . , B E SPA LO V , D . F . , ZAPO RO ZHETS, V . M . , KANTOR, S . A . , L E IP U N SK A Y A , D . I . , S U L IN , V . V . , FELDMAN, I . I . , S H IM E L E V IC H ,

Y U . S . , " P r e s e n t S t a t e an d U se o f B a s i c N u c le a r G e o p h y s ic a l M e th o d s f o r I n v e s t i g a t i n g R o c k s a n d O r e s " , A t . E n e r g y R e v ie w ,IA E A , V ie n n a , 4 ( 2 ) , ( 1 9 6 6 ) , 5 9 - 1 1 1 .

K E Y S , W .S . , BOULOGNE, A . R . , "W e l l L o g g in g w i t h C a l i f o r n i a - 2 5 2 " , SPW LA 1 0 th A n n u a l L o g g in g S ym p o s iu m , N o u s t o n , T e x a s , ( 1 9 6 9 ) .

CZU BEK, J . A . , " R e c e n t R u s s ia n an d E u ro p e a n D e v e lo p m e n ts i n N u c le a r G e o p h y s ic s A p p l ie d t o M in e r a l E x p l o r a t i o n an d M i n i n g " ,

L o g A n a l y s t , N o v -D e e . ( 1 9 7 1 ) , 2 0 -3 4 .

G J IB E L S , S . R . , "N e u t r o n A c t i v a t i o n A n a l y s i s o f O re s a n d M in e r a l s , M in e r . S e i . E n g . , 5 _ (4 ), ( 1 9 7 3 ) , 3 0 4 -3 4 8 .

E I S L E R , P . L . , H UPPERT , P . , W Y L IE , A .W ., " L o g g i n g o f C o p p e r i n S im u la t e d B o r e h o le s b y Gamma S p e c t r o m e t r y " , G e o e x p lo r a t io n , 9_,( 1 9 7 1 ) , 1 8 1 -1 9 4 :

H OYER, W .A . , LO CKE , G . A . , " L o g g i n g f o r C o p p e r b y I n s i t u N e u t r o n A c t i v a t i o n A n a l y s i s " , A IM E A n n u a l M e e t in g , S a n F r a n c i s c o ,R e p r in t N o . 7 2 - L - 2 8 ( 1 9 7 2 ) .

LANDSTRÖM, 0 . , C H R IS T E L L , R . , K O S K I, K . , " F i e l d E x p e r im e n t s on th e A p p l i c a t i o n o f N e u t ro n A c t i v a t i o n T e c h n iq u e s t o I n s i t u B o r e h o le A n a l y s i s " , G e o e x p lo r a t i o n , 1Ю, (1 9 7 2 ) 2 3 .

МОХНАМ, R . M . , S E N F T L E , F . E . , BOYNTON, G . R . , " B o r e h o l e A c t i v a t i o n A n a l y s i s b y D e la y e d an d C a p tu re Gamma R a y s U s i n g a 2 S 2 c f N e u t ro n S o u T c e " , E c o n o m ic G e o lo g y , 6^7, ( 1 9 7 2 ) , 5 7 9 - 5 9 1 .

NARGOLW ALLA, S . S . , KUNG, A . , LEGRADY, O . J . , ST R E V E R , J . ,

C S IL L A G , A . , S E IG E L , H .O . , " N u c l e a r M e t a lo g G ra d e L o g g in g i n M in e r a l D e p o s i t s " , i n I n t e r n a t i o n a l S ym p o siu m on N u c le a r T e c h n iq u e s i n E x p l o r a t i o n , E x t r a c t i o n an d P r o c e s s i n g o f M in e r a l R e s o u r c e s , IA E A , V ie n n a , ( 1 9 7 7 ) , i n p r e s s .


LAU BER, A . , LANDSTROM, О . , " A G e ( L i ) B o r e h o le P ro b e f o r I n s i t u G am m a-ray S p e c t r o m e t r y " , G e o p h y s ic a l P r o s p e c t i n g , 20_, ( 1 9 7 2 ) , 8 0 0 - 8 1 3 .

TAN N ER, A . B . , МОХНАМ, R .M . , S E N F T L E , F . E . , B A IC K E R , J . A . , " A P ro b e f o r N e u t r o n A c t i v a t i o n A n a l y s i s i n a D r i l l H o le U s i n g

C f a n d a G e ( L i ) D e t e c t o r C o o le d b y a M e l t i n g C r y o g e n " ,N u c l . I n s t . M e t h . , 1 0 0 , ( 1 9 7 2 ) , 1 - 7 .

D IB B S , H . P . , "T h e A p p l i c a t i o n o f N e u t r o n A c t i v a t i o n A n a l y s i s t o th e D e t e r m in a t io n o f C o p p e r i n M i n e r a l s " , C IM T r a n s . , 7 3 , ( 1 9 7 0 ) , 1 0 2 -1 0 8 .

K R ISH N A N , S . S . , J . R a d i o a n a l y t i c a l C hem ., 1 ¿ , ( 1 9 7 3 ) , 1 6 5 -1 7 2 .

P r o c . o f a S ym p o s iu m on "U s e o f N u c le a r T e c h n iq u e s i n th e M e a su re m e n t an d C o n t r o l o f E n v ir o n m e n t a l P o l l u t i o n " , S a l z b u r g , IA E A , V ie n n a , ( 1 9 7 0 ) .

P r o c . o f t h e S e c o n d I n t e r n a t i o n a l C o n fe re n c e o n N u c le a r M e th o d s i n E n v ir o n m e n t a l R e s e a r c h " , U n i v e r s i t y o f M i s s o u r i , ER D A , R ep . C O N F -7 4 0 7 0 1 , ( 1 9 7 4 ) . -

R E C E N T A N A L Y T I C A L A P P L I C A T I O N S

O F N E U T R O N - C A P T U R E

G A M M A - R A Y S P E C T R O S C O P Y

J .A . L U B K O W IT Z , M . H E U R T E B I S E ,

H . B U E N A F A M A

I n s t i tu to V e n e z o la n o d e In v e s t ig a c io n e s

C ie n t íf ic a s ,

C e n tr o d e P e tr ó le o y Q u ím ic a ,

C a ra ca s , V e n e z u e la

Abstract

RECEN T A N A LYTICA L APPLICATIONS O F N EUTRON -CAPTURE GAMM A-RAY SPECTROSCOPY.

The measurement o f the prompt photons released from the compound nucleus following the absorption o f a neutron is a complem entary technique to activation analysis. The technique used with reactors requires the design o f a well-collimated and thermalized neutron beam. A high-resqlution G e(Li), high-efficiency detector is required in conjunction with a sample gamma-ray collim ator. The technique has been applied to the analysis o f Cr, Mn, Fe,Ni, Ag, Aw, Ti, Zn, V , Co and Cu in alloys at concentrations ranging from 5 -6 5 % . The relative standard deviations ranged from 1 .9 -8 .2 % . A ccuracy errors ranged from 0 .8 -8 .9 % . Prompt gamma rays have been found useful in the analysis o f Co, Mo and Ni in desulphurization catalysts. Titanium oxide is used as internal standard. Standard deviations o f 5% were typical for concentration ranges o f 2 -1 5 % . Comparison o f the results with those o f activation analysis showed that the two m ethods differed by 0 —13% for 11 samples. The water content can be obtained simultaneously with the determination o f Co, Mo or Ni in the same sample. A relative standard deviation o f 3.6% was obtained for a water content o f 12.65% .

IN T R O D U C T IO N

In n e u tr o n -c a p tu r e g a m m a -ra y s p e c tr o s c o p y , th e p ro m p t g a m m a -ra y

in te n s ity is d e p e n d e n t o n th e c a p tu r e c r o s s -s e c t io n and in d e p e n d e n t o f th e ra d io

a c t iv e p r o p e r t ie s o f th e p r o d u c t n u c le u s . T h e s e tw o fa c ts d if f e r e n t ia te th e

te c h n iq u e fro m r a d io a c t iv a tio n a n a ly s is w h ic h re lie s o n th e g ro u n d -s ta te p ro

p e r t ie s o f th e p r o d u c t n u c le u s . In n u m e ro u s ca se s th e p r o d u c t n u c le u s m a y

h a v e a s h o r t h a lf - l i fe , a lo n g h a lf - l i fe , o r a s ta b le c o m p o s it io n o r i t m a y n o t b e

a g a m m a -e m itte r a t a ll. I t is a lso p o s s ib le th a t th e ta rg e t n u c le u s h a s a sm all

i s o to p ic a b u n d a n c e . In th e s e ca se s it is a d v a n ta g e o u s to u se p r o m p t g a m m a ra y s .

125

126 L U B K O W IT Z et al.

S in c e th e l if e t im e o f th e e x c i te d c o m p o s ite n u c le u s is sm a ll ( 1 0 -9 — 1 0 " 12 s ) ,

th e c h a r a c te r is t ic g a m m a -ra d ia tio n m u st b e m e a su re d d u rin g n e u tr o n b o m b a r d

m e n t . T h e w ith d ra w a l o f n e u tr o n s th ro u g h a b e a m p o r t is an e s s e n t ia l,p a r t o f

th e sy s te m .

T h is p a p e r d ea ls w ith th e a p p lic a t io n o f n e u tr o n -c a p tu r e g a m m a -ra y

s p e c tr o s c o p y to th r e e ty p e s o f p ro b le m s :

I. A n a ly s is o f a llo y s [ 1 , 2 ]

I I . A n a ly s is o f C o , M o an d N i in d e s u lp h u riz a tio n c a ta ly s ts [ 3 ]

I I I . D e te r m in a t io n o f m o is tu r e in c a ta ly s ts [ 4 ^ 6 ]

T H E O R Y

D u rin g th e ir ra d ia tio n o f a m a tr ix c o m p o s e d o f e le m e n ts А , В , C , ... i,

th e n u m b e r o f r e a c t io n s p e r s e c o n d , dN /dt, f o r a g iv en e le m e n t is e x p r e ss e d as:

d N. — = Ф a N ( 1 )

dt

w h e re Ф is th e th e r m a l n e u tr o n f lu x , •

a is th e m ic r o s c o p ic c r o s s -s e c tio n an d :

N is th e n u m b e r o f ta rg e t a to m s o f th e e le m e n t in q u e s t io n .

T h u s , th e n u m b e r o f p ro m p t-g a m m a e v e n ts m e a su re d pér. s e c o n d is g iv en b y :

С - Ф a N I e ( 2 )

w h e re I is th e a b s o lu te in te n s ity , and

6 is th e e f f ic ie n c y .

S in c e th e se n s it iv ity f a c to r S is d e fin e d as S = I ст/M, w h e re M is th e a to m ic

m ass o f th e e le m e n t , an d s in c e th e a b s o lu te e f f ic ie n c y e c a n b e r e d e fin e d as

e — к в, w h ere к is a c o n s ta n t an d в is th e re la t iv e e f f i c ie n c y , i t is p o s s ib le to

re w rite E q . ( 2 ) as

С = Ф S W A 0 к ( 3 )

w h e re W is th e w e ig h t o f th e e le m e n t in q u e s t io n an d ■

A is th e A v o g a d ro ’s n u m b e r .

I f th e m a tr ix is c o m p o s e d o f sev e ra l e le m e n ts А , В , C , . .. i , th e n th e su m

o f th e m a sse s o f th e c o n s t i tu e n ts e x p re ss e d in a n y u n its is g iv en b y :

1 = W A + W B + W c + ........W¿ ( 4 )

N E U T R O N - C A P T U R E G A M M A - R A Y S P E C T R O S C O P Y 127

T h e w e ig h t p e r c e n t o f c o n s t i tu e n t A is g iven b y :

W AW * + ----------------------------------------------- " 1 0 0 % ( 5 )

A (W A + W B + W C + ... W j) w

B y so lv in g E q .( 3 ) fo r W and s u b s titu tin g i t in to E q . ( 5 ) , o n e g e ts :

C a / S a ^ aW a = -------- ;---------------------- ;------------------------ ;-------- 1 0 0 % ( 6)

(C A /SA 0 A + c B / sB 0 B + - c i/s i0 i)

E q u a t io n ( 6) w as u sed fo r th e d e te r m in a t io n o f m e ta ls in a llo y s . T h e re la tiv e

e f f ic ie n c y в m u st b e e x p e r im e n ta l ly e v a lu a te d as a fu n c t io n o f e n e rg y . T h e

S v alu es w ere o b ta in e d fr o m tw o v e ry c o m p le te r e fe r e n c e s o f p ro m p t-g à m m a

tr a n s it io n s [ 7 , 8].

E q u a t io n ( 1 ) c a n a lso b e e x p re ss e d in te rm s o f th e a c c u m u la te d c o u n ts

in a g iven t im e in te rv a l d u e t o n u c le a r r e a c t io n s w ith an e le m e n t in th e c a ta ly s t.

F o r e x a m p le , fo r a c a ta ly s t c o n ta in in g h y d ro g e n an d an in te r n a l s ta n d a rd su ch

as T i 0 2 , E q . ( l ) c a n b e e x p re ss e d as:

Н - к ц а ^ д Ф ( 7 )

T i = k 2ff2W X i02<ï> ( 8)

w h ere th e c o n s ta n t к is a f u n c t io n o f th e b r a n c h in g r a t io , d e te c to r e f f ic ie n c y

an d th e a to m ic m ass. H an d T i a re th e m e a su re d c o u n ts d u e to th e h y d ro g e n

m ass W H an d th e t ita n iu m o x id e m ass W T ¡ o 2 p re s e n t in th e sa m p le . D iv id in g

E q s ( 2 ) an d ( 3 ) y ie ld s :

H K W h

T i W T i o 2( 9 )

w h e re К = k t а г /к2 а 2 . T h is e q u a t io n sh o w s th a t th e c o u n ts r a t io f o r a h o m o

g e n e o u s sa m p le is in d e p e n d e n t o f th e f lu x and f lu x v a r ia tio n s . I f th e r a t io ,

T i/ W x jo 2 , is re p la c e d b y th e s y m b o l S , th e t ita n iu m o x id e s p e c if ic c o u n ts ,

E q . ( 4 ) ca n b e r e w r it te n as:

Ъ SV

К)00

- L iF

» ? " О:• & • •• • .

:o ;: 9.

rCd: (1mm THICKNESS)

POOL

LUСИоо

FIG.1. Diagram o f the collimation o f the horizontal tube N o.6 o f the R V-l reactor.

LUBK

OW

ITZ

et al.


T h e p e r c e n ta g e o f h y d ro g e n in th e sa m p le is g iven b y :

W H 1 0 0% H = -------------- ( 1 1 )

w

w h ere W is th e sa m p le w e ig h t. R e p la c in g W H in E q . ( l 1 ) b y its v a lu e in E q .( lO )

y ie ld s :

H X 1 0 0

( % H ) K = ^ r - ( 1 2 )

T h is e q u a t io n is used fo r c a lc u la t in g th e h y d ro g e n c o n te n t in th e sa m p le a f te r

f ir s t e v a lu a tin g th e c o n s ta n t К fro m a s ta n d a rd sa m p le c o n ta in in g a w eigh ed

a m o u n t o f t ita n iu m o x id e .

E q u a t io n ( 9 ) c a n b e u sed to c a lc u la te th e w e ig h t o f C o , M o a n d N i p re se n t

in th e sa m p le w h e n s ta n d a rd s are p re p a re d w ith d if fe r e n t m e ta l c o n c e n tr a t io n s

b u t w ith a c o n s ta n t w e ig h t o f T i 0 2 . E q u a t io n ( 7 ) c a n b e u sed to d e te r m in e

% H o r % m e ta l w h e n k n o w n a m o u n ts o f a sta n d a rd a re ad d ed to th e sa m p le .

T h e s e e q u a t io n s w e re u sed in th e a n a ly s is o f c a ta ly s ts and in th e d e te r m in a t io n

o f m o is tu re in c a ta ly s ts .

N E U T R O N C O L L IM A T IO N

T h e h o r iz o n ta l b e a m u sed w as fo r m e d in th e h o r iz o n ta l tu b e T -6 o f th e

R V -1 r e a c to r . T h e n e u tr o n b e a m w as c o ll im a te d b y m e a n s o f tw o h e a v y

c o n c r e te c o ll im a to r s , as sh o w n in F i g . l . In th e e x te r n a l c o ll im a to r , a sin g le

b is m u th c r y s ta l ( 1 5 -c m lo n g , 5 .0 cm in d ia m e te r ) p la ce d in th e c e n tr a l o r if ic e

re d u c e d th e d ir e c t f is s io n -g a m m a r a d ia tio n , e p ith e rm a l and fa s t n e u tr o n s to an

a c c e p ta b le le v e l. B e h in d th is in e u tr o n c o ll im a to r , a lea d d is c an d a b o r a te d -

p a ra ff in d is c ( e a c h 2 3 -c m lo n g an d 2 3 cm in d ia m e te r ) w e re u sed t o re d u c e th e

re m a in in g g a m m a r a d ia t io n an d fa s t n e u tro n s . A t th e e x it -e n d o f th e b e a m a

co m p re sse d L iF s lab (2 -c m th ic k ) serv ed as a f in a l n e u tr o n a b s o r b e r . A t 5 6 cm

fro m th e r e a c to r w all a b è a m -c a tc h e r w as p la ce d w h ic h c o n ta in e d 2 m 3 o f

b o ra te d p a r a ff in . H y d ro g e n p ro m p t-g a m m a ra y s w e re e lim in a te d b y a c o n c r e te

sa fe ty sh ie ld ( 4 0 -c m w id e ).

N e u tr o n f lu x c h a r a c te r is t ic s

In th e c e n tr e o f th e b e a m a f lu x o f 4 .7 5 • 1 0 7 n - c r n -2 . s _1 an d a C d ra t io

o f 3 0 .5 w ere m e a su re d u sin g g o ld d e te c to r s . T h e f lu x d e v ia t io n m e a su re d u sing


5 -m m g o ld d is c s w as < 1 0 % in a n a re a o f 6 .1 c m 2 . T h e s e c h a r a c te r is t ic s p e rm it

1- to 5-g sa m p le s to b e irra d ia te d w ith o u t s u b je c t io n to f lu x in h o m o g e n e ity .

M e a s u re m e n t o f p ro m p t g a m m a ray s

T o e lim in a te th e g am m a ra y s arisin g fro m th e n e u tr o n c o ll im a to r and

n e u tr o n b e a m -c a tc h e r , a g a m m a c o ll im a to r w as b u ilt to en su re th e m e a s u r e m e n t

o f th e sa m p le g am m a ra y s u n d e r c o n d it io n s w h e re l i t t le o f th e b a c k g ro u n d

r a d ia t io n re a c h e s th e d e te c to r . T h is c o ll im a to r c o n s is ts o f tw o in d iv id u a l lead

b r ic k s h av in g a c irc u la r e n tr a n c e slit sm a lle r th a n th e e x i t s lit . T h e e x i t s lit o f

o n e b lo c k c o in c id e s in size w ith th e e n tr a n c e slit o f th e n e x t b lo c k . E a c h

b lo c k is 5 c m in le n g th , a n d a t 2 c m o f th e la s t b lo c k a lead c a s t le w a s b u ilt to

h o u s e th e G e ( L i ) d e te c to r . T h e lead c a s t le h a s a c irc u la r o p e n in g o f 4 .4 cm

an d th e en d fa c in g th e d e te c to r h a s a c irc u la r o p e n in g o f 5 .4 c m . A t th e en d o f

th is o p e n in g th e d ia m e te r o f th e h o le re m a in s c o n s ta n t a t 6.0 cm , th u s c o in c id in g

w ith th e d e te c to r d im e n s io n s . T h e p o s it io n an d d ia m e te r o f th e o p e n in g s o f

th e s e b lo c k s h a s b e e n c a lc u la te d so th a t a s p h e r ic a l c o n e a rises h av in g i ts a p e x a t

1 2 .7 cm fro m th e fir s t b lo c k w h ile th e o th e r en d h a s a s u r fa c e c o in c id in g w ith

th e a c t iv e d e te c to r s u r fa c e . T h e sa m p le is p la c e d a t a p o in t o f th is c o n e hav in g

a 1.0-c m d ia m e te r .

A 0 .1 -m m -th ic k a lu m in iu m tu b e , w h ic h ca n a c c o m m o d a te c ir c u la r p e lle ts ,

p o w d e rs an d liq u id s , w as used as sa m p le h o ld e r . T h e sa m p le h o ld e r w as su s

p en d ed a b o v e th e b e a m p a th b y a s te e l w ire c o v e re d w ith sh ie ld in g c la y . D e f le c te d

an d d if f r a c te d n e u tro n s re a c h in g th e G e ( L i ) d e te c to r w ere av o id ed b y co v erin g

th e lead c o ll im a to r s w ith c o m p re sse d L iF s la b s o f 2 -c m th ic k n e s s . T h e d e te c to r

w as lo c a te d 2 3 cm fro m th e c e n tr e o f th e b e a m . T h e n e u tr o n an d g a m m a

c o ll im a to r s w ere lo c a te d in th e sa m e p la n e b u t a t 9 0 ° o f e a c h o th e r . T h e g am m a

r a d ia tio n s w ere m e a su red w ith a 9 6 - c m 3 G e ( L i ) d e te c to r (P/C 4 4 : 1 , 1 .7 6 k e V

r e s o lu t io n a t 1 .3 3 M e V ) co u p le d to a 4 0 9 6 - c h a n n e l p u lse h e ig h t a n a ly se r . T h e

m u lt ic h a n n e l a n a ly se r w as c a lib ra te d in th e ra n g e o f 5 .0 — 9 .0 9 6 M e V w h ic h is

th e re g io n o f in te r e s t f o r th e e le m e n ts s tu d ie d . I r o n , w ith its th r e e tw in p e a k s ,

n ic k e l w ith its h ig h -e n e rg y g a m m a ra y s and m e rc u ry w ith its h ig h s e n s it iv ity ,

p e r m it th e re g io n o f in te r e s t to b e c a lib ra te d to th e n e a re s t k e V b y u sin g th e

ta b u la te d v a lu es o f D u ffe y e t a l. [ 7 ] . A d iag ram o f th e a rra n g e m e n t is sh o w n

in F ig .2 . T h e p u lse h e ig h t a n a ly se r w as c a lib r a te d in th e ran g e o f 0 .5 — 2 .5 M e V

f o r C o an d M o d e te r m in a t io n s a n d in th e ra n g e o f 5 .0 —9 .0 9 6 M e V fo r N i

d e te r m in a t io n s . C o b a lt w as m e a su re d a t 5 5 6 , M o a t 7 7 8 , an d T i a t 1 3 8 1 k e V .

C o b a lt an d M o c o u n ts w ere o b ta in e d w ith g o o d p re c is io n d u rin g a 1 0 -m in

ir r a d ia tio n -c o u n tin g t im e . F o r N i a n a ly s is , a 3 0 -m in ir r a d ia tio n -c o u n tin g t im e

p e r m its th e a c c u m u la t io n o f s u ff ic ie n t c o u n ts b y in te g ra tin g th e c o u n ts o f th e

8 9 9 9 - k e V g a m m a tr a n s it io n w ith th e c o rre s p o n d in g sin g le- and d o u b le -e s c a p e

p e a k s . In th is ca se th e T i is m e a su red a t 6 2 4 9 k e V (s in g le -e sc a p e p e a k ) . H y d ro g e n


FIG.2. Top view o f the detector shield and gamma collimator.

w as m e a su re d b y in te g ra tin g th e c o u n ts o b ta in e d a t 2 2 3 2 k e V . T h e T i p e a k a t

1 3 8 1 k e V w as u sed in th e m o is tu r e d e te r m in a t io n s . E x p e r im e n ta l d e ta ils o n

sa m p le t r e a tm e n t an d m o d e o f s ta n d a rd iz a tio n a re g iven in R e fs [ 1 —6 ] .

I. A P P L IC A T IO N S IN T H E A N A L Y S I S O F A L L O Y S

A . D e v e lo p m e n t o f th e m o d e l

I t is p o s s ib le to q u a n t i ta te th e e le m e n ta l c o m p o s it io n o f a llo y s w ith o u t th e use

o f a s ta n d a rd . T h e m e th o d is b a se d o n th e o b s e r v a tio n th a t a re la t io n s h ip

e x is ts b e tw e e n th e p e a k a re a s re p re s e n tin g d if fe r e n t m a c r o -c o n s t i tu e n ts and

th e ir c o n c e n tr a t io n [ 9 ] . T h e m e th o d is in d e p e n d e n t o f f lu x , d e p re s s io n s and

se lf-s h ie ld in g n e u tr o n e f fe c t s o f a h o m o g e n e o u s m a tr ix . W e h av e s u c c e s s fu lly

L U B K O W IT Z et al.

FIG.3. Relative efficiency o f the detector fo r fu ll peak.

F I G . 4 . S i n g l e - e s c a p e p e a k c o u n t s t o f u l l p e a k c o u n t s r a t i o a s a f u n c t i o n o f e n e r g y .


FIG.5. Double-escape peak counts to fu ll peak counts ratio as a function o f energy.

FIG. 6. Relative efficiency curve o f the Ge(Li) detector fo r single-escape and doubleescape peaks.


ap p lied th is m e th o d to th e a n a ly s is o f g o ld a llo y s in c o n v e n tio n a l n e u tr o n

a c t iv a tio n a n a ly sis .

A s sh o w n b y E q . ( 6) , i t is n e c e ssa r y to m e a su re a c c u r a te ly th e re la tiv e

e f f ic ie n c y в fo r th e e le m e n ts b e in g a n a ly se d . F ig u re 3 sh o w s th e fu ll p e a k

re la t iv e e f f ic ie n c y as a f u n c t io n o f e n e rg y in th e ra n g e o f 5 .0 — 9 .0 M e V . S in c e

s in g le -e sca p e and d o u b le -e s c a p e p e a k s are g e n e r a lly m o re in te n s e th a n th e c o r

re s p o n d in g fu ll p e a k , th e ir u se is e f fe c t iv e in o b ta in in g an a n a ly s is w ith g re a te r

se n s it iv ity . T o e s ta b lis h th e re la t iv e e f f ic ie n c y o f s in g le -e sca p e p e a k s , a s tu d y o f

th e r a t io o f sin gle-escap e; p e a k c o u n ts to fu ll p e a k c o u n ts a t d if f e r e n t en e rg ie s

w as m a d e , as sh o w n in F ig .4 . T h e fu n c t io n is l in e a r in th is re g io n an d is d e scrib e d

b y th e e q u a t io n , R = 0 .3 0 3 7 ( ± 0 . 0 0 5 7 ) E —0 .5 0 2 3 ( ± 0 . 0 3 9 2 ) . T h e c o r r e la t io n

c o e f f ic ie n t is 0 .9 9 8 1 , in d ic a tin g a g o o d f i t . A s im ila r e x p e r im e n t w as p e r fo rm e d

to m e a su re th e ra t io o f d o u b le -e s c a p e p e a k c o u n ts to fu ll p e a k c o u n ts as a

fu n c t io n o f e n e rg y , and th e re la t io n s h ip is sh o w n in F ig . 5 . T h e e q u a t io n

o b ta in e d w as R = 0 .3 2 1 9 ( ± 0 . 0 3 3 5 ) E — 3 1 1 3 ( ± 0 . 2 2 7 ) . T h e c o r r e la t io n c o e f

f ic ie n t o b ta in e d w as 0 .9 4 9 8 . T h e large u n c e r ta in t ie s o b ta in e d in th e s lo p e ,

in te r c e p t an d c o r r e la t io n c o e f f ic ie n t m a d e th e u se o f th e d o u b le -e s c a p e p e a k

n o t a m e n a b le to a n a ly t ic a l p u rp o s e s . F u r th e r s tu d y is in p ro g ress to u n d e rs ta n d

th e re a s o n s fo r th e p o o r c o r r e la t io n b e tw e e n th e d o u b le -e s c a p e p e a k an d th e

fu ll p e a k . I t h a s t o b e p o in te d o u t th a t th e d o u b le -e s c a p e p ea k / fu ll p e a k r a t io

w as m e a su re d a t th e sam e t im e as th e s in g le -esca p e/ fu ll p e a k r a t io . O n e p o ss ib le

e x p la n a tio n lie s in th e fa c t th a t th e s ta t is t ic a l d is tr ib u t io n fo r th e o c c u r r e n c e

o f a m o re c o m p le x p h e n o m e n o n su ch as d o u b le -e s c a p e sh o u ld h av e a la rg e r t

sta n d a rd d e v ia t io n th a n th a t o f a re la tiv e ly s im p le r p ro c e s s su c h as s in g le -e sca p e .

B y u tiliz in g th e fu ll p e a k re la tiv e e f f ic ie n c y , th e s in g le -esca p e/ fu ll p e a k

r a t io an d th e d o u b le -e sca p e / fu ll p e a k r a t io as a fu n c t io n o f e n e rg y , it w as p o s

s ib le to c o n s tr u c t th e re la tiv e e f f ic ie n c y as a fu n c t io n o f e n erg y fo r th e d o u b le -

an d sin g le -e sca p e p e a k s . T h is fu n c t io n is sh o w n in F ig .6. I t is im p o r ta n t to

o b se rv e th a t th e a r b itr a r y u n its used in e x p re ss in g th e re la tiv e e f f ic ie n c ie s o f

th e d o u b le -e s c a p e and sin g le -e sca p e p e a k s v ersu s e n e rg y are th e sa m e as th o s e

u sed in e x p re ss in g th e fu ll p e a k re la tiv e e f f ic ie n c y v ersu s e n e rg y cu rv e . I t is th u s

p o ss ib le to c o m p a r e m e a s u r e m e n ts m a d e b y u sin g th e fu ll p e a k , s in g le -e sca p e

a n d d o u b le -e s c a p e p e a k s . I t c a n b e o b se rv e d th a t as th e fu ll p e a k re la tiv e

e f f i c ie n c y as a fu n c t io n o f e n e rg y v aries b y a f a c to r o f 3 in th e ra n g e o f

5 .0 — 9 .0 M e V , th e v a r ia t io n o f th e e f f ic ie n c y o f th e s in g le -e sca p e p e a k v ersu s

e n e rg y is o n ly 2 0 % u n d e r th e sa m e e x p e r im e n ta l c o n d it io n s . C o n s e q u e n t ly , th e

u se o f th e s in g le -e sca p e p e a k in cre a se s o v e ra ll s e n s it iv ity o f th e d e te r m in a t io n

p a r t ic u la r ly w h e n h ig h -e n e rg y g am m a ra y s are m e a su re d . T h e p o o r re s u lts

o b ta in e d w ith th e d o u b le -e s c a p e p e a k s led u s to u se th e m o n ly in q u a lita t iv e

m e a s u r e m e n ts . A n e s t im a t io n o f th e a b s o lu te d e te c to r e f f ic ie n c y w as o b ta in e d

b y u sing c a lib ra te d so u rc e s o f 88Y , 60C o , 24N a and 49C a. F r o m th e c a l ib r a t io n

cu rv e , a d e te c to r e f f ic ie n c y o f 2 .8 • 1 0 _s w as o b ta in e d a t 5 M e V b y in te r p o la t io n .


FIG. 7. Typical spectrum o f a Maraging steel sample.

T A B L E I . R E S U L T S O F T H E A N A L Y S I S O F S Y N T H E T IC S A M P L E S

Sample No. Ni(%)

Cu(%)

Co(%)

Fe(%)

Ti(%)

Zn(%)

V(%)

Mn(%)

1 18.08 28.79 14.68 32.14 6.31

1 17.88 29.28 15.56 31.31 6.45

1 18.21 27.05 15.14 33.76 5.85

1 18.00 29.81 14.92 31.74 6.13

1 18.77 28.57 14.97 32.40 5.29

1 17.98 28.67 14.39 32.63 6.32

18.67 30.79 15.92 29.35 5.28

57.32 9.72 32.95

Average 18.23 28.99 15.08 31.90 5.95

Standarddeviation 0.35 1.16 0.52 1.36 0.49

True composition (%) 20.02 28.66 14.72 30.70 5.90 58.76 9.35 31.88

Error (%) -8 .9 4 + 1.15 +2.45 +3.91 +0.85 -2 .4 +3.96 +3.36


T A B L E I I . R E S U L T S O F T H E A N A L Y S I S O F A N i-1 0 -C r -2 0 .5 A L L O Y

S A M P L E

Trial No. Ni

(%)

Cr

(%)

Fe

(%)

Mn

(%)

1 10.46 20 .80 6 4 .1 0 4.71

2 9 .13 19.11 6 6 .7 7 4 .9 9

3 9 .1 9 21 .26 6 4 .1 9 4.65

4 11.23 18.76 65 .0 9 4.91

Average 10.18 20 .0 0 ' 6 5 .0 4 4.81

Standard deviation 0 .8 9 1.25 1.24 0.16

NBS reported composition (%) 10.10 20.51 6 2 .9 0 4 .62

Error (%) - 0 .8 0 - 2 .4 0 + 3 .4 0 + 4 .11

B . Q u a n tita t iv e a n a ly s is

F ig u re 7 sh o w s a sp e c tr u m o b ta in e d fr o m th e ir ra d ia tio n o f a M arag in g

s te e l c o n ta in in g F e ( 6 9 .8 % ) , N i ( 1 9 .0 % ) an d C o ( 7 .3 % ) .

T a b le I sh o w s th e r e s u lts o b ta in e d fo r th e a n a ly s is o f th e a r t i f ic ia l sa m p les .

T h e d a ta in d ic a te th a t g o o d r e p r o d u c ib il i ty is o b ta in e d . T a b le I a lso sh o w s th e

re s u lts o b ta in e d fo r th e Z n , V a n d M n sa m p le p re p a re d . T h e a c c u r a c y o b ta in e d is

g o o d . T a b le I I sh o w s th e re s u lts o b ta in e d fo r a N i-1 0 -C r -2 0 .5 s te e l sa m p le and

c h a r a c te r iz e s th e a b il i ty o f th e m e th o d to a n a ly se s te e l sa m p les . I t sh o u ld b e

p o in te d o u t th a t th e r e is 1.88% o f im p u r it ie s in th e s te e l.

T h e m e th o d d e sc r ib e d c a n b e s u c c e s s fu lly ap p lied to th e q u a n tita t iv e

a n a ly s is o f m a c r o -c o n s t i tu e n ts in a llo y s p ro v id e d th a t th e y ca n a ll b e m ea su red

b y n e u tr o n -c a p tu r e g a m m a -ra y s p e c tr o s c o p y . T h is im p lie s a g o o d c o m p r o m is e

b e tw e e n th e re la tiv e c o m p o s it io n o f th e sa m p le and th e S v a lu es o f th e c o m

p o n e n ts . W h en th is te c h n iq u e ca n b e a p p lie d , it h a s th e ad v a n ta g e o f b e in g

s im p le an d it d o e s n o t re q u ire a n y ty p e o f c o r r e c t io n fo r s e lf -a b s o r p t io n

sh ie ld in g , f lu x in h o m o g e n e ity d if fe r e n c e s b e tw e e n sa m p le and s ta n d a rd , e tc .

M o re o v e r, th e m e th o d ca n g e n e r a lly b e ap p lied to d e te r m in e m a ss r a t io s o f

c o m p o n e n ts . O n c e th e c o n c e n tr a t io n o f o n e o f th e s e c o n s t i tu e n ts is k n o w n b y

a p p ly in g a n y m e th o d , i t is th e n p o s s ib le to q u a n t ita te th e o th e r m e a su ra b le

c o m p o n e n ts . F u r th e r m o r e , th e m e th o d is a m e n a b le to c o m p u te r t r e a tm e n t o f

d a ta an d c a lc u la t io n s o f c o m p o s it io n .


I I . A P P L IC A T IO N T O T H E A N A L Y S I S O F C o , M o A N D N i IN

H Y D R O -D E S U L P H U R IZ A T IO N C A T A L Y S T S

A . Im p o r ta n c e of th e a n a ly s is

T h e m o s t im p o r ta n t c a ta ly s ts in th e p e tr o le u m in d u s try are th o s e w h ic h

c o n ta in C o -M o , M o -N i an d C o -M o -N i. T h e c a rr ie r fo r th e s e m e ta ls is u su a lly

a m ix tu r e o f s y n th e t ic 7-a lu m in a an d sm a ll a m o u n ts o f s ilic a . T h e r a t io o f

th e s e th r e e e le m e n ts in th e c a ta ly s t d e te r m in e s to a g re a t e x t e n t , a m o n g o th e r

v a ria b le s , th e c a ta ly t ic a c t iv ity . A g re a t in te r e s t e x is ts in th e p re p a r a t io n o f th e

c a ta ly s ts b y im p r e g n a tio n , c o -p r e c ip ita t io n , an d o th e r te c h n iq u e s w ith v ary in g

c o n c e n tr a t io n s o f th e s e th r e e e le m e n ts in th e 7-a lu m in a -s ilic a m a tr ix . T h e s e

c a ta ly s ts are im p o r ta n t fo r th e h y d ro -d e s u lp h u r iz a tio n an d h y d r o tr e a tin g

p ro c e ss e s . F u r th e r m o r e , i t is o f e c o n o m ic a l in te r e s t to s tu d y th e p o is o n in g o f

th e c a ta ly s ts a f te r p ro c e ss in g o f c ru d e s a t h ig h te m p e ra tu re s an d p re ssu re s .

T h e s e r e q u ire m e n ts m u s t b e m e t fo r a c c u r a te a n a ly s is o f C o , M o a n d N i.

B . P ro m p t g a m m a -ra y s p e c tr a

F ig u re 8 sh o w s th e sp e c tr u m o f a ty p ic a l a lu m in a c a ta ly s t sa m p le c o n

ta in in g C o O an d M 0O 3. I t c a n b e o b se rv e d th a t a 1 0 -m in ir r a d ia tio n is s u ff ic ie n t

to o b ta in th e p r e c is io n fo r th e c o u n ts o f C o , T i an d M o . I t sh o u ld b e p o in te d

o u t th a t th e M o p e a k a t 7 7 8 k e V is w e ll re so lv e d f ro m th e C o p e a k a t 7 8 6 k e V .

T h e re la t iv e ly h ig h c o u n ts a c c u m u la te d a t 2 2 2 3 k e V a re d u e to th e h y d ro g e n

p re s e n t in th e sa m p le . T h is p e a k , as w ill b e sh o w n la te r , is u sed to d e te r m in e

th e m o is tu re c o n te n t o f th e sa m p le . T h e d e la y e d g a m m a tr a n s i t io n o f a lu m in iu m

a t 1 7 7 9 k e V c a n a lso b e o b se rv e d . F ig u re 9 sh o w s a ty p ic a l s p e c tr u m o f an

a lu m in iu m c a ta ly s t c o n ta in in g N i. P e a k s aris in g fr o m th e sa m p le d u e to N i, T i ,

A l and C o a re o b se rv e d in c o n ju n c t io n w ith P b p e a k s arisin g f r o m th e G e ( L i)

sh ie ld in g .

A 1 0 -m in ir r a d ia tio n o f sa m p le N o .9 y ie ld s 2 8 1 0 ± 1 4 5 , 5 2 7 4 ± 1 3 4 and

6 5 8 7 ± 1 1 8 c o u n ts fo r C o , M o an d T i , re s p e c tiv e ly . A f te r a 3 0 -m in irra d ia tio n ,

th e sa m p le y ie ld s 1 0 7 1 ± 6 3 c o u n ts o b ta in e d a t 8 9 9 9 , 8 4 8 8 an d 7 9 7 7 k e V d ue

to n ic k e l. T h e T i c o u n ts o b ta in e d w ere 3 6 2 9 ± 5 8 a t 6 2 4 9 k e V .

T ita n iu m o x id e is a g o o d in te r n a l s ta n d a rd b e c a u s e it c a n b e e a s ily and

h o m o g e n e o u s ly m ix e d w ith th e sa m p le . F u r th e r m o r e , i ts n u c le a r p r o p e r t ie s

are a lso a d v a n ta g e o u s . I t s h ig h c r o s s -s e c t io n , c o u p le d w ith th e f a c t th a t it

p re s e n ts th re e p r o m in e n t p e a k s a t 6 7 6 0 k e V ( 5 4 % ) , 6 4 1 8 k e V ( 3 6 % ) an d

1 3 8 1 k e V ( 66% ) y ie ld in g a g o o d sp e c tr u m c o n tr a s t , m a d e th is e le m e n t a

ju d ic io u s c h o ic e as in te r n a l s ta n d a rd . T h u s , i t ca n b e u sed in b o t h e n e rg y

ra n g es s tu d ie d .

COUN

TS

/ 10

min

2000

1 0 0 0

ENERGY, keV

FIG.8. Typical catalyst spectrum measured from 500 to 2500 ke V.

LUBK

OW

ITZ

et al.

CO

UN

TS

/3

0

min

FIG.9. Typical catalyst spectrum measured from 5000 to 9000 ke V.

NEU

TRO

N-C

APTU

RE

GA

MM

A-RA

Y SP

EC

TR

OSC

OP

Y

TABLE III. LINEARITY OF CALIBRATION CURVES FOR COBALT, MOLYBDENUM AND NICKEL

Cobalt Molybdenum Nickel

Mass ratio, Counts ratio, Mass ratio, Counts ratio, Mass ratio, Counts ratio,Со/ТЮ2 Co/Ti М0/ТЮ2 Mo/Ti N i/Ti02 Ni/Ti

0 .1801 0 .5 2 5 9 0 .4 9 2 3 a 0 .2 6 3 7 0 .1 2 3 7 0 .2271

0 .3 0 5 5 e 0 .8 7 5 2 0 .6 0 6 6 a 0 .3 0 7 9 0 .2 6 5 8 0 .5 1 0 3

0 .4 9 0 9 a 1 .5396 0 .6 6 3 0 0 .3 2 1 9 0 .3 9 6 2 0 .7 5 8 5

0 .5 8 7 3 1 .7040 1 .0 4 9 5 a 0 .5 0 6 8 0 .5 2 9 9 0 .9 7 4 2

0 .6 5 2 3 a 2 .1 2 5 9 1 .2 2 8 0 a 0 .6 3 7 2 0 .6 8 2 4 1 .2406

0 .9 0 1 8 e 2 .7 6 3 0 2 . 1 1 1 0 e 0 .9 8 9 4

0 .9 0 7 4 2 .6 8 2 0 2 .3 5 9 3 1 .0 7 8 0

Ï .0 5 0 0 3 .3 0 7 8 5 .0 0 0 0 e 2 .4 0 8 8

Slope 3 .1 3 1 2 (0 .1 0 7 7 )b 0 .4 7 4 8 (0 .0 6 9 6 6 ) 1 .802 (0 .0 4 2 6 6 )

Intercept - 0 .0 4 3 3 4 (0 .0 7 5 0 1 )b 0 .0 8 7 8 7 (0 .0 1 5 3 5 ) 0 .0 2 1 8 4 (0 .0 1 8 9 7 )

Correlation coefficient 0 .9 9 6 5 0 .9 9 9 3 0 .9 9 9 2

a Added as oxide; remaining ones added as metal. b Values in parenthesis are standard deviations. c M atrix is powdered aluminium.

LUBK

OW

ITZ

et al.

N E U T R O N - C A P T U R E G A M M A - R A Y S P E C T R O S C O P Y

TABLE IV. STANDARD ADDITION ANALYSIS

141

Cobalt countsb Mass o f Molybdenum Mass o fCoO added counts0 M0O3 added(mg) (mg)

2071 ' 0 4 4 1 4 0

5749 23 .2 9 5599 4 8 .5 5

9197 39 .0 7 8556 151.13

12978 62 .2 7 11304 2 55 .45

Cobaltmassintercept

1 1 .0 2 mgM6O3massintercept

160.3 mg

% CoOa (dry basis) 1.2 2

% M0 O3 (dry basis) 13.93

a Sample weight 1151 mg on dry basis (sample N o.9). b Molybdenum counts constant. c Cobalt counts constant.

C. S ta n d a rd c a l ib r a t io n cu rv es

T h e d a ta o b ta in e d fo r th e a p p lic a t io n o f E q .( 9 ) a re sh o w n in T a b le I I I .

T h e d a ta w ere o b ta in e d f r o m p re p a re d a r t i f ic ia l a lu m in a sa m p le s sp ik e d w ith

C o , C o O , M o , M 0O 3 an d N i. T h e d a ta sh o w th a t th e c a l ib r a t io n is lin e a r ,

in d ic a tin g th a t th e m e ta l/ T i0 2 m ass r a t io is d ir e c t ly p r o p o r tio n a l to th e

m e ta l/ T i c o u n ts r a t io . T h e s e c a l ib r a t io n cu rv es w ere r e p r o d u c ib le f r o m d ay to

d ay so th a t it w as a c c e p ta b le to p re p a re o n e sta n d a rd w h e n a n a ly s in g sam p les .

S ta n d a rd s p re p a re d b y u sin g th e m e ta l o r an o x id e fo r m in an a lu m in a o r

a lu m in iu m m a tr ix y ie ld e d c o u n ts r a t io s th a t d id n o t d e p a rt f ro m lin e a r ity ,

in d ic a tin g th a t n o m a tr ix e f fe c ts are o b se rv e d .

D . S ta n d a rd a d d itio n s tu d y

T h e s ta n d a rd a d d itio n m e th o d w as e m p lo y e d to c h e c k th e p re v io u s re s u lts

o b ta in e d b y u sin g th e s ta n d a rd c a l ib r a t io n cu rv e s as w e ll as th e p o s s ib le in te r

fe r e n c e s in th e C o an d M o d e te r m in a t io n s . M o re o v e r , th e u se o f a T i 0 2 -m a tr ix

c o m b in a t io n m a y c a u se sh a d o w in g e f fe c ts . S in c e th e a c t iv a tio n c r o s s -s e c t io n

is a fu n c t io n o f n e u tr o n e n e rg y , re s o n a n c e p e a k s (w h e n p r e s e n t) m a y in te r fe r e

w ith th e n e u tr o n -e le m e n t in te r a c t io n . T h is s i tu a t io n , o n th e o th e r h a n d , ca n

b e c o m p lic a te d b y th e f a c t th a t th e w a te r -m o d e ra te d r e a c to r e m p lo y e d in th is


w o rk h a s a d a ily c y c l ic o p e r a t io n o f 6 h d u rin g w h ic h th e w a te r te m p e ra tu re

m a y in c r e a s e ca u sin g a s h if t in th e c ro s s -s e c tio n -n e u tr o n -e n e rg y f u n c t io n and th u s

v a ry in g th e n e u tro n -s a m p le e le m e n t in te r a c t io n . T h e s ta n d a rd a d d it io n m e th o d

sh o u ld c o n f ir m th e p re s e n c e o r a b s e n c e o f th e s e e f f e c ts . T a b le IV sh o w s th e

r e s u lts fo r C o O and M o 0 3 o b ta in e d fo r a ty p ic a l c o m m e rc ia l c a ta ly s t . A

c o m p a r is o n o f th e % C o O an d % M o 0 3 o b ta in e d b y th e c a l ib r a t io n te c h n iq u e ,

as sh o w n in T a b le I I I , w ith th e v alu es o b ta in e d in T a b le IV , sh o w s an a b s o lu te

d if fe r e n c e o f 0 .1 0 % f o r th e M o 0 3 c o n t e n t an d a d if fe r e n c e o f 0 .1 1 % f o r th e

C o O c o n te n t . T h is in d ic a te s th a t th e tw o m o d e s are in a g re e m e n t. M o re o v e r ,

th e c h o ic e o f T i 0 2 as in te r n a l s ta n d a rd is a p p r o p r ia te fo r th e a n a ly s is . N ic k e l

p ro m p t-g a m m a tra n s itio n s u sed o c c u r in th e ra n g e o f 7 .9 7 7 — 8 .9 9 9 M e V and

are fr e e o f in te r fe r e n c e s ; th u s th e sta n d a rd a d d itio n te c h n iq u e w as n o t e m p lo y e d .

E . C o m p a r is o n o f p r o m p t g a m m a a n a ly s is w ith n e u tr o n a c t iv a tio n

a n a ly s is (N A A )

T h e re s u lts o f a n a ly s in g tw e lv e c o m m e r c ia l c a ta ly s ts b y p ro m p t g a m m a -ra y

s p e c tr o s c o p y and N A A are p re s e n te d in T a b le V . A sta n d a rd r e fe r e n c e c a ta ly s t

is a t p re s e n t n o t a v a ila b le so it is d if f ic u lt to assess a c c u r a c y . H o w e v e r, th e

tw o m e th o d s d o sh o w a s a t is fa c to r y a g re e m e n t. In th e C o d e te r m in a t io n , fo u r

sa m p le s sh o w a d if fe r e n c e o f g re a te r th a n 1 0 % b y th e N A A a n a ly s is as r e fe r e n c e

f o r th e c a lc u la t io n . In th e M o 0 3 d e te r m in a t io n , th r e e sa m p les sh o w a re la tiv e

d if fe r e n c e o f 1 0 % o r m o re . I t is in te re s tin g to n o te th a t th e av erag e o f th e s e

re la t iv e d if fe r e n c e s w ith reg ard to sign is + 1 .8 3 % fo r th e C o O d e te r m in a t io n s

an d + 1 . 3 2 % fo r th e M o 0 3 d e te r m in a t io n s , in d ic a tin g th a t n e ith e r m e th o d

sh o w e d a n y b ia s . S a m p le N o . 1 3 is a s y n th e t ic c a ta ly s t w h ic h w as p re p a re d b y

im p r e g n a tio n o f a b o u t 1 0 0 g o f 7-a lu m in a . T h e c o n c e n tr a t io n o f th e im p re g n a

tin g M o 0 3 s o lu t io n w as d e te r m in e d b y N A A an d th e w ash s o lu t io n s w ere

a n a ly se d b y a to m ic a b s o r p tio n . T h u s , a M o 0 3 c o n te n t c o u ld b e c a lc u la te d .

T h e p e r c e n ta g e d if fe r e n c e s o f th e M o 0 3 d e te r m in a t io n o n th is sa m p le w ere

6 .3 8 % an d 5 .1 7 % as d e te r m in e d b y p ro m p t g a m m a -ra y s p e c tr o s c o p y an d N A A ,

re s p e c tiv e ly . O n ly th r e e o f th e c o m m e r c ia l c a ta ly s ts a n a ly se d c o n ta in e d N i, and

th e N iO c o n te n t o f th e c a ta ly s ts w as co m p a r e d w ith th e c o rre s p o n d in g a n a ly sis

b y th e g ra v im e tr ic te c h n iq u e u s in g d im e th y lg ly o x im e . T h e d if fe r e n c e s b e tw e e n

th e n ic k e l d e te r m in a t io n s b y b o t h m e th o d s an d b y th e g ra v im e tr ic m e th o d

are b e tw e e n 4 an d 5% . T h e p r o m p t g a m m a -ra y sta n d a rd d e v ia tio n e x p e r im e n ta lly

o b ta in e d b y s ta t is t ic a l t r e a tm e n t o f th e d a ta o f sev e ra l d if f e r e n t a n a ly se s o f th e

sa m p le is q u ite s im ila r to th a t o b ta in e d b y th e s ta t is t ic a l t r e a tm e n t o f c o u n tin g

d a ta . T h is w o u ld se e m to im p ly t h a t th e u se o f th e in te r n a l s ta n d a rd is e f fe c t iv e

in c o r r e c t in g f o r g e o m e tr y e f fe c t s f ro m sa m p le to sa m p le , f lu x g ra d ie n ts w ith in

th e sa m p le , and f lu x v a r ia t io n s w ith t im e . C a lc u la te d sta n d a rd d e v ia tio n s o n

c o u n tin g s ta t is t ic s in N A A a re a b o u t 2 % as c o m p a r e d w ith 5 .6 % f o r th e rea l


T A B L E V . A N A L Y S I S O F C A T A L Y S T S A M P L E S B Y P R O M P T

G A M M A -R A Y S P E C T R O S C O P Y (P -7 ) A N D N A A

Sample% CoO

P-7 NAA

% M0O 3

P-7 NAA

% NiO

P-7 NAAGravim etric

1 3.32(0 .2 0 )1

3.06(0 .0 5 )2

13.21(0 .6 5 )1

12.81(0 . 1 2 ) 2

a

2 1.74 1.87 8 .59 9 .04 4.21 3 .82 4 .04(0 .0 9 )1 (0 .0 6 )2 (0 .5 3 ) 1 (0 .1 3 )2 (0 .2 3 )2 ( 0 .1 7 )2 (0 .0 7 )

3 < 0 .0 5 0.02 15.03 13.23 3.71 3 .3 9 3 .58(0 .7 5 ) 1 ( 0. 1 2)2 (0 .2 l )2 ( 0 . 1 2 ) 2 (0 .0 4 )

4 3.61(0 .1 7 )1

3 .3 9(0 .0 6 )2

8 .79(0 .5 5 ) 1

8.66(0 .1 3 )2

a

5 2.71( 0. 12 )2

2 .47(0 .0 5 )2

13.23(0 .66)2

11.85(0 .1 3 )2

a

6 3 .19(0 .1 3 )2

2.71(0 .0 5 )2

14.49(0 .5 8 )2

14.61( 0 . 12 ) 2

a

7 2 .89(0 .1 4 )2

2 .59(0 .0 6 )2

16 .14 ( 0 .6 1)2

15.29 (0. 1 l )2

a

8 3 .70(0 .1 5 )2

3 .58(0 .0 5 )2

1 1 .2 2(0 .5 3 )2

10.20(0 .1 3 )2

a

9 1.33 1.51 14.03 14.88 2.68 2 .78 2 .59(0 .0 7 )1 (0 .0 7 )1 (0 .6 0 )1 (0 .6 3 )1 (0 . 1 l ) 1 ( 0 .1 7 )1 (0 .0 3 )

10 4 .7 8 (0 .1 7 )2

4 .7 8(0 .0 5 )2

14.27(0 .5 8 )2

14.21 ( 0. 1 l )2

a

11 3.85(0 .1 5 )2

3 .5 4(0 .0 5 )2

13.01(0 .6 2 )2

13.86( 0. 12 )2

a

12 3.36(0 .1 5 )2

2 .7 9(0 .0 5 )2

13.91(0 .6 2 )2

15.44 (0. 1 l )2

a

13b a 11.78(0 .6 0 )2

11.92( 0. 12 )2

a

a Indicates m etal is not reported as a constituent by manufacturer. b Synthetic catalyst o f 12.57% M0O 3 .1 Experim ental standard deviation.2 Standard deviation calculated on the basis o f counting statistics.

T A B L E V I , D E T E R M I N A T I O N O F H Y D R O G E N B Y N E U T R O N - C A P T U R E G A M M A -R A Y S P E C T R O S C O P Y

Sample name Hcounts

Ticounts

Ti02weight(mg)

S ratio(counts Ti/mg ТЮ2)

Sample weight, W (g)

HW100

S

%Htheoretical

%Hfound

%H20 'incatalyst

Potassium biphthalate 2179 3461 85.8 40.3 0.6927 7798 2.468 a

Potassium biphthalate 2015 . 2551 64.7 39.4 0.6273 8146 2.468 a

Cupferron 2397 5171 95.1 54.4 0.2336 18873 5.847 5.84

Zirconium oxychloride 8H20 2604 4188 155.3 27.0 0.5987 16127 5.005 4.99

Sodium barbital 2775 5688 141.6 40.2 0.3990 17314 5.378 5.36

Sodium molybdate 2H2 0 1226 4391 109.7 40.0 0.5676 5396 1.666 1.67

Benzoic acid 3977 - 5419 148.5 36.5 0.6482 16814 4.953 5.20

Na2 EDTA 2H20 2849 2555 51.5 49.6 0.3665 15669 4.874 4.85

Catalyst 1 1255 3370 82.6 40.8 0.7310 4208 b 1.30 11.6

Catalyst 1 1375 2940 71.1 4.13 0.7608 4374 b 1.35 ■ 12.1

a Reference material. b Not determinable.

144 LU

BKO

WITZ

et

al.


s ta n d a rd d e v ia tio n . T h is m a y im p ly th a t th is m e th o d is a f fe c te d b y f lu x v aria

t io n s in sa m p le an d sta n d a rd d u rin g ir ra d ia tio n b y c o u n tin g g e o m e tr y , an d b y

p h y s ic a l d if fe r e n c e s b e tw e e n s ta n d a rd an d sam p les.

F . C o n c lu s io n s

T h e p ro m p t g a m m a -ra y a n a ly s is o f c a ta ly s ts is a p re c is e , ra p id an d n o n

d e s tru c tiv e a n a ly s is o f h y d ro -d e s u lp h u r iz a tio n c a ta ly s ts . T h e t im e re q u ire d fo r

a C o an d M o d e te r m in a t io n is 1 0 m in , an d an a d d itio n a l 3 0 m in is re q u ire d fo r

a N i a n a ly sis . T h e c o rre s p o n d in g N A A a n a ly sis re q u ire s a b o u t 3 —4 d a y s .

F u r th e r m o r e , b e c a u s e o f th e a lu m in a -s ilic a m a tr ix , th e m in e r a liz a tio n o f th is ty p e

o f c a ta iy s t is a lso r a th e r t im e -c o n s u m in g . T h e a p p lic a t io n s o f p r o m p t g am m a-

ra y s fo r r o u t in e a n a ly s is u t il iz in g a r e a c to r n e u tr o n b e a m hav e b e e n fe w , p erh a p s

b e c a u s e o f th e c o m p le x i ty o f o b ta in in g a s u ita b le c o ll im a te d n e u tr o n b e a m w h o se

c h a r a c te r is t ic s d e p e n d o n th e n e u tr o n s o u rc e u sed . H o w ev er, o n c e th e b e a m

b e c a m e a v a ila b le , th is w o rk sh o w e d th a t p r o m p t g a m m a ra y s ca n b e u s e fu l in

a n a ly s in g sa m p les c o n ta in in g e le m e n ts w h o se ir ra d ia tio n an d c o u n tin g c h a ra c

te r is t ic s m a y m a k e th e m la b o r io u s to a n a ly se b y c o n v e n tio n a l N A A . T h is w o rk

h a s a lso sh o w n th a t th e u se o f e n e rg y s p e c tr a in th e ra n g e o f 5 0 0 — 2 5 0 0 k e V

is c o n v e n ie n t fo r r o u t in e a p p lic a t io n s .

I I I . D E T E R M IN A T IO N O F W A T E R IN H Y D R O -D E S U L P H U R IZ A T IO N

C A T A L Y S T S

A . In tr o d u c t io n

T h e d e te r m in a t io n o f h y d ro g e n b y n e u tr o n -c a p tu r e g a m m a -ra y s p e c tr o

s c o p y w as a p p lie d to c o a l b y R a sm u ss e n an d H u k a i [ 1 0 ] . W e h av e u sed p ro m p t

g a m m a ra y s to d e te r m in e m o is tu r e in h ig h ly so rp tiv e s u r fa c e s su c h as a lu m in a s ,

z e o lite s an d im p re g n a te d a lu m in a c a ta ly s ts . C a ta ly s ts b a sed o n 7-a lu m in a are

k n o w n to a b s o r b 6- 20% o f th e ir w e ig h t in w a te r , d e p e n d in g o n s u r fa c e

c h a r a c te r is t ic s an d e n v ir o n m e n ta l fa c to r s .

B . A p p lic a tio n to th e d e te r m in a t io n o f h y d ro g e n in o rg a n ic c o m p o u n d s

T h e u se o f E q .( 1 2 ) re q u ire s th e d e te r m in a t io n o f К w h ic h w as d e te r m in e d

b y u sing p o ta s s iu m b ip h th a la te as a s ta n d a rd . O n c e th is c o n s ta n t w as k n o w n ,

th e p e r c e n ta g e o f h y d ro g e n w as d e te r m in e d in a s e le c te d n u m b e r o f c o m p o u n d s .

T h e d a ta are sh o w n in T a b le V I . In a ll ca se s th e c o u n ts o f th e h y d ro g e n p e a k

a t 2 2 3 2 k e V an d th e c o u n ts o f th e T i 0 2 p e a k a t 1 3 8 1 k e V w e re u sed in c o n

ju n c t io n w ith E q . ( 1 2 ) . T a b le V I sh o w s th a t th e a g re e m e n t is e x c e l le n t b e tw e e n

T A B L E V I I . S T A N D A R D A D D IT IO N M E T H O D F O R T H E M O I S T U R E D E T E R M I N A T I O N O F C A T A L Y S T

N o . l B Y N E U T R O N - C A P T U R E G A M M A -R A Y S P E C T R O S C O P Y

TrialNo.

Ha(counts)

Ti(counts)

Ti02 weight (mg)

S ratio(counts Ti/mg ТЮ2)

H100

S

H20 added (mg)

1 1814 2130 45.4 46.9 3866 0

2 1986 1971 47.5 41.5 4786 18.7

3 2492 2202 49.8 44.2 5635 39.2

4 2933 2705 57.'8 46.8 6267 58.5

5 3054 2835 62.6 45.3 6743 77.9

6 3392 2429 53.2

Slope

Intercept

Correlationcoefficient

% h 2o

45.7

3 5 .6 + 1 .9 9

4 0 5 7 ± 177

0.9938

11.4%

7429 97.1

a Weight of catalyst was 1.000 g.


th e % H fo u n d an d th e c o rre s p o n d in g th e o r e t ic a l v a lu e c a lc u la te d f ro m th e

m o le c u la r w e ig h t a n d c o m p o s it io n . T h e d a ta a lso sh o w th a t T i 0 2 is a s u ita b le

in te r n a l s ta n d a rd fo r th e h y d ro g e n d e te r m in a t io n .

C . S ta n d a rd a d d it io n s tu d ie s

T h e sta n d a rd a d d it io n m e th o d w as u sed to c h e c k th e re s u lt o b ta in e d b y

d ir e c t c o m p a r is o n w ith s ta n d a rd s . I t c a n b e o b se rv e d f ro m E q . ( 1 2 ) th a t a p lo t

o f H X 1 00/ SW v ersu s w eig h t o f w a te r ad d ed to id e n tic a l a l iq u o ts o f th e sam p le

sh o u ld y ie ld a s tra ig h t lin e w h o se in te r c e p t p e r m its th e c a lc u la t io n o f th e

o r ig in a l c o n te n t o f w a te r in th e sa m p le . T h e d a ta o b ta in e d sh o w g o o d lin e a r ity ,

as ca n b e se e n fr o m T a b le V I I . T h e c o n te n t o f w a te r in c a ta ly s t N o . l d e te r -

' m in e d b y th is m e th o d is 1 1 .4 % w h ic h is in g o o d a g re e m e n t w ith th e v alu es

o b ta in e d b y s ta n d a rd c o m p a r is o n , as sh o w n in T a b le V I.

D . C o m p a r is o n o f m o is tu r e d e te r m in a t io n s in c a ta ly s ts b y p r o m p t

g a m m a -ra y s p e c tr o s c o p y , th e r m o g r a v im e tr ic a n a ly s is (T G A ) and

o v e n d ry in g

B e c a u s e it is n o t fe a s ib le to a c q u ir e a r e fe r e n c e h y d ro -d e s u lp h u r iz a tio n

c a ta ly s t w ith a sta n d a rd w a te r c o n te n t an d s in c e in th e p r o m p t g a m m a -ra y a n a ly sis

it is assu m ed th a t th e h y d ro g e n m e a su re d is s o le ly d u e to w a te r , th e p ro m p t

g a m m a -ra y a n a ly se s w e re c h e c k e d b y th e r m o g r a v im e tr ic a n a ly se s ( T G A ) . T h e

c a ta ly s ts s u b je c te d to T G A c o n ta in e d 1 0 - 1 5 % M o 0 3 a n d th u s c o u ld n o t b e

h e a te d to te m p e ra tu re s a b o v e 5 0 0 ° C s in c e lo sse s o f M ó w o u ld o c c u r . A c o m

p a ris o n o f th e re s u lts o b ta in e d b y p r o m p t g a m m a -ra y a n a ly se s a n d th e w e ig h t

lo ss o b ta in e d a t 5 0 0 ° C is sh o w n in T a b le V I I I . T h e d if fe r e n c e s b e tw e e n th e

p ro m p t-g a m m a d e te r m in a t io n s and th e w e ig h t lo sse s o b ta in e d a t 5 0 0 ° C seem

fa ir ly c o n s ta n t an d a v erag e 9 .2 0 % lo w e r fo r th e T G A s tu d y .

T h e lo w e r v a lu es a re d u e to th e f a c t th a t w a te r re m a in s a t 5 0 0 ° C and

c a n n o t b e e lim in a te d w ith o u t d e c o m p o s in g th e M o 0 3 in th e c a ta ly s t . T h is w as

c o n f ir m e d b y h e a tin g a n o n -im p re g n a te d c a ta ly s t s u p p o r t to te m p e r a tu r e s o f

7 0 0 ° C w h e re , r e p o r te d ly , 7-a lu m in a lo s e s a ll i ts w a te r c o n te n t . T h e re s u lt

o b ta in e d is in g o o d a g re e m e n t w ith th e p ro m p t-g a m m a v a lu e . T h e 2 0 0 ° C

d if fe r e n c e y ie ld s a p e r c e n ta g e d if fe r e n c e w e ig h t lo ss o f 9 .0 5 % w h ic h is s im ila r

to th e re la t iv e p e r c e n ta g e w a te r d if fe r e n c e a s d e te r m in e d in th e c a ta ly s t a t

5 0 0 ° C an d b y p ro m p t g a m m a -ra y a n a ly s is . F u r th e r m o r e , a n a tu ra lly o c c u rr in g

a lu m in o -s il ic a te o r z e o li te a lso y ie ld s a w e ig h t loss w h ic h , e x p re ss e d as p e r c e n t

w a te r , is a lso in g o o d a g re e m e n t w ith th e p ro m p t-g a m m a d e te r m in a t io n . O v ën

d ry in g y ie ld s c o n s id e r a b ly lo w e r re s u lts th a n th o s e o f th e o th e r tw o m e th o d s

sh o w in g th a t th is t e c h n iq u e is u n s a t is fa c to r y in t o ta l ly re m o v in g th e w a te r

c o n t e n t in th e c a ta ly s t .


T A B L E V I I I . C O M P A R IS O N O F M O I S T U R E D E T E R M IN A T IO N S B Y

N E U T R O N C A P T U R E G A M M A -R A Y S P E C T R O S C O P Y (N C ), T G A A N D

O V E N D R Y IN G

Sample

% H20 determined

NC TGAa Oven dried

Catalyst 1 11.86 11.30 8.08

2 12.65 11.33 8.62

3 22.66 20.54 16.81

4 12.03 10.82 9.06

5 19.27 . 17.31 15.06

Zeolite 22.79 22.10 -

Catalyst support 16.88 15.08 -(16.58)b

a Weight loss at 500°C. b Weight loss of catalyst support at 700°C.

I V . O T H E R R E C E N T D E V E L O P M E N T S

A p p lic a tio n s o f n e u tr o n c a p tu re are n o t t o o n u m e ro u s a lth o u g h th e y are

b e c o m in g m o re n u m e ro u s . R e c e n t ly , G la d n e y e t a l. [ 1 1 ] d e te r m in e d В an d C d

n o n -d e s tru c tiv e ly . O w in g to th e la rg e c r o s s -s e c t io n o f th e s e tw o e le m e n ts

( 3 8 4 0 an d 2 4 5 0 b a rn s , r e s p e c t iv e ly ) , th e s e r e s e a rc h e rs w ere a b le to d e te r m in e

th e tw o e le m e n ts a t c o n c e n tr a t io n s o f less th a n 1 p p m w ith a sta n d a rd d ev ia

t io n o f 3 —7% . H e n k e lm a n an d B o r n [ 1 2 ] h av e r e c e n t ly d iscu ssed th e

p r o b le m a tic s o f d e te rm in in g B o r o n using th e 10B ( n ,a ) 7L i r e a c tio n .

P o u ra g h a b a g h e r an d P r o f io [ 13 ] r e c e n t ly r e p o r te d th e d e te r m in a t io n o f V

in c ru d e s in a s tre a m b y u tiliz in g a 252C f s o u rc e . V a n a d iu m p r o m p t g a m m a -ra y

tr a n s it io n s w e re u sed to a n a ly se th e c ru d e a t c o n c e n tr a t io n s o f a b o u t 100 p p m .у

Z w ittlin g e r [ 1 4 ] r e p o r te d th e a n a ly se s o f a llo y s o f s im ila r c o m p o s it io n

to th e o n e s re p o r te d in th is w o rk . H o w e v e r, b e c a u se o f th e c h a r a c te r is t ic s o f

th e b e a m , sa m p le s o f 5 0 g an d ir r a d ia t io n t im e s o f a b o u t 1 0 h w ere re q u ire d .

T h e m e th o d u sed sta n d a rd c o m p a r is o n w h ic h re q u ire s th a t th e t o t a l m a c r o

s c o p ic c r o s s -s e c tio n o f b o t h th e sta n d a rd an d th e sa m p le m u st b e v e ry s im ila r.

T h is im p lie s a p r io r k n o w le d g e o f th e e le m e n ta l c o m p o s it io n o f th e sa m p le .

W ith th e a d v e n t o f 252C f s o u rc e s , i t is p o ss ib le th a t th e a p p lic a t io n s o f

p ro m p t g a m m a -ra y s w ill in c r e a s e , s in c e it is e a s ie r to o b ta in a th e r m a liz e d b e a m

r e la t iv e ly fr e e o f in te r fe r in g g a m m a ra d ia tio n .


R E F E R E N C E S

[1] H E U R T E B ISE , M., LUBKOW ITZ, J .A ., J . Radioanal. Chem. 31 (1 9 7 6 ) 503.[2] H E U R T E BISE , M., LUBKOW ITZ, J.A ., Trans. Am. Nucl. Soc. 21 2 (1 9 7 5 ) 60.[3 ] H E U R T E BISE , M., BUENAFAM A, H., LUBKOW ITZ, J.A „ Anal. Chem. 4 8 13

(1 9 7 6 ) 1971.[4] H E U R T E BISE , M., LUBKOW ITZ, J .A ., Anal. Chem. 48 14 (1 9 7 6 ) 2 143 .[5 ] H E U R T E BISE , M., LUBKOW ITZ, J.A ., IV IC , Center o f Petroleum and Chemistry,

Report No. 16, Sep. 1975.[6 ] H E U R T E BISE , M., LUBKO W ITZ, J.A ., Proc. Modern Trends in Neutron Activation

Analysis, Munich, Sep .1975.[7] D U F F E Y , D., EL-K A D Y, A ., SE N FT L E, F .E ., Nucl. Instrum. Methods 8 0 (1 9 7 0 ) 149.[8 ] SE N FT L E, F .E ., M OO RE, H.D., L EEP, D .B ., EL-K A D Y , A., D U F F E Y , D ., Nucl.

Instrum. Methods 93 (1 9 7 1 ) 425 .[9] H E U R T E BISE , M., LUBKOW ITZ, J .A ., Anal. Chem. 45 (1 9 7 3 ) 47.

[10] RASM USSEN , N.C., HUKAI, Y ., Trans. Am. Nucl. Soc. 12 (1 9 6 7 ) 29.[1 1 ] GLA D N EY, E .S ., JU R N E Y , E ., C U R TIS, D .b ', Anal. Chem. 4 8 14 (1 9 7 6 ) 2139.[1 2 ] HENKELMAN, R ., BO RN , H J . , J . Radioanal. Chem. 16 (1 9 7 3 ) 4 7 3 .[1 3 ] R EZA POURAGHABAGH ER, P RO FIO , E ., Anal. Chem. 46 (1 9 7 4 ) 1223.[1 4 ] ZW ITTLIN G ER, H., J . Radioanal. Chem. 14 (1 9 7 3 ) 147.

A P P L I C A T I O N S O F P O S I T R O N A N N I H I L A T I O N

P. H A U T O J Ä R V I , A . V E H A N E N

H e ls in k i U n iv e rs ity o f T e c h n o lo g y ,

E s p o o , F in la n d

Abstract

APPLICATIONS O F PO SITRO N ANNIHILATION.The paper reviews various applications o f the positron annihilation technique. When

positrons from radioisotopes are injected into a condensed medium they rapidly slow down to thermal energies, live a relatively long tim e in equilibrium with the surrounding m atter and finally annihilate with electrons almost always into two gamma quanta. By studying this annihilation radiation with m ethods developed in nuclear physics, inform ation can be obtained about the state o f the annihilating electron-positron pair and thus about the electronic structure o f the medium. The lifetim e distribution o f positrons is measured with a coincidence technique using fast scintillators and time-to-pulse height conversion. The mean lifetim e o f free positrons varies from 0.1 to 0.5 ns depending on the electron density o f the medium. The momentum o f an electron-positron pair is studied by measuring the angular correlation o f the annihilation quanta. The deviation o f the tw o gamma rays from collinearity is determined almost totally by the momentum o f the electron, especially if the positron is free. Thus, the angular correlation curve reflects the m omentum distribution o f electrons in the medium. This techique is powerful, e.g. in Ferm i surface studies o f metals and alloys. Positrons are found to be trapped in crystal lattice defects like metal vacancies, voids and dislocations. This phenomenon is widely utilized in the determination o f vacancy form ation energies and in the study o f void form ation. The annihilation radiation from the trapped positrons contains also unique inform ation about the'in tem al electronic structure o f defects. In molecular substances the positron can capture an electron while slowing down and form a positronium atom . The form ation probability and lifetim e o f positronium depends strongly on the properties o f the medium. Because o f its hydrogen-like structure the positronium atom takes part in chemical reactions with different kinds of atoms and molecules. Positronium chem istry deals with the chemical aspects o f positron annihilation. The medical applications are concentrated on the development o f positron cameras. With these instrum ents it is possible to image the distribution of positron emitters like 11C, 13N and lsO and thus to follow various physiological processes in the human body.

1. IN T R O D U C T IO N

P o s it r o n p h y s ic s is c o n c e r n e d w ith a n n ih ila t io n p h e n o m e n a o f lo w -e n e rg y

p o s itr o n s in m a tte r . T h e e x is te n c e o f th e p o s itr o n w as p re d ic te d b y D ir a c [ 1 ]

in 1 9 3 0 an d i t w as e x p e r im e n ta l ly o b se rv e d b y A n d e rs o n [ 2 ] in 1 9 3 2 . T h e b ir th

an d ra p id g ro w th o f p o s itr o n p h y s ic s o c c u r r e d in th e e a r ly 1 9 5 0 s as th e a p p lic a

b i l i ty o f p o s itr o n a n n ih ila t io n t o th e s tu d y o f th e e le c t r o n ic s tr u c tu r e o f m a tte r

w as re a liz e d . S in c e th e n th e f ie ld h a s g ro w n v e ry ra p id ly , as sh o w n in F ig . 1 [3 ]

151

152 H A U T O J Ä R V I and V E H A N E N

Y E A R

FIG .l. The number o f annually published papers dealing with the study o f low-energy positrons and positronium [.?].

w h ic h in d ic a te s th e n u m b e r o f a n n u a lly p u b lish e d p a p e rs . T h e g r o w th r a te is a t

p re s e n t a b o u t 2 0 % a y e a r . T h e re a s o n s f o r th e rap id g ro w th lie in th e u n iq u e

in fo r m a t io n o b ta in e d b y p o s itr o n s , e .g . o n c r y s ta l la t t i c e d e fe c ts . A t th e sam e

t im e , c o m m e r c ia l an d in e x p e n s iv e d e v ic e s b e c a m e a v a ila b le .

T h e r e h a v e b e e n sev era l in te r n a tio n a l c o n fe r e n c e s sp e c ia lly d e v o te d to

p o s itr o n a n n ih ila t io n s tu d ie s . T h e f ir s t w as h e ld in D e t r o i t , U S A , in 1 9 6 5 [ 4 ] , th e

se c o n d in K in g s to n , C a n a d a [ 5 ] , th e th ird in Ö ta n ie m i, F in la n d [6] a n d th e m o s t

r e c e n t in H e ls in g ^ r, D e n m a r k in 1 9 7 6 [ 7 ] . T h e f i f t h c o n fe r e n c e w ill b e h e ld in

J a p a n in 1 9 7 9 an d a lre a d y th e 1 9 8 2 c o n fe r e n c e h a s b e e n sc h e d u le d f o r th e U S A .

P O S IT R O N A N N I H I L A T IO N 153

T h e p o s i tr o n a n n ih ila t io n te c h n iq u e p ro v id e s sev e ra l a d v a n ta g e s in th e stu d y

o f m a tte r . I t p ro v id e s a n o n -d e s tru c tiv e te s t in g o f m a te r ia ls b e c a u s e t h e in fo r m a tio n

is c a rr ie d o u t o f th e m a te r ia l w ith p e n e tr a tin g a n n ih ila tio n q u a n ta . T h e a re a

sc a n n e d b y p o s itr o n s is ty p ic a lly c a . 3 0 m m 2 . T h e la y e r th ic k n e s s is

ca . 3 0 — 1 5 0 m g/ cm 2 d e p e n d in g o n th e p o s itr o n e m it te r b u t b e in g q u ite in se n sitiv e

t o th e a to m ic n u m b e r o f th e sa m p le . T h e p re p a r a t io n o f sa m p le s is re la t iv e ly

ea sy an d in so m e a p p lic a t io n s a lso ‘in s i tu ’ s tu d ie s , fo r e x a m p le o n d y n a m ic

p h e n o m e n a a t e le v a te d te m p e ra tu re s , a re p o s s ib le . S e v e ra l rev iew s o n p o s itr o n

p h y s ic s in th e s tu d y o f c o n d e n s e d m a t te r hav e b e e n p u b lis h e d [8— 11 ].

2 . P O S IT R O N M E T H O D

2 .1 . P o s itr o n e x p e r im e n t

W h e n e n e r g e tic p o s itr o n s f r o m r a d io is o to p e s are in je c te d in to a c o n d e n se d

m e d iu m th e y f ir s t s lo w d o w n to th e r m a l e n e rg ie s in a v e ry s h o r t t im e , o f th e

o r d e r o f 1 p s [ 1 2 ] . T h e p o s itr o n th e n liv es in th e m e d iu m in th e r m a l e q u ilib r iu m

fo r a so m e w h a t lo n g e r p e r io d an d th e m e a n l if e t im e is in th e ra n g e 0.1 to 0 .5 ns ,

c h a r a c t e r is t i c 'o f e a c h m a te r ia l . F in a l ly , th e p o s itr o n a n n ih ila te s w ith a n e le c tr o n

o f th e su rro u n d in g m e d iu m p r e fe r e n t ia l ly in to tw o 5 1 1 -k e V g a m m a q u a n ta .

T h e a b o v e p ic tu r e is d is to r te d in so m e m e d ia w h ere p o s itr o n iu m fo r m a t io n

o c c u r s d u rin g th e s lo w in g d o w n o f th e p o s itr o n . T h is p h e n o m e n o n , h o w e v e r,

is t r e a te d s e p a ra te ly in S e c t io n 5 .

F ig u re 2 sh o w s th e s c h e m a tic c o u rs e o f th e p o s itr o n a n n ih ila t io n e x p e r im e n t

in w h ic h th e m o s t c o m m o n ly u s e d r a d io is o to p e 22N a is in v o lv e d . A lm o s t

s im u lta n e o u s ly w ith th e p o s itr o n , th e r a d io is o to p e e m its an e n e r g e tic ( 1 .2 8 -M e V )

p h o to n u sed as th e b ir th sig n a l. T h e l if e t im e o f th e p o s itr o n c a n th u s b e

m e a su re d as th e t im e d e la y b e tw e e n th e b ir th an d th e a n n ih ila t io n g am m as.

No-22У

1280 keV

П П ! ) T \ } n n n u n

© e+у «■— y/VT— ' * 1— W — * Y

511 keV © e" 511 keV

FIG.2. The positron experiment. Positrons from radioactive isotopes penetrate into the material, slow down and annihilate with electrons. The gamma radiation gives information on the structure o f the material.


B y m ea su rin g th e an g le b e tw e e n th e a n n ih ila tin g q u a n ta w e c a n d e d u c e th e

m o m e n tu m o f t h e e le c t r o n -p o s it r o n p a ir c h a r a c te r iz in g th e e le c t r o n ic s tr u c tu r e

o f th e m e d iu m . T h e m o t io n o f th e a n n ih ila tin g p a ir ca u se s a D o p p le r -s h if t to th e

a n n ih ila tio n r a d ia tio n and th is is se e n in an a c c u r a te e n e rg y m e a su r e m e n t o f o n e

o f th e p h o to n s .

2 .2 . A n n ih ila tio n o f f r e e p o s itr o n

T h e a n n ih ila tio n p r o c e s s is a r e la t iv is tic e v e n t w h ere th e m a sses o f th e

p o s itr o n and th e e le c tr o n are c o n v e r te d in to e le c t r o m a g n e t ic e n e rg y , th e a n n ih ila

t io n p h o to n s . F r o m th e in v a r ia n c e p r o p e r t ie s o f th e e le c tr o m a g n e t ic in te r a c t io n

w e c a n d eriv e sev eral s e le c t io n ru le s fo r a n n ih ila tio n . O n e -g a m m a a n n ih ila t io n

is p o s s ib le o n ly in an e x te r n a l f ie ld an d h a s n o p r a c t ic a l m ea n in g . T h e m o s t

im p o r ta n t p ro c e s s is th e tw o -g a m m a a n n ih ila tio n . H ig h e r-o rd e r p h e n o m e n a m a y

b e n e g le c te d u n le ss so m e s e le c t io n ru le fo r b id s th e a n n ih ila tio n in to tw o -g a n im a

q u a n ta . T h is s itu a t io n is t r e a te d in S e c t io n 5 .

F r o m th e n o n -re la t iv is tic l im it o f th e tw o -g a m m a a n n ih ila tio n c r o s s -s e c t io n

(D ir a c [ 1 3 ] ) , w e o b ta in th e a n n ih ila t io n p r o b a b ili ty p e r u n it t im e o r th e

a n n ih ila tio n ra te

У 2 у = n I o c n e& ( ! )

w h ich is in d e p e n d e n t o f th e v e lo c ity o f th e p o s itr o n . H e re , r 0 is th e c la s s ic a l

e le c t r o n ra d iu s, с th e v e lo c ity o f l ig h t and n eg is th e e le c t r o n d e n sity a t th e s ite

o f th e p o s itr o n . B y m e a su rin g th e a n n ih ila t io n ra te J 2y, o n e d ir e c tly o b ta in s

th e e le c t r o n d e n s ity n eg , i .e . th e p o s itr o n serv es as a te s t p a r t ic le fo r th e e le c t r o n

d e n s ity o f th e m e d iu m . H o w e v e r, b e c a u s e o f th e ir o p p o s ite ch a rg e , s tro n g

C o u lo m b a t t r a c t io n e x is ts b e tw e e n e le c t r o n s an d th e p o s itr o n . C o n s e q u e n tly ,

th e e le c t r o n d e n s ity neg w ill b e e n h a n c e d f r o m th e e q u ilib r iu m e le c tr o n d e n s ity

in m a t t e r due to th e C o u lo m b scre e n in g o f th e p o s itr o n . C a lc u la t io n o f th e s e

p o s itr o n -e le c tr o n c o r r e la t io n s is a d if f ic u lt m a n y -b o d y p ro b le m .

T h e k in e t ic e n e rg y o f th e a n n ih ila tin g p a ir is ty p ic a lly a fe w e le c t r o n v o lts .

In th e ir c e n tr e -o f-m a s s fra m e th e p h o to n e n e rg y in th e tw o -g a m m a a n n ih ila tio n

is m 0c 2 = 5 1 1 k e V , an d th e p h o to n s are m o v in g e x a c t ly in o p p o s ite d ir e c tio n s .

B e c a u s e o f th e n o n -z e ro m o m e n tu m o f th e a n n ih ila tin g p a ir th e p h o to n s d e v ia te

f r o m c o -lin e a r ity in th e la b o r a to r y fra m e . S im p le r e la t iv is tic e n e rg y -m o m e n tu m

c o n s e r v a t io n ru le s y ie ld a re s u lt

0 = p x /m0 c ( 2)

w h e re 1 8 0 ° — 0 is th e a n g le b e tw e e n th e tw o p h o to n s in th e la b o r a to r y an d p j

is th e m o m e n tu m c o m p o n e n t o f th e e le c t r o n -p o s it r o n p a ir tra n sv erse to th e

P O S IT R O N A N N I H I L A T IO N 15 5

(3+- SOURCE AND SAMPLES

1.28 MeV 0.511 MeV

FIG.3. The schematic diagram o f the fast-slow coincidence system used in the positron lifetime measurements.

p h o to n e m is s io n d ir e c t io n . U su a lly , в is very sm all (6 < 1 ° ) an d E q . ( 2 ) is valid .

B e c a u s e th e m o m e n tu m o f th e th e r m a liz e d p o s itr o n is a lm o s t z e r o [ 1 2 ] , th e

m e a su re d a n g u la r c o r r e la t io n cu rv e d e sc r ib e s th e m o m e n tu m d is tr ib u t io n o f th e

a n n ih ila te d e le c t r o n s in m a tte r .

A s m e n tio n e d p re v io u s ly , th e e n e rg y o f th e a n n ih ila t io n p h o to n s u ffe r s a

D o p p le r -s h if t , b e c a u s e th e e le c tr o n -p o s itr o r i p a ir w as o r ig in a lly m o v in g in th e

la b o r a to r y . W ith s im ila r a n a ly s is as a b o v e th e e n erg y d e v ia tio n f r o m 5 1 1 k e V

e q u a ls

5 E = “ c p L ( 3 )

w h e re p L is n o w th e lo n g itu d in a l m o m e n tu m c o m p o n e n t o f t h e a n n ih ila tin g p a ir .

2.3. Lifetime measurement system

F ig u re 3 sh o w s a s c h e m a tic d iag ram o f th e p o s itr o n l if e t im e s p e c tr o m e te r .

T h e m o s t c o m m o n p o s i tr o n e m it te r is 22N a. T h e l if e t im e is m e a su re d as th e t im e

d e la y b e tw e e n th e 1 .2 8 - M e V and 5 1 1 -k e V p h o to n s sh o w n in F ig .2 . T h e l ife t im e


s p e c tr o m e te r is a fa s t-s lo w c o in c id e n c e s y s te m c o n v e n tio n a lly u sed in n u c le a r

p h y s ic s . T h e p o s itr o n s o u rc e is p re p a re d b y e v a p o ra tin g a fe w m ic r o c u r ie s o f

a q u e o u s 22N aC l s o lu t io n o n a th in m e ta l l ic o r p la s t ic fo i l ( ty p ic a l ly 1 m g/ cm 2)

an d i t is su rro u n d e d b y tw o p ie c e s o f th e sa m p le m a te r ia l.

T h e d e te c to r s c o n s is t o f fa s t p la s t ic s c in ti l la to r s c o u p le d to fa s t p h o t o

m u ltip lie r tu b e s . T h e fa s t sig n a ls ta k e n f r o m th e a n o d e s o f th e p h o to m u lt ip l ie r s

are th e n fe d to c o n s ta n t- fr a c t io n t im in g d is c r im in a to rs to p ro d u c e t im e signals.

T h e s e are th e n g u id ed to a t im e -to -a m p litu d e c o n v e r te r ( T A C ) , w h o se o u tp u t

a m p litu d e is p r o p o r tio n a l to th e t im e d if fe r e n c e b e tw e e n th e tw o in p u t sig nals .

T h e T A C p u lses are th e n c o l le c te d in to th e m u lt ic h a n n e l a n a ly se r .

T h e s lo w c h a n n e ls in F ig .3 e x is t o n ly to c h e c k th a t th e e n e rg ie s o f th e tw o

p h o to n s d e te c te d w ere c o r r e c t an d th a t th e tw o p u lses o r ig in a te fro m th e sam e

p ro c e s s . T h e c o rre s p o n d in g e n e rg y re g io n s are s e le c te d w ith th e s in g le -c h a n n e l

a n a ly se rs .

In p r a c t ic e , th e r e are ra n d o m an d s y s te m a t ic e rro rs in th e s y s te m a n d th e

m e a su re d l if e t im e sp e c tr u m b e c o m e s a c o n v o lu tio n o f th e id e a l s p e c tr u m and

a n in s tr u m e n ta l r e s o lu t io n f u n c t io n . T h e la t t e r c a n b e m e a su re d b y re p la c in g

th e p o s itr o n s o u rc e b y a 60C o g a m m a s o u rc e w ith o u t to u c h in g o th e r s e tt in g s o f

t h e sy s te m . T h is s o u rc e e m its tw o a lm o s t s im u lta n e o u s p h o to n s in a c a s c a d e .

T h e m e a su re d r e s o lu t io n fu n c t io n tu rn s o u t to b e n e a r ly G a u s sia n sh a p ed . A t im e

r e s o lu t io n o f 3 0 0 p s (F W H M ) is ty p ic a lly o b ta in e d w ith a v a ila b le c o m m e r c ia l

e q u ip m e n t. T h e re s o lu t io n d e p e n d s o n th e w id th s o f th e e n e rg y w in d o w s in th e

s lo w ch a n n e ls . T h e b e s t r e s o lu t io n o b ta in e d is a b o u t 1 7 0 p s w ith 3 3 % e n e rg y

w in d o w s [ 1 4 ].

T h e r e s o lu t io n d e sc r ib e d a b o v e a llo w s m e a s u r e m e n ts o f p o s itr o n l if e t im e

s p e c tr a in th e fo r m e x p ( —X t) , w h e re t h e m e a n p o s itr o n l if e t im e r = X -1 is

~ 1 0 0 p s o r m o re . T h e m e a su re d cu rv e ca n th e n b e f it te d w ith c o m p u te r p r o

g ram s t o a n a ly se th e l if e t im e v a lu e r . U su a lly , c o m p le x s p e c tr a w ith m a n y

e x p o n e n t ia l te rm s a re o b se rv e d .

In c e r ta in ca se s d e s c r ib e d , e .g . in S e c t io n 4 , th e r e is so m e s ta t is t ic a l ad v a n ta g e

in m e a su rin g th e m e a n l if e t im e o f p o s itr o n s as th e t im e d is p la c e m e n t b e tw e e n th e

c e n tr o id s o f th e l if e t im e s p e c tr u m and th e 60C o r e s o lu t io n cu rv e . T h is d em a n d s

s im u lta n e o u s m e a s u r e m e n t o f th e s e tw o cu rv es re a liz e d w ith a r o u te r -m ix e r

s y s te m [ 1 5 ] .

2 .4 . A n g u la r c o r r e la t io n a p p a ra tu s

T h e a n g le b e tw e e n th e a n n ih ila t io n p h o to n s is m e a su re d w ith a s y s te m

d e sc r ib e d s c h e m a tic a lly in F ig .4 . T h e p h o to n s a re d e te c te d w ith u su a l N a l(T l)

d e te c to r s an d th e ang le is d e te r m in e d b y th e p o s i t io n o f th e d e te c to r s an d th e

le a d c o ll im a to r s .


PHA CO IN C PHA

SC A L E R I S C A L E R f S C A L E R

FIG. 4. The angular correlation apparatus with long-slit geometry.

T h e p o s itr o n s o u rc e is u s u a lly 22N a, 64C u o r 58C o w ith a n a c t iv ity o f 1 0 m C i

o r m o re . T h e p o s itr o n s p e n e tr a te in to th e sa m p le an d a n n ih ila te th e r e . T h e lead

sh ie ld s h in d e r th e p h o to n s a n n ih ila tin g in th e s o u rc e f r o m re a c h in g th e d e te c to r s .

T h e r e s o lu t io n o f th e e q u ip m e n t is d e te r m in e d a lm o s t t o ta l ly b y th e g e o m e tr y

o f th e d e te c to r s an d c o ll im a to r s . A ty p ic a l s lit w id th in f r o n t o f th e d e te c to r s is

0 .5 m rad .

T h e d is c r im in a to rs are tu n e d a t 5 1 1 -k e V p h o to n e n e rg ie s an d th e d ev ic e

s im p ly c o u n ts th e c o in c id e n c e p u ls e s as a fu n c t io n o f th e a n g le 6Z. T o m in im iz e

th e e rr o rs o n e u s u a lly m e a su re s sev e ra l ru n s o v e r th e t o ta l a n g u la r ra n g e , ty p ic a lly

± 1 5 m rad .

T h e g e o m e tr y in F ig .4 is c a lle d th e lo n g -s lit g e o m e tr y , a c c o rd in g t o th e sh ap e

o f th e c o ll im a to r s . T h e d e v ic e c a n n o t reso lv e e ith e r th e a n g u la r d e v ia t io n in th e

y -d ir e c t io n o r th e D o p p le r- s h ift in th e x -d ir e c t io n an d c o n s e q u e n tly th e

c o in c id e n c e c o u n tin g r a te b e c o m e s

OO

N ( 0 Z) = С i f dp x d p y p (p x , P y , 0 Z m 0 c ) ( 4 )

w h ere p ( p x ,p y ,p z ) is th e m o m e n tu m d is tr ib u t io n o f th e a n n ih ila t io n p ö s itr o n -

e le c t r o n p a irs in th e s tu d ie d m e d iu m . T y p ic a lly , an a n g u la r c o r r e la t io n m e a su re

m e n t ta k e s sev e ra l d a y s w ith th e c o in c id e n c e c o u n tin g ra te a b o u t 10 s' 1 a t 6Z = 0.


A lso p o in t-s lit g e o m e tr ie s a re u sed to m e a su re tw o -d im e n sio n a l m o m e n tu m

d e n s it iè s , b u t th e a c c u m u la t io n ra te o f d a ta is v e ry s lo w d u e to g e o m e tr ic a l

c o n d it io n s . T h e la te s t s te p in th e d e v e lo p m e n t o f a n g u la r c o r r e la t io n d e v ic e s is

th e tw o -d im e n sio n a l d e te c to r sy s te m c o n s is t in g o f a m u lt ic o u n te r sy s te m o r a

p a ir o f p o s itio n -s e n s itiv e d e te c to r s [ 1 6 , 1 7 ]. W ith su c h d e v ices th e tw o -

d im e n s io n a l m o m e n tu m d is tr ib u t io n c a n b e m e a su red in a re a s o n a b le t im e .

FIG.5. The system fo r measuring the Doppler-broadened annihilation line.

2 .5 . A n n ih ila t io n l in e s p e c tr o m e te r

A s d e sc r ib e d p re v io u s ly , th e m e a s u r e m e n t o f th e 5 1 1 -k e V p h o to n e n erg y

d is tr ib u t io n c o m in g fr o m th e s tu d ie d m e d iu m is id e n tic a l w ith th e a n g u la r

c o r r e la t io n m e a s u r e m e n t in th e lo n g -s lit g e o m e tr y . F ig u re 5 sh o w s a ty p ic a l

in s ta l la t io n . T h e s o u rc e and sa m p le s a re p re p a re d in th e sa m e w ay as fo r l ife t im e

m e a s u r e m e n ts 'a n d th e a n n ih i la t io n r a d ia t io n is d e te c te d c o n v e n tio n a lly w ith a

h ig h -re s o lu tio n G e (L i) d e te c to r . T h e e f f ic ie n c y is ro u g h ly a h u n d re d t im e s b e t t e r

th a n th a t o f th e an g u la r c o r r e la t io n sy s te m s in c e n o c o in c id e n c e r e q u ire m e n ts

e x is t . T y p ic a lly , a o n e -h o u r m e a s u r e m e n t w ith a 5 juCi 22N a s o u rc e is e n o u g h fo r

s u ff ic ie n t s ta t is t ic a l a c c u r a c y . T h e d isa d v a n ta g e , h o w e v e r, is th e r e s o lu t io n o f th e

sy s te m . T h e b e s t G e ( L i) d e te c to r s h av e an e n e rg y r e s o lu t io n o f a b o u t 1 .2 k e V

a t th e 5 1 4 -k e V lin e o f 85S r , c o rre s p o n d in g to an a n g u la r r e s o lu t io n o f a b o u t

4 m ra d , i .e . an o r d e r o f m a g n itu d e w o rse th a n th e r e s o lu t io n o f th e a n g u la r c o r r e

la t io n d ev ice . A ls o , e le c t r o n ic s ta b il i ty p r o b le m s a rise , a re la t iv e ly sm all d r ift

d u rin g th e m e a s u r e m e n t c a n se v e re ly d e s tro y th e in fo r m a t io n o b ta in e d . T h e

d ig ita l s p e c tr u m s ta b iliz e r sh o w n in F ig .5 is e s s e n tia l, i f re lia b le re s u lts are

re q u ire d .

T h e r e is a lso a d isa d v a n ta g e in in te r p r e t in g th e re s u ltin g cu rv e in p h y s ic a l

te r m s b e c a u s e o f th e p o o r r e s o lu t io n . S o m e d e c o n v o lu t io n p r o c e d u r e s h av e b e e n

d e v e lo p e d b u t th e ir r e l ia b i l i ty is lim ite d . T h e c o n s e q u e n c e o f th e a b o v e f a c t is

t h a t th e D o p p le r-re s u lts c a n o n ly b e u sed to d e fin e a lin e -sh a p e p a r a m e te r

c h a r a c te r iz in g th e m e a su re d cu rv e . O n e e x a m p le o f su c h a p a ra m e te r S = C/A

( P O S IT R O N A N N I H I L A T IO N 159

zz<iu

Zz>Ou

S =

r =

l - c - l

I I

C H A N NE L N U M B E R

FIG.6. The Doppler-broadened 511-keVannihilation line flower) and the describing the instrumental resolution (upper) [-/<?].

зэоо'

&sSr 514-keV line

Cou

nts

per

Ch

an

ne

l

FIG. 7. Positron lifetim e spectrum in sodium metal [19].

160 H

AU

TO

JÄR

VI

and V

EH

AN

EN


is g iv en in F ig .6 [ 1 8 ] , w h ich sh o w s th e D o p p le r : cu rv e to g e th e r w ith th e in s tru

m e n ta l r e s o lu t io n fu n c t io n o b ta in e d w ith th e m o n o e n e r g e tic 85 S r g a m m a so u rc e .

T h u s , th e D o p p le r - te c h n iq u e c a n b e u se d to fo l lo w v a rio u s p h e n o m e n a w h ich

re q u ire m e a s u r e m e n ts o f m a n y sta g es o f sa m p les in a c o m p a r a tiv e ly s h o r t t im e .

3 . E L E C T R O N IC S T R U C T U R E O F I D E A L M E T A L S

3 .1 . T h e a n n ih ila t io n ra te

W e sh a ll n o w tr e a t th e p r o b le m o f a th e r m a liz e d p o s itr o n in a p e r fe c t m e ta l

la tt ic e . T h e e le c tr o n s in s im p le m e ta ls c a n b e ro u g h ly d iv id ed in to n e a r ly fre e

v a le n c e e le c t r o n s a n d t ig h tly b o u n d c o r e e le c t r o n s . T h e p re s e n c e o f t h e d en se

fr e e e le c t r o n gas to ta lly p re v e n ts p o s itr o n iu m fo r m a t io n . T h e re p u ls iv e in te r a c t io n

b e tw e e n th e p o s itr o n an d th e io n s re s u lts in sm all o v e rla p p in g o f th e p o s itr o n and

c o r e e le c t r o n w ave fu n c tio n s . T h u s , th e a n n ih ila tio n r a te X is a lm o s t to ta lly

d e te r m in e d b y th e v a le n c e e le c t r o n d e n s ity and is a lso t im e in d e p e n d e n t. A l ife

t im e m e a su r e m e n t th e n p ro d u c e s a s in g le -e x p o n e n tia l s p e c tr u m e x p ( — X t)

a n a lo g o u s ly to th e ra d io a c t iv e d e c a y p ro c e s s . F ig u re 7 [ 1 9 ] sh o w s a ty p ic a l

l if e t im e s p e c tr u m in a p u re w e ll-a n n e a le d m e ta l .

T h e d e p e n d e n c e o f th e a n n ih ila t io n ra te Л o n th e v a le n c e e le c t r o n d e n s ity

nvag is c o n v e n tio n a lly e x p r e ss e d in te rm s o f a d e n s ity p a r a m e te r r s , w h ic h is

d e fin e d b y 4/3 7rr| = nÿ1̂ . F ig u re 8 sh o w s th e m e a su re d a n n ih ila t io n ra te s in

v a rio u s m e ta ls as a fu n c t io n o f r s . T h e th e o r e t ic a l cu rv e 1 in th e f ig u re is c a l

c u la te d fr o m E q . ( l ) w ith n e re p la c e d b y n va£ . W e se e , h o w e v e r , th a t th e resu ltin g

cu rv e is fa r to o lo w an d i ts r s-d e p e n d e n c e is t o o s te e p . T h e re a s o n fo r th is d is

c re p a n c y is p a r t ly th e sm a ll c o n t r ib u t io n o f c o r e a n n ih ila tio n s and m a in ly th e

m a n y -b o d y p o s itr o n -e le c t r o n c o r r e la t io n s m e n tio n e d p re v io u s ly . C u rv e 2 in

F ig .8 c o rre s p o n d s to th e re s u lts o f m a n y -b o d y c a lc u la t io n s a p p lie d t o a p o s itr o n

in an e le c t r o n gas [ 2 0 ] . F o r s im p le m e ta ls th e a g re e m e n t is q u ite g o o d , b u t in

th e n o b le an d t r a n s it io n m e ta ls th e m e a su re d a n n ih ila t io n ra te s are m u c h h ig h e r

b e c a u s e o f th e b ig o v erla p o f th e p o s itr o n an d th e c o r e e le c tr o n s .

3 .2 . A n g u la r c o r r e la t io n s tu d ie s

T h e a n g u la r d is tr ib u t io n o f th e a n n ih ila tio n p h o to n s N ( 0 Z) m e a su re d w ith

th e lo n g -s lit a p p a ra tu s is an in v e r te d p a ra b o la in th e c a se o f fr e e e le c t r o n s w ith a

sp h e r ic a l F e r m i s u r fa c e b e c a u s e th e a re a o f th e c r o s s -s e c t io n o f th e F e r m i sp h ere

a t a p la n e p z = 0 z m o c d e c re a se s q u a d r a tic a lly . T w o e x a m p le s o f a n g u la r c o r r e la

t io n cu rv e s in a lu m in iu m [ 2 1 ] an d c o p p e r [ 2 2 ] are g iven in F ig .9 . T h e cu rv e s

are se e n to c o n s is t o f tw o p a r ts , th e in v e r te d p a ra b o la an d a b r o a d e r , ro u g h ly

G a u ssia n -sh a p e d c o n t r ib u t io n w h ic h is d u e to a n n ih ila tio n s w ith th e c o r e e le c tr o n s .


FIG.8. Experimental and theoretical annihilation rates in simple metals as a function o f the electron density parameter rs. Curve (1 ) is the one-electron approximation and curve (2) is a result o f many-body calculations [20].

ANGLE IN MILLIRADIANS (8 = Pz/m0c)

F I G . 9 . T h e a n g u l a r c o r r e l a t i o n c u r v e s f o r A l a n d C u [ 2 1 , 2 2 ] .


A n g le Between Annihilation Photons in M illirodions

FIG .l 0. The angular correlation curves in copper single crystals in various crystallographic directions. A special short-slit geometry is used where the p^-integration in Eq.(4) is restricted inside the Fermi sphere [23].


[ 0 0 1 ]

F IG .ll . The three-dimensional spin-polarization contour diagram in momentum space measured in ferromagnetic iron with polarized positrons [24].

FIG. 12. Point-slit angular correlation in an aluminium single crystal measured with a multicounter system. The solid curve represents an OPW calculation, the dashed curve the Wigner-Seitz model [16].


T h e in te r s e c t io n o f th e s e tw o p a rts d ir e c t ly g ives th e F e r m i an g le Op c o rre s p o n d in g

to th e F e r m i m o m e n tu m . F ig u r e 9 sh o w s a lso a s ig n if ic a n t d if fe r e n c e in th e c o re

a n n ih ila t io n f r a c t io n and a sm a ll d if fe r e n c e in th e w id th s o f th e p a r a b o la s b e tw e e n

th e tw o m e ta ls . T h e s e e f fe c ts a re d u e to d if fe r e n t c o r e an d v a le n c e e le c t r o n

d e n s it ie s in th e m e ta ls .

I n c o n tr a s t t o th e l if e t im e d is tr ib u t io n , th e a n g u la r c o r r e la t io n cu rv es seem

to b e su rp ris in g ly in s e n s it iv e to m a n y -b o d y e f fe c ts . T h u s , a s im p le o n e -e le c tr o n

d e s c r ip tio n q u ite a d e q u a te ly c h a r a c te r iz e s th e m e a su re d cu rv es. T h e r e fo r e ,

p o s itr o n s a re p a r t ic u la r ly s u ita b le in s tu d ie s o f a n is o tro p ie s in F e r m i su r fa c e s ,

w h e n sin g le c r y s ta l sa m p le s are u sed . S u c h an e x a m p le is g iven in F ig . 1 0 [ 2 3 ]

w h e re th e n e c k s in th e c o p p e r F e r m i s u r fa c e are c le a r ly v is ib le .

B y c o m b in in g sev e ra l lo n g -s lit m e a s u r e m e n ts o n e c a n r e c o n s tr u c t th e th re e -

d im e n s io n a l m o m e n tu m d e n s ity p($)- F ig u re 11 [ 2 4 ] sh o w s a re s u lt f o r p (p )

in c o n to u r d iag ram r e p r e s e n ta t io n . T h e a d d itio n a l p e c u la r ity in th e fig u re is th a t

i t d e n o te s th e e le c t r o n sp in p o la r iz a t io n d iag ram p up(p ) — p doWn (p ) in fe rro m a g

n e tic iro n o b ta in e d b y u sin g p o la r iz e d p o s itr o n s .

T h e d e v e lo p m e n t o f tw o -d im e n s io n a l a n g u la r c o r r e la t io n d e v ic e s h a s e n a b le d

u s to a ch ie v e s ig n if ic a n t d e ta ils in th e m o m e n tu m d e n s ity s tu d ie s . F ig u r e 12

g ives a p o in t-s l i t a n g u la r c o r r e la t io n cu rv e o f a lu m in iu m m e a su re d w ith a m u lti

c o u n te r sy s te m [ 1 6 ] .

T h e p o s itr o n s tu d ie s o f m o m e n tu m d e n s itie s hav e en la rg e d a m p ly . N o w

a d a y s th e F e r m i ra d iu s c a n b e m e a su re d w ith 0 .1 % a c c u r a c y [ 2 5 ] . T h e sam e

a c c u r a c y is o b ta in e d w ith m o r e c o n v e n tio n a l m e th o d s . H o w e v e r, w ith a llo y s o r

im p u re m e ta ls th e o th e r m e th o d s fa il b e c a u s e o f th e s h o r t s c a t te r in g m e a n fre e

p a th o f e le c tr o n s . In th e s e m a te r ia ls th e p o s itr o n m e th o d o f fe r s u n iq u e in fo r m a

t io n an d th e s u b je c t is u n d e r e x te n s iv e s tu d y [ 2 6 ] .

4 . S T U D I E S O F M E T A L L A T T I C E D E F E C T S

4 .1 . T ra p p in g o f p o s itr o n s

T h e b e h a v io u r o f p o s itr o n s in s im p le m e ta ls is q u ite w ell u n d e r s to o d as

d iscu sse d in th e p re v io u s c h a p te r . H o w e v e r, i t w as fo u n d th a t v a rio u s p h y s ic a l

s ta te s o f th e m e ta l c o u ld d ra s t ic a lly c h á n g e a n n ih ila tio n p ro p e r t ie s . A s th e m e ta l

w as m e c h a n ic a lly d e fo rm e d , th e m e a n life t im e in cre a se d [ 2 7 , 2 8 ] an d th e an g u lar

c o r r e la t io n cu rv e b e c a m e s ig n if ic a n t ly n a r ro w e r [ 2 9 , 3 0 ] . T h e s e e f f e c t s are c le a r ly

v is ib le in F ig s 1 3 , 1 4 [ 2 2 ] a n d 15 [ 3 1 ] . In a m o re d e ta ile d s tu d y o n d e fo rm e d

a lu m in iu m [ 2 8 ] th e l if e t im e s p e c tr u m w as fo u n d to c o n ta in tw o e x p o n e n t ia l

l i f e t im e c o m p o n e n ts . T h e e f fe c t o f c r y s ta l d e fe c ts o n p o s itr o n b e h a v io u r w as

se e n to s a tu r a te a t re la t iv e ly lo w ,d egrees o f d e fo r m a tio n , th e l if e t im e s p e c tru m

b e c a m e o n e -e x p o n e n tia l ag a in , b u t th e l if e t im e v a lu e w as h ig h e r .

CO

UN

TS

PER

CH

AN

NEL


C H A N N E L

FIG. 13. Positron lifetime spectra in aluminium with different degrees o f deformation.

FIG.14. The e ffec f o f deformation on the angular correlation curve in copper. The fitted Gaussian and parabolic parts are shown in both cases [22].


10-

«2 6L

ш Zz4

5 4LcШ Q.inKzЭ 2U о о

D oppler - b roadened a nn ih ila tio n

line sh a pe s in annea led ( ° )4 and h eav ily ro lle d ( • ) copper

(Peaks n o rm a liséd to equal area

= 4 .5 2 x 10® c o u n ts )

— A - В

L = С - В

•£.y>

3 9 5 0 82 9 6 128 139

CHANNEL NUMBER

FIG.15. The Doppler-curves in deform ed and annealed copper [31].

T h e e x p la n a t io n f o r th e s e p h e n o m e n a is th e tra p p in g o f p o s itr o n s b y c r y s ta l

d e fe c ts . A t lo w -d e fe c t c o n c e n tr a t io n s o n ly so m e p o s itr o n s g e t tra p p e d an d in th is

re g io n th e a n n ih ila t io n c h a r a c te r is t ic s d e p e n d se n s itiv e ly o n th e n u m b e r o f

d e fe c ts . W h e n a ll p o s itr o n s are tra p p e d , s a tu r a tio n is re a c h e d . T h e d e fe c t s th a t

h av e b e e n fo u n d t o tra p p o s itr o n s are v a c a n c ie s , d is lo c a t io n s , v a c a n c y c lu s te r s

o r v o id s an d s u rfa c e s . In th e d is o rd e re d re g io n s th e p o s itiv e c h a rg e o f io n s is

re d u c e d an d th e re s u ltin g r e d is tr ib u tio n o f fr e e e le c t r o n s le a d s to a n e t n e g a tiv e

e le c t r o s ta t ic p o te n t ia l a t tr a c t iv e to th e p o s itr o n . I f th e p o te n t ia l is s tro n g e n o u g h

th e p o s itr o n s u ffe rs a t r a n s it io n f r o m th e B lo c h - l ik e s ta te t o th e tra p p e d , lo c a liz e d

s ta te . T h e e le c t r o n d e n s ity a t th e s ite o f th e d e fe c t is sm a lle r a n d c o n s e q u e n tly th e

p o s itr o n l if e t im e in c r e a s e s . T h e sa m e re a s o n a lso le a d s to th e n a r ro w in g o f th e

a n g u la r c o r r e la t io n cu rv e , s in c e th e lo c a l F e r m i m o m e n tu m d e c re a s e s [ 2 1 ] . T h e s e

fe a tu r e s c a n b e a lso q u a n t ita t iv e ly u n d e r s to o d o n th e b a s is o f th e e le c t r o n ic

s tr u c tu r e o f la t t i c e d e fe c ts [ 3 2 , 3 3 ] .


FIG .l 6.' Arrhenius plots fo r the determintion o f vacancy formation energies with the positron method fo r several metals [37].

4 .2 . D e te r m in a t io n o f d e fe c t c o n c e n tr a t io n s

T h e p o s itr o n tra p p in g c a n b e u til iz e d to stu d y th e d e n s ity o f d e fe c t s in th e

sa m p le . A c c o r d in g to a sim p le tra p p in g m o d e l [ 3 4 —3 6 ] th e r e a re tw o ty p e s o f

p o s itr o n s , e i th e r fr e e o r tra p p e d . T h u s , a n y a n n ih ila tio n c h a r a c te r is t ic (p e a k

c o u n tin g ra te in a n g u la r c o r r e la t io n cu rv e , m e a n l ife t im e , th e sh a p e o f a n n ih ila tio n

lin e e t c . ) , w h ic h d e p e n d s lin e a r ly o n th e f r a c t io n o f p o s itr o n s in th e tra p p e d s ta te ,

c a n b e u sed to s tu d y th e d e n s it ie s o f th e re la t iv e d e fe c ts .. T h e p o s itr o n te c h n iq u e

h a s p ro v e d t o b e a v ery se n sitiv e an d p o w e r fu l m e th o d an d it h a s b e e n e x te n s iv e ly

ap p lied e s p e c ia lly fo r th e d e te r m in a t io n o f v a c a n c y fo r m a t io n e n e rg ie s in m e ta ls .

T h is is s im p ly d o n e b y m e a su rin g so m e p o s i tr o n p a r a m e te r as a f u n c t io n o f

te m p e ra tu re . F ig u re 1 6 [ 3 7 ] sh o w s th e re s u ltin g A rrh e n iu s p lo t fo r d if fe r e n t

m e ta ls . E a c h s lo p e g ives d ir e c t ly th e v a c a n c y fo r m a t io n en e rg y .

T h e a d v a n ta g es o f th e p o s itr o n m e th o d lie in th e fa c t th a t i ts s e n s it iv ity

s ta r ts a lre a d y fr o m th e v a c a n c y c o n c e n tr a t io n s o f a b o u t 10'6 an d th u s th e c o n t r i

b u t io n o f d iv a c a n c ie s is q u ite sm all. E v e n re a s o n a b le e s t im a te s fo r th e d iv a ca n cy

b in d in g e n e rg y h av e b e e n o b ta in e d [ 3 8 ] . F o r m o re d e ta ils o n th e a p p lic a t io n s

t o th e d e te r m in a t io n o f th e v a c a n c y fo r m a t io n e n e rg ie s se e , f o r e x a m p le , W e st [ 3 9 ] .

P O S IT R O N A N N I H I L A T IO N 1 6 9

FIG. 17. The e ffec t o f voids on the angular correlation curve o f molybdenum. The voids were produced during high-dose fast-neutron irradiation [41]. (Open circles with voids, crosses without voids.)

500

400 -

3 0 0 -

LUX

2 0 0 -

1 0 0100 200 300 400A N N E A L IN G T E M P E R A T U R E Г С )

FIG. IS. The void formation during isochronal annealing o f electron-irradiated molybdenum. The experimental points are from Ref. [45] and the void size was calculated in Ref. [46].


P o s itr o n a n n ih ila tio n in irra d ia te d an d a n n e a le d m e ta ls h a s s h o w n re m a rk a b ly

d if fe r e n t b e h a v io u r f r o m a n n ih ila t io n o f p o s itr o n s tra p p e d in v a c a n c ie s o r d is

lo c a t io n s . O n th e o th e r h a n d , sm a ll c a v it ie s o r v o id s are k n o w n t o a g g lo m e ra te

f r o m v a c a n c ie s u n d e r h ig h -d o se fa s t-n e u tr .o n ir ra d ia tio n [ 4 0 ] . D r a s t ic ch a n g e s in

th e a n g u la r c o r r e la t io n an d l if e t im e cu rv es h a v e b e e n o b se rv e d in m o ly b d e n u m

u n d e r s im ila r c o n d it io n s [ 4 1 , 4 2 ] . T h e l i f e t im e o f p o s itr o n s in c r e a s e d b y a f a c to r

o f fo u r an d th e a n g u la r c o r r e la t io n cu rv e n a rro w e d t o a b o u t o n e h a lf , as sh o w n

in F ig . 17 [4 1 ]. T h e v o id s w ith a d ia m e te r o f a b o u t 3 0 A w ere a lso seen in an e le c t r o n

m ic r o s c o p e . I t w as su g g e sted [ 4 3 , 4 4 ] th a t in su ch b ig v o id s th e p o s itr o n is tra p p e d

in a s u r fa c e s ta te .

T h e n e x t step in th e v o id s tu d ie s w as to re d u c e th e ir r a d ia t io n d o se an d to

u se e le c t r o n s in s te a d o f fa s t n e u tro n s . E ld ru p e t a l. [ 4 5 ] re v e a le d th a t th e m o t io n

o f v a c a n c ie s d u rin g stag e I I I a n n e a lin g c a u se d s u b m ic r o s c o p ic v o id f o r m a t io n a t

re la t iv e ly sm a ll ir r a d ia t io n d o ses . A f te r a n n e a lin g a t h ig h e r te m p e r a tu r e s t h e v o id

s tr u c tu r e c o a rs e n e d an d b e c a m e v isib le in th e e le c t r o n m ic r o s c o p e , to o . T h e

l i f e t im e re s u lts w ere c o r r e la te d w ith th e v o id s iz e [ 4 6 ] ; th e re s u ltin g a n n e a lin g

cu rv e p r e d ic ts th e n u m b e r o f v a c a n c ie s c lu s te r e d in th e v o id , as sh o w n in F ig . 1 8 .

T h u s , p o s itr o n s c a n re v e a l v o id s f r o m th e v e ry b e g in n in g , lo n g b e fo r e th e y

b e c o m e v is ib le in th e e le c t r o n m ic r o s c o p e . I n th is sen se th e p o s itr o n a n n ih ila tio n

m e th o d is a u n iq u e to o l. In p r a c t ic e , v o id fo r m a t io n is a se r io u s p r o b le m in th e

c o n s t r u c t io n o f n u c le a r r e a c to r s an d p o s itr o n s are n o w b e in g u sed f o r th e s e

s tu d ie s , se e fo r e x a m p le R e f . [ 4 7 ] ,

4.3. Gustering of vacancies into voids

4.4. Studies of deformed metals

A s p o s itr o n s are se n sitiv e t o v a rio u s d e fe c ts p ro d u c e d d u rin g p la s t ic d e fo rm a

t io n o f m e ta ls , th e d y n a m ic p r o p e r t ie s o f d if fe r e n t d e fe c ts and th e ir in te r a c t io n s

c a n b e s tu d ie d . T h e to p ic s in th is f ie ld a re th e re c o v e ry o f m e ta l d e fe c t s d u rin g

a n n e a lin g an d re la te d p h e n o m e n a . N u m e ro u s a n n e a lin g s tu d ie s h a v e r e c e n t ly

b e e n m a d e a n d n e w fe a tu r e s a re fo u n d w ith p o s itr o n s .

F ig u re 1 9 sh o w s an is o c h r o n a l a n n e a lin g s tu d y o f tw o p la s t ic a lly d e fo rm e d

ir o n s w ith d if f e r e n t d eg rees o f im p u r it ie s [ 4 8 ] . T h e p o s itr o n p a r a m e te r s , l ife t im e

an d a n a n n ih ila t io n p a r a m e te r S d e fin e d in F ig .6, c h a n g e re m a rk a b ly a t a b o u t

6 0 0 ° C , w h ic h is th e r e c r y s ta ll iz a t io n te m p e r a tu r e o f iro n . T h u s , th e m a in

r e c o v e ry a t th is te m p e r a tu r e sh o w s th a t th e tra p p in g o f p o s itr o n s is c a u se d b y

d is lo c a tio n s . A n o th e r in te r e s t in g fe a tu re is th e d if fe r e n c e b e tw e e n th e tw o

cu rv es. T h e p u re r ir o n sh o w s p a r t ia l re c o v e ry a t a b o u t 2 5 0 ° C , w h ic h is a b s e n t

in th e iro n c o n ta in in g m o re im p u rit ie s , m a in ly c a r b o n . T h e e x p la n a tio n h e r e is

th e re o rd e rin g o f d is lo c a t io n s tru c tu re (p o ly g o n iz a t io n p ro c e s s ) in th e p u re iro n .

p o s it r o n An n i h il a t io n 171

180

¡Ü 160

шu.

2ОIX UO

if>оcl

120

tn 0.65а .ыы

<СИ<о.

0.64

ша.<x1Л

0.63

200 400 600 800ANNEALING TEMPERATURE (*С)

FIG.19. The recovery o f positron lifetime and annihilation lineshape parameter S (defined in Fig.6) in two deform ed irons during isochronal annealing [48\

T h e c a r b o n in te r s t i t ia ls a re k n o w n to m ig ra te in to d is lo c a t io n su rro u n d in g s

a n d th is c a u se s th e b lo c k in g o f th e d is lo c a t io n s tr u c tu r e an d c o n s e q u e n tly

th e a b s e n c e o f th e lo w -te m p e r a tu r e r e c o v e r y .

D lu b e k e t al. [ 4 9 ] h av e s tu d ie d th e b e h a v io u r o f tw o n ic k e ls w ith d if fe r e n t

p u r itie s d u rin g th e p la s t ic d e fo r m a tio n . T h e in te r e s t in g re s u lt w as th e o b se r v a tio n

th a t th e p re s e n c e o f im p u r it ie s c a ta ly s e d c lu s te r in g o f v a c a n c ie s .

O th e r s tu d ie s m o re o r ie n te d to th e m e ta llu r g ic a l f ie ld a re sh o w n in F ig s 2 0

an d 2 1 [ 5 0 ] d ea lin g w ith fa tig u e d am ag e in c o m m e r c ia l s te e ls . In F i g .2 0 th e

s te e l w as e x p e c te d t o s h o w fa tig u e h a rd e n in g an d in F i g . 2 1 fa tig u e s o fte n in g . T h e

m e a n p o s itr o n l if e t im e in d e e d sh o w e d s y s te m a t ic b e h a v io u r w ith r e s p e c t to th e s e

e f fe c ts . T h u s , p o s itr o n s c a n b e u sed t o p r e d ic t fa ilu re s in c o m m e r c ia l m a te r ia ls .

ME

AN

P

OS

ITR

ON

L

IFE

TIM

E

f (p

s)


FIG.20. Fatigue hardening in a soft commercial steel studied by positrons [50].

FIG.21. Fatigue softening in a hard commercial steel studied by positrons [50].


I t sh o u ld b e e m p h a s iz e d th a t th e c h o ic e o f p a ra m e te rs u se d to d e s c r ib e th e

c h a n g e s d u rin g th e k in e t ic s tu d ie s is e s s e n tia l. T h e p a ra m e te rs sh o u ld b e

s ta t is t ic a l ly a c c u r a te , ra p id ly o b ta in a b le and lin e a r w ith re s p e c t to th e stu d ied

e f fe c t . T h e p a ra m e te rs o b ta in e d a f te r te d io u s m a th e m a tic a l m a n ip u la tio n s fro m

th e m e a su re d cu rv es, su c h as m u lt ic o m p o n e n t l ife t im e s an d th e r e la t iv e in te n s it ie s ,

e t c . , sh o u ld n o t b e u sed .

4 .5 . In te r n a l s tr u c tu r e o f la t t i c e d e fe c ts

B e c a u s e th e p o s i tr o n a n n ih ila te s in a lo c a liz e d s ta te in s id e a d e fe c t su c h as

a v a c a n c y , th e a n n ih ila t io n c h a r a c te r is t ic s r e f le c t th e e le c t r o n ic s tr u c tu r e o f th e

d e fe c t . A v a c a n c y is n o t an e m p ty h o le in th e la t t i c e , b u t th e e le c t r o n d e n s ity

in th e c e n tr e is a b o u t 3 0 % o f th e b u lk e le c t r o n d e n s ity . T h is c a lc u la t io n [ 3 3 ]

is c o n f ir m e d b y c o m p a r in g th e re s u ltin g a n n ih ila tio n ra te s w ith th e e x p e r im e n ta l

v alu es.

T h e e le c t r o n d e n s ity in s id e th e v o id s is , h o w e v e r , c a lc u la te d t o re a c h z e ro

v ery fa s t as th e v o id size in c r e a s e s [ 4 6 ] . T h is g ives c o n s id e ra b le s u p p o r t to th e

id e a s d e scrib e d p re v io u s ly th a t th e p o s itr o n in te r a c ts s tro n g ly w ith th e void

s u r fa c e [ 5 1 ] .

T h e p o s itr o n s ta te s in d is lo c a t io n s a re th e o r e t ic a lly m o re c o m p le x . I t h a s

n o t b e e n v e r if ie d w h e th e r th e p o s itr o n c a n m o v e fr e e ly a lo n g th e d is lo c a t io n lin e

o r w h e th e r it is lo c a liz e d a t th e jo g s o r v a c a n c ie s in s id e th e d is lo c a t io n s . M o re

e x p e r im e n ta l a n d th e o r e t ic a l w o rk in th is f ie ld is n e e d e d .

5 . P O S IT R O N S IN M O L E C U L A R S U B S T A N C E S

5 .1 . P o s itro n iu m fo r m a t io n

P o s itr o n s in m o le c u la r s u b s ta n c e s c a n c a p tu r e an e le c t r o n f r o m th e su rro u n d

ing m e d iu m an d a p o s itr o n iu m a to m , th e b o u n d s ta te o f th e p o s i tr o n -e le c t r o n

p a ir , is fo rm e d . A s th e s iz e o f th e p o s itr o n iu m is a b o u t tw o t im e s th a t o f th e

h y d ro g e n a to m , th e p o s itr o n iu m fo r m a t io n o c c u r s in a m o le c u la r m e d iu m w h e re

c o m p a r a tiv e ly la rg e e m p ty re g io n s e x is t .

T h e fo r m a t io n p ro c e s s o f p o s itr o n iu m is a t p r e s e n t u n d e r d is c u ss io n . T w o

s im p le m o d e ls h av e b e e n p re s e n te d . T h e “ O re g a p ” m o d e l [ 5 2 ] s ta te s th a t

p o s itr o n iu m fo r m a t io n is m o s t p r o b a b le w h en th e p o s itr o n e n e rg y lie s w ith in th e

so -c a lle d O re gap , w h e re n o o th e r e n e rg y tr a n s fe r p ro c e s s is p o s s ib le . A c c o r d in g

to th e r a d io c h e m ic a l “ sp u r” m o d e l [ 5 3 ] , p o s itr o n iu m fo r m a t io n is a r e a c t io n

b e tw e e n th e fr e e p o s itr o n an d th e e le c t r o n s in th e sp u r p ro d u c e d d u rin g th e s lo w in g

d o w n o f th e p o s itr o n its e lf .


Angle between photons (m rad)

FIG.22. Parapositronium peak in the angular correlation curves o f SÍO2 powders with different grain size [54\

T h e g ro u n d s ta te s o f p o s itr o n iu m are th e s in g le t * S s ta te o r p a ra p o s itro n iu m

an d th e t r ip le t 3S s ta te o r o r th o p o s itr o n iu m . T h e l ife t im e o f p a ra p o s itro n iu m in

tw o -p h o to n a n n ih ila tio n is 1 2 5 p s, a b o u t th e sam e as th e fre e p o s itr o n l if e t im e

in m e ta ls . H o w e v e r, th e tw o -p h o to n a n n ih ila t io n is fo rb id d e n b y th e s e le c t io n

ru le s in th e c a s e o f o r th o p o s itr o n iu m . T h u s , i t d e c a y s v ia th r e e p h o to n e m is s io n s

w ith a l if e t im e o f 1 4 0 n s , m o re th a n th r e e o rd e rs o f m a g n itu d e lo n g e r th a n th e

p a r a p o s itr o n iu m l ife t im e . A c o m p e tin g m e c h a n is m ca lle d th e p ic k -o ff-a n n ih ila t io n

is a lw a y s p re s e n t w h e n th e o r th o p o s it r o n iu m a to m m o v e s in a m e d iu m . In th e

p ic k - o f f p ro c e s s th e p o s itr o n in th e p o s itr o n iu m a to m a n n ih ila te s w ith a n o th e r

e le c t r o n o f o p p o s ite sp in an d tw o -g a m m a a n n ih ila t io n re s u lts . C o n s e q u e n t ly ,

o r th o p o s itr o n iu m life t im e s in m a te r ia ls a re o f th e o r d e r o f a fe w n a n o s e c o n d s .

A s is q u ite e v id e n t , th e a n g u la r c o r r e la t io n cu rv e an d th e l ife t im e sp e c tr u m

a re c o m p le x in th e p re s e n c e o f p o s itr o n iu m . T h e s e lf -a n n ih ila t io n o f p ara

p o s itr o n iu m ca u se s a p e a k a t sm a ll an g les in th e a n g u la r c o r r e la t io n cu rv e , b e c a u se

th e c e n tr e -o f-m a s s o f th e p a ra p o s itro n iu m a to m h a s a sm all m o m e n tu m in th e

la b o r a to r y fra m e . O n e e x a m p le o f th e p a ra p o s itro n iu m p e a k is sh o w n in F i g .2 2 ,

w h ere p o s itr o n iu m fo r m a t io n o c c u r s in S i 20 p o w d e r [5 4 ] .

B e c a u s e o f th e a n o m a lo u s ly lo n g l if e t im e o f o r th o p o s itr o n iu m , o th e r

c o m p e tin g a n n ih ila tio n m e c h a n is m s m a y fu r th e r d e c re a s e th e o r th o p o s itr o n iu m

p ic k - o f f l ife t im e . S u c h q u e n c h in g p ro c e s s e s a re th e o r th o -p a r a c o n v e r s io n an d

c h e m ic a l r e a c t io n s o f th e p o s itr o n iu m . T h e o r th o -p a r a c o n v e r s io n m a y b e d ue

to e le c t r o n e x c h a n g e b e tw e e n th e p o s itr o n iu m and su rro u n d in g m o le c u le s w h ile

c h e m ic a l r e a c t io n s are m o re c o m p le x in c lu d in g th e fo r m a t io n o f e +A " ty p e

m o le c u le s . P o s itro n iu m c h e m is tr y s tu d ie s th e s e p h e n o m e n a .


LUZz<Io

LLtû_

ZDO<J

C H A N N E L N U M B E R

FIG.23. Positron lifetime spectra in cyclohexane, where positronium is formed. The long tails represent the orthopositronium decay. The chemical reaction o f positronium with added iodine molecules is seen to decrease the orthopositronium p ick-o ff lifetime [57].

C O N C E N T R A T IO N O F IO D IN E (M )

FIG.24. Orthopositronium decay rates as a function o f the iodine concentration. The slopes in each case denote the chemical reaction rate constants [57].


5.2. Positronium chemistry

T h e o r th o p o s itr o n iu m d e c a y ra te an d its re la t iv e in te n s ity d e p e n d s tro n g ly

o n th e p r o p e r t ie s o f th e m e d iu m . B e c a u s e i ts h y d ro g e n -lik e n a tu re a n d re la t iv e ly

lo n g l if e t im e , th e p o s itr o n iu m c a n ta k e p a r t in c h e m ic a l r e a c tio n s . T h is to p ic

h a s b e e n d iscu sse d in m o re d e ta il in R e fs [ 1 0 ,5 5 ,5 6 ] .

F ig u r e 23, sh o w s a n e x a m p le o f th e s tu d ie s o f o r th o p o s it r o n iu m r e a c tio n s

w ith io d in e m o le c u le s [ 5 7 ] . T h e c h e m ic a l r e a c t io n b e tw e e n p o s itr o n iu m and

io d in e c a u se s a c o n s id e ra b le d e cre a se in th e m e a su re d l ife t im e . T h e m e a su re d

d e c a y r a te is d e scrib e d b y

^meas ~ ^-pick-off ^-react (5 )

w h e re X react is th e r e a c t io n r a te o f o r th o p o s itr o n iu m . A t lo w re a g e n t c o n c e n

t r a t io n s C , w e hav e

^ re a c t — к • С ( 6)

w h e re к is th e r e a c t io n r a te c o n s ta n t , a n im p o r ta n t p a ra m e te r in c h e m is try .

W e see f r o m F ig .2 3 th a t as io d in e is d isso lv ed in c y c lo h e x a n e s o lv e n t th e

o r th o p o s it r o n iu m l ife t im e d e cre a se s . A fu r th e r a d d itio n o f p y r id in e to th e s o lu t io n

ca u se s th e b in d in g o f io d in e t o a c o m p le x m o le c u le and th e r e a c t io n p r o p e r t ie s

c h a n g e s lig h tly . F ig u re 2 4 sh o w s th e m e a su re d d e c a y ra te s as a f u n c t io n o f io d in e

c o n c e n tr a t io n in b o th ca se s . T h e lin e a r d e p e n d e n c e p re d ic te d b y E q .( 5 ) is se e n

a n d th e s lo p e s give th e c h e m ic a l r e a c t io n ra te c o n s ta n ts к in a b s o lu te t im e sca le .

5.3. Study of glasses with positrons

T h e f ir s t g lass s tu d ie s c o n c e r n e d q u a r tz , w h e re p o s itr o n iu m fo r m a tio n

o c c u r r e d in th e a m o rp h o u s s ta te b u t n o t in th e c r y s ta l l in e fo r m [ 5 8 ] . P o s itro n s

h a v e b e e n ap p lied to v a rio u s p r o b le m s lik e g lass t r a n s itio n te m p e ra tu re s [ 5 9 ] ,

p o s itr o n b e h a v io u r as a fu n c t io n o f g lass c o m p o s it io n in b in a ry g lasses [ 6 0 ]

a n d s tu d ie s o f d y n a m ic p ro c e s s e s lik e liq u id -liq u id p h a s e -s e p a r a tio n and

c r y s ta l l iz a t io n [ 6 1 —6 5 ] .

I n th e l ith iu m s il ic a te ( L i 20 - S i 0 2 ) g lass th e p h a s e -s e p a r a tio n is a fa s t p ro c e s s

w h e re n e a r ly p u re S i 0 2 d ro p le ts are se g re g a te d f r o m th e b u lk g lass. T h e l if e t im e

s p e c tr a c a n b e d iv id ed in to th r e e e x p o n e n t ia l c o m p o n e n ts an d th e lo n g -life t im e

in te n s ity a s s o c ia te d w ith th e o r th o p o s itr o n iu m p ic k - o f f a n n ih ila t io n is se e n to

b e p r o p o r tio n a l to th e re la tiv e v o lu m e o f th e S i 0 2 d ro p le ts , as sh o w n in F i g .2 5

[ 6 4 ] . T h u s th e o r ig in o f th e lo n g -life t im e c o m p o n e n t is th e S i 0 2 d r o p le t p h a se .

A s th e g lass is h e a t t r e a te d a t m o d e ra te te m p e ra tu re s , th e d ro p le t p h a se

re m a in s th e sa m e an d th e l ith ia - r ic h b u lk p h a se c ry s ta ll iz e s . C o n s e q u e n t ly ,


L i20 CO NCEN TRAT IO N OF G L A S S (m o l% )

О 10 20 30 ¿0

VO LU M E F R A C T IO N O F S IL IC A -R IC H P H A S E -

FIG.25. P ick-off annihilation intensity in the phase-separating LiÔ-SiO-i glass as a function o f the relative volume o f the separated SiOi-droplets [64].

HEAT TREATMENT TIME IN HOURS

FIG.26. Kinetic curves o f crystallization in a L i20-Si02 glass at different heat-treatment temperatures. Changes in the lifetime parameter “A ” are proportional to volume crystallinity [65].

c h a n g e s in th e l i f e t im e s p e c tr u m are se e n o n ly as a d e c re a s e o f th e in te r m e d ia te

l i f e t im e in te n s ity . T h u s , th e in te n s ity c a n b e u sed to d e te c t c h a n g e s in th e

c r y s ta l l in ity . H o w e v e r, a s ta t is t ic a l ly m o re a c c u r a te p a r a m e te r “ A ” [ 6 3 ] , w h ich

w as sh o w n to d e p e n d lin e a r ly o n th e c r y s ta l l in ity b y c o m p a r a tiv e X -r a y d if f r a c t io n

m e a s u r e m e n ts , w as u sed . F ig u r e 2 6 [ 6 5 ] sh o w s th e k in e t ic cu rv es m e a su re d w ith

th e “ A ” -p a r a m e te r a t d if f e r e n t te m p e r a tu r e s , an d th u s th e a c t iv a tio n e n erg y o f

c r y s ta l l iz a tio n c a n b e d e te r m in e d .

178 HAUTOJÄRVI and V E H A N E N

LOG (t)

FIG.27. Kinetic curves of crystallization in different LÍ2O-SÍO2 glasses at constant temperature. The changes in the Doppler-parameter “S" (see Fig.6) are proportional to volume crystallinity. The scales are chosen in such a way that the slopes of the fitted straight lines give the morphology index of crystallization [65].

The phase-separation occurring before crystallization has a strong influence on the parameters describing the crystallization process. This is seen in Fig.27. which shows crystallization kinetics at constant temperature measured in three different lithium silicate glasses with different fractions of the Si02 droplets.The scale is chosen in such a way that a mathematical model, the Johnson—Mehl- Avrami equation, describing the crystallization [65] is a straight line and the slope denotes the morphology index process of crystal growth. This is seen to vary with the glass composition.

The phase-separation has a strong effect on the activation energy of volume crystallization as well as on the morphology index. On the other hand, the activation energy associated with the linear crystal growth stays constant. Thus, positrons can reveal information on the connection between phase-separation and the crystallization process, which is important in the production of glass- ceramic materials.

6. MEDICAL APPLICATIONS OF POSITRON ANNIHILATION

Positrons can be used in cancer localization, in metabolic studies and in meson beam collimation inside the human body. The basic problem is to deduce the distribution of positron emitters from the annihilation radiation. Figure 28

POSITRON ANNIHILATION 179

FIG.28. Schematic representation of the positron camera.

shows the principle of the positron camera for this determination. Two position- sensitive gamma detectors are used in coincidence to determine the two annihilation quanta emitting in opposite directions. Thus, the pair of detection coordinates determines the straight line, where the positron emitter existed. The advantage of the positron camera is the lack of collimators needed in conventional gamma cameras. The three-dimensional distribution of the positron emitter can be calculated. Some mathematical and. instrumental questions are discussed in Refs [17, 66, 67].

The most useful application of positron cameras is perhaps the metabolic studies in the human body. A superior method for brain metabolism studies is to label a suitable chemical (CO, glucose etc.) with positron emitters. The reason is that the only suitable radioactive isotopes among carbon, oxygen and nitrogen are the positron-active isotopes n C, 13N and 150 . Thus, the labelling technique can be utilized with any organic compounds. The practical limitation is the lifetime of these isotopes (20 min or less). A cyclotron for the isotope production must be situated in close connection with the positron camera.

7. FUTURE DEVELOPMENT

As seen in Fig. 1, the amount of research on positron physics shows continuous growth. The instruments needed are cheap, commercially available and normally exist in well-equipped nuclear physics laboratories. A further increase in the work in this field is expected.


The new two-dimensional angular correlation devices provide further possibilities in electronic structure studies of alloys and non-metallic compounds. The lattice defect studies have enabled us to understand the positron behaviour in disordered regions. Thus, it is possible to apply the method to more technological problems like deformed metals, radiation damage, fatigue and other metallurgical applications. In molecular substances the positronium formation and its interaction with the surrounding medium are still, to some extent, open questions. A totally new area of applications, the use of positrons in the study of surfaces, is at present breaking out [68].

In conclusion, we may state that although the field of positron annihilation is rather wide, it still keeps growing. There are plenty of opportunities and interesting applications to anyone who wants to enter this field.

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[41 ] MOGENSEN, O., PETERSEN, K., C O T TE R ILL , R.M.J., HUDSON, B., Nature (London) 2 39 (1 9 7 2 ) 98.

[42 ] C O TTE R ILL, R.M.J., M AC KE N ZIE , I.K ., SMEDSKJAER, L., TRUM PY, G.,

T R Ä F F, J.H .D .L., Nature (London) 239 (1972) 99.

[43 ] HODGES, C.H., STOTT, M.J., Solid State Commun. 12 (1 9 7 3 ) 1153.

[44 ] N IEM IN EN , R., M A N N IN E N , M., Solid State Commun. 15 (1974) 403.

[45 ] ELDRUP, M., MOGENSEN, O.E., EVANS, J.H., J. Phys. F 6 (1976) 499.

[46 ] H A U TO JÄ R V I, P., H E IN IÖ , J „ M A N N IN E N , M., N IE M IN E N , R., Phil. Mag. 35 (to be published).

[4 7 ] GAUSTER, W.B., J. N ucl. Mater. 62 (1976) 118.

[48 ] H A U TO JÄ R V I, P., V E H AN E N , A ., M IK H A L E N K O V , V.S., App l. Phys. 11 (1976) 191.[49 ] D LU BEK, G., BRÜ M M ER, O., HENSEL, E., Phys. Status Solidi A 34 (1976) 737.

[50 ] LY N N , K.G., BYR N E, J.G., M etall. Trans., A 7 (1976) 604.

[51 ] N IEM IN EN , R., M A N N IN E N , M., in Positrons in Solids (H A U T O JÄ R V I, P., Ed.),

Springer-Verlag, Heidelberg (1977), to be published in Topics in Current Physics.

[5 2 ] ORE, A., Univ. i Bergen, A rb o k Naturvitenskap, Rekke No. 9 (1949).

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[5 4 ] STELT, F.R., V A R L A S H K IN , P.G., Phys. Rev. В 5 (1972) 4265.[55 ] ACHE, H.J., Angew. Chem. In t. Ed. Engl. 11 (1972) 179.

[56 ] G O LD A N S K II, V .l., SH A N TA R O VIC H , V.P., Appl. Phys. 3 (1974) 335.

[5 7 ] L E V A Y , B., H A U T O JÄ R V I, P., J. Phys. Chem. 76 (1972) 1951.

[58] B E LL, R.E., G R A H A M , R .L., Phys. Rev. 90 (1953) 644.

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[6 0 ] SINGH, K.P., WEST, R.N., PA U L, A ., J. Phys. С 9 (1976) 305.

[61 ] H A U T O JÄ R V I, P., PAJAN N E, E., J. Phys. С 7 ( 1974) 3817. -

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DO RIKENS, M., J. Phys. С 8 (1975) 393.

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Non-Crystalline Solids (FR IS C H AT, G.H., Ed.) Trans. Tech. Publications, Germany (1977)

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[66 ] BROW NELL, G.L., BU R N H AM , C.A., in Instrum entation in Nuclear Medicine, V o l.2

(H IÑ E , G.J., SORENSON, J.A ., Eds), Academic Press, New Y ork (1974) p p .1 3 5 -1 5 9 .

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A P P L I C A T I O N S O F T H E R M A L

N E U T R O N S C A T T E R I N G

G. KOSTORZ Institut Laue-Langevin,Grenoble, France

Abstract

APPLICATIO NS OF T H E R M A L N EU TR O N SCATTERING.

A lthough in the past neutrons have been used quite frequently in the study o f condensed

m atter, a more recent development has lead to applications o f therm al neutron scattering in

the investigation o f more practical rather than purely academic problems. Physicists, chemists,

materials scientists, biologists, and others have recognized and demonstrated that neutron

scattering techniques can yield supplementary in fo rm ation which, in many cases, could not be

obtained w ith other methods. The paper illustrates the use o f neutron scattering in these areas

o f applied research. No attem pt is made to present all the aspects o f neutron scattering which

can be found in textbooks. From the vast amount o f experim ental data, on ly a few examples

are presented fo r the study o f structure and atom ic arrangement, “ extended” structure, and

dynam ic phenomena in substances o f current interest in applied research.

INTRODUCTION

The use of thermal neutrons in scattering experiments started more than thirty years ago [1 ], and many outstanding and unique results have been obtained on structural and dynamical properties of a great number of substances [2—6].As the neutron carries no net charge, scattering of thermal neutrons by an assembly of atoms is mainly controlled by the interaction of neutrons with the nuclei which can be described by the Fermi pseudopotential, and the interaction of the neutron magnetic moment with magnetic moments of electrons. The theory of thermal neutron scattering is based on the use of the first Born approxim- mation, and the scattering cross-section can be written (see, e.g., Marshall and Lovesey [5] for details)

d2a к ->■dS2dE ftk0

where Í2 is a solid angle, E is energy, k0 is the wave-vector of the incident neutrons, and k0 = |k0|, к is the wave-vector of the scattered neutrons, and к = |k I; -ft has its usual meaning. The quantity describes the neutron-target interaction. It

183

184 K O S T O R Z

W ave-leng th (X )

i_______i______ i______ i______ i----- —i10 1 10"' 10“2 10~3 10~4

Energy feV)

FIG.l. Differential neutron flux as a function of wave-length for different moderators at the High Flux Reactor of the Institut Laue-Langevin, Grenoble, France [7 ].

has the dimension of area, and for purely nuclear scattering jrf is related directly to the appropriately averaged single-nucleus cross-section which is independent of the scattering vector Q defined by .

Q = k0- k (2)

If magnetic scattering is considered, ^depends on the direction and the magnitude of Q and on the polarization of the incident neutron beam. The quantity ,9*(Q,gj) is called scattering function (or scattering law) and has the dimension of time. The angular frequency cj in the argument is determined by

ftco = E0- E (3)

where E0 and E are the energies of incident and scattered neutrons. 5^(Q,cj) describes the physical processes of the sample probed in the scattering experiment and is related, by Fourier transformation, to the time-dependent correlation functions of the scattering system. Equation (1) implies that correlations between the processes contained in л/ and in ,9̂ '(Q,co) can be neglected which is in fact permitted in many cases of interest [5].

T H E R M A L N E U T R O N SCATTERING 185

FIG.2. The energy-momentum plane for neutron scattering (a) in 1970, (b) in 1976, (cj expected ranges accessible with neutrons from pulsed sources (top) and with ultra-cold neutrons (bottom). Replotted from White [8 ].

Most of the neutrons used in scattering experiments today are produced in nuclear reactors, as described in textbooks (e.g. Bacon [6]). Beam tubes serve to extract neutrons in thermal equilibrium with the moderator (at a temperature slightly above room temperature). For special applications, smaller moderators for different temperatures are incorporated to modify the available spectrum of neutrons to be used for scattering studies. As an example, Fig.l shows the three different spectra obtained from the High Flux Reactor of the Institut Laue- Langevin [7]. A hot source (graphite at about 2000 K) and a cold source (D2 at 25 K) provide “hot” and “cold” neutrons of a mean wave-length of roughly 0.9 and 6 Â, in addition to the thermal neutrons (~2 Â) from the D20 moderator.The range of useful wave-lengths thus extends from 0.6 to 20 Â (and higher for special applications), with corresponding incident energies from 230 to 0.2 meV. As these wave-lengths cover the range of interatomic distances and the neutron energies are similar to typical.energies of excitation in condensed matter, thermal neutrons offer ideal possibilities to study the structure and dynamical properties of solids, liquids, etc. The recent progress in expanding the range of accessible values of scattering vector and energy transfer is documented in Fig.2, adapted from White [8]. This progress is the combined result of more powerful sources and more sophisticated instrumentation for scattering experiments. Figure 2 also indicates the expected ranges of Q and AE for ultra-cold neutron sources [9] and pulsed spallation sources [10] which are currently being developed.

186 K O S T O R Z

FIG.3. Coherent scattering length as a function of atomic weight for some elements and isotopes. Replotted from Schmatz et al. [1 1 ].

Neutron sources have a lower luminosity than X-ray or gamma sources, and larger samples are usually necessary. As absorption is normally small, large sample volumes can in fact be studied, and this may also be advantageous if average bulk properties are to be investigated. For the same reason, neutrons are particularly suited for scattering experiments under extreme conditions (temperature, pressure, etc.) where samples have to be contained and surrounded by other materials.

Although neutron scattering experiments may be expensive and time- consuming, they offer information that cannot (or only indirectly or with greater difficulty) be obtained from other experiments. Inelastic neutron scattering is the most direct and detailed method to study dynamical phenomena as documented in a series of conference proceedings published by the IAEA.

The magnetic interaction yields unique details on magnetism on a microscopic scale [5], and the non-systematic variation of the nuclear scattering cross-sections with isotopic mass can be used to produce scattering contrast between otherwise indistinguishable atoms. Figure 3 [11] shows the coherent scattering length b, which is related to the coherent scattering cross-section, acoh, by

CTcoh = 4îrb2 (4)

for bound nuclei and atoms (i.e. averaged according to the natural abundance of isotopes of one chemical species) of the lighter elements. The large difference of b for hydrogen and deuterium has led to a variety of applications in chemistry and biology, as is discussed later. Incoherent scattering studies also benefit from


the high incoherent scattering cross-section of H, whereas D has a much lower incoherent cross-section and can be used to replace H where coherent phenomena are investigated. Similar use can be made of the widely different scattering length of, e.g. nickel isotopes, and neighbouring elements (e.g. Al-Mg), which show no contrast with electromagnetic radiation, can be distinguished.

As stated already, neutron scattering experiments have been performed for over thirty years, and it is not the purpose of this paper to review any of these activities. A bibliography is available covering the literature from 1932 to 1974 [12], and recent references may be consulted for more current information (e.g.Ref. [1 ] and all subsequent papers). With the availability of more neutron scattering instruments, neutron scattering has become a more frequently used method in applied research or research leading to applications. In the following, an attempt is made to illustrate how neutrons are used in the fields of physical chemistry, materials science, and biology. The applied aspect of neutron scattering is emphasized, and examples are given for experiments relating to structure (atomic arrangements), “extended” structure (large-scale inhomogeneities), and dynamics (excitations and atomic or molecular motion). As in any developing field, a coherent picture will only emerge after the situation has been reviewed from several different points of view and, quite naturally, only one can be offered at a time.

STRUCTURE AND ATOMIC ARRANGEMENTS

Coherent elastic scattering yields information about the time-averaged local arrangement of scattering centres with a spatial resolution corresponding to 5 = 2n/Qmax, where Qmax is the maximum scattering vector used in the experiment. For any spectrometer, the upper limit of Qmax is 2k0, and a large range of Q values may be desirable for precise structural studies, especially for amorphous materials and liquids.

For ideal crystalline substances, coherent elastic scattering occurs at Bragg peaks only, i.e. for Q = т, where r is a vector of the reciprocal lattice. This condition can be expressed by the Bragg equation

nX = sin в (5)

where dhkg is the interplanar distance for planes { hkß}, n is the order of .the reflection, and в is half the scattering angle. For simple systems, a few Bragg peaks are sufficient to determine the crystallographic structure, but several thousands of them may be necessary for a complete analysis of molecular crystals. Neutron crystallography is specifically indicated if the position of light atoms (of small scattering cross-section for electromagnetic radiation) is to be studied, or if the magnetic structure is to be determined.

188 K O S T O R Z

FIG.4. Stereoscopic drawing of the dimeric unit of di(p-chlorophenyl)dithiophosphinic acid. The two C\iHbCl-xP(S)SH molecules are linked by one S-H... S hydrogen bond (from Krebs et al. [ 13 p .

As an example for a structural investigation to reveal the position of protons in an organic molecule, the work of Krebs et al. [13] is quoted on a small (3 mm3) crystal of di(p-chlorophenyl) dithiophosphinic acid. This acid crystallizes in the space group PI with two molecules per unit cell. Figure 4 shows the dimeric unit, two Ci2H8Cl2 P(S)SH molecules linked by one S-H ... S hydrogen bond.The proton positions were obtained from a difference Fourier synthesis using X-ray results for the heavier atoms and 1798 recorded neutron reflections. Whereas all hydrogen atoms in the aromatic ring are well localized as expected, the protons of the sulfhydryl groups occupy three partially filled positions which are cystallographically different.

Some of the structural work in materials science has recently been summarized [14], both for nuclear and magnetic scattering, and a major review is forthcoming [15]. Here, less common efforts in applied research are emphasized.

A large amount of work on materials concerns multi-phase systems obtained by phase separation or compaction. As neutrons sample large volumes, it is possible to investigate the structure of minority phases. For example, the binary alloy Al-Mg decomposes near room temperature if the magnesium concentration exceeds about 8 at.% [16]. The coherent Guinier-Preston zones of about 50 Â


radius form an ordered structure which could not be determined unambiguously from X-ray and electron diffraction results. Neutron diffraction measurements on a set of samples after room temperature aging and reversion indicate L 12-type (Cu3Au) order [17]. This leads to Mg concentrations of 25 at.% in the zones and 7.5 at.% in the matrix after complete ageing. In another diffraction study, the interlayer spacing c0 of graphite nodules in a high-carbon steel were measured after different stages of graphitization [18]. From the change of c0 upon stress relief in the surrounding matrix, the pressure insidé the nodules can be estimated.

Structure studies of minority phases represent only one example of the general possibilities of structure determinations under extreme conditions mentioned earlier. As a variety of materials are available which are very transparent for neutrons, cryostats, furnaces [19], pressure cells [20], sample containers for liquids etc. can be made and used without major difficulties in many cases. Other examples where the large sample volume is the key point are texture studies (extensively reviewed by Szpunar [21]), structure determinations of surface layers [8,22], and kinetic measurements in intercalation [23] and adsorption processes.

Monolayers, multilayers or clusters (droplets) of crystalline or amorphous structure may be formed upon physisorption. Neutron diffraction (and also quasielastic scattering, see later) can be used in all these cases if sufficient surface area is exposed to the neutron beam. The structure of deuterated ammonia adsorbed on precooled graphon (graphitized carbon of about 300 Â mean particle diameter and a surface area of 86 m2/g) was recently stiidied [8,24] as a function of adsorbed amount and temperature (between 80 and 200 K). The diffraction patterns showed that ammonia forms large crystallite sheets with the structure of bulk ND3 above a certain critical coverage (~0.25, expressed in hypothetical monolayer units) but a fraction always remains amorphous. Figure 5 [8] shows the diffraction pattern from ammonia adsorbed on graphon, measured at 100 К and at several higher temperatures. A change of the height and width of the diffraction peaks indicates annealing phenomena, while the integrated coherent scattering decreases with increasing temperature until, at a temperature Tms (150 to 180 K, depending on the coverage), all Bragg peaks have disappeared. The ammonia crystallites melt at Tms < Tm where Tm ;= 197 К is the melting point of bulk ammonia.

The structure of nitrogen adsorbed on grafoil (exfoliated graphite, surface area 20 m2/g) has been investigated by Kjems et al. [22]. In this case, continuous monolayers are formed and, despite the low intensities, very significant results, e.g. on the first monolayer structure and on the transition from one to two monolayers, were found.

■ Although crystallographic structure determination provides essential information for many applied problems, the deviations from the ideal crystal structure control many practical properties as well, and neutron scattering has

190 KO S T O R Z

i\ - 100K 1 \zaeШ (Eft l\1 ' X

J V S£ 50a*r-

V i —■

L— A5»и§ 408mО

(1 ' l 1 ¡1! 4

î!

l68K ! \ L

U > ' " y

t 30 1/111 •1 1 1 175 К П

i ' AZUJ►-Z1 1 1 1 ! i, K 4 - . o /

g 20 ас >— э

1

179K A \zо £ 10 UJ ^ 7 X ' Г77К J-nt—3

°,\

0 201 l .J.30 40 5C

SCATTERING ANGLE. 26

FIG.5. Diffraction patterns at different temperatures from graphon after adsorption of deuterated ammonia. The sharp lines are the diffraction peaks of ND3 (from White [8]Л

been used successfully to study short-range order or clustering phenomena in alloys as well as strain fields around substitutional and interstitial solute atoms. Under simplified conditions, the diffuse coherent elastic scattering for a subtitutional binary alloy A-В can be written as [25—27]

f e ) =NJC(Q)I2 lbA - b B +ib i"(Q)-Q|2 (6)VdSÎ/diff s

where Ns is the number of atoms, C(Q) is the Fourier component of the compositional fluctuations as introduced by Krivoglaz [25], b is the average scattering length of the alloy, and s (Q) is the Fourier transform of the displacement field around one solute atom (Debye-Waller factors have been neglected). With neutrons, it is possible to study compositional fluctuations alone by preparing alloys with b = 0. This has been done by several authors [28—30] for the system Cu-Ni, and the state of local clustering has been characterized for different heat treatments.

A knowledge of the displacements around solute atoms may be interesting for several reasons, e.g. for an understanding of the solute atom-dislocation interaction. If the solute site is unknown, its symmetry and lattice distortion strength


FIG.6. Diffuse scattering of Nb-2.54 at.% D at 293K for Q parallel [111]. The open circles are interpolated values obtained from the measured data in the (Oil ) plane of the crystal. Four theoretical curves are shown:

........................... octahedral sites, nearest-neighbour forces only

........................... octahedral sites, nearest and next-nearest neighbour forces----------------------- tetrahedral sites, nearest-neighbour forces only

tetrahedral sites, nearest and next-nearest neighbour forces. From Bauer et al. [32].

can be obtained from diffuse scattering on single crystals (obviously, dilute solutions and bA = bB are desirable). Bauer et al. [31] have shown that deuterium (used instead of hydrogen which would contribute too much incoherent scattering) in niobium occupies tetrahedral interstitial sites, and the displacement scattering can be described by a model with forces on nearest and next-nearest neighbours. Figure 6 shows one example of more recent results [32] including larger scattering vectors to confirm the earlier conclusion. Similar work on Nb-0 is in progress [33]. The { 001} plane was investigated for pure Nb and two different oxygen concentrations. Pure Nb showed constant (incoherent) scattering for Q below the Bragg condition,whereas for the Nb-0 samples diffuse peaks at (1/2 1/2 0), (1 /2 0 0 ) and (1 0 0 ) were found.

It should be stressed that diffuse scattering is a well-established method with X-rays, and that neutrons are not always advantageous, especially if there is a large amount of incoherent scattering. If the coherent contrast is high enough, however, an evaluation of neutron diffuse scattering should be more straightforward as there are no form factor problems, and inelastic scattering can be separated.

In concluding this section on atomic arrangements, the possibilities of neutron scattering in research on amorphous and vitreous systems (metallic and non-metallic), molten salts and liquid alloys are mentioned only briefly. Neutron diffraction is particularly useful in the analysis of amorphous ferromagnets [34,35],

192 K O S T O R Z

Scattering angle 20

FIG. 7. The scattered intensity due to compositional fluctuations Scc/Q) for a liquid “null- matrix" alloy (b = 0), 1 Li-28 at.% Ag at 300°C. The peak at about 12°and subsequent oscillations of SCC(Q) are characteristic for short-range order, i.e. a preference for unlike nearest neighbours (from Ruppersberg [38]).

especially if polarized neutrons are available [36]. Diffraction measurements on liquid alloys are of special value for the study of compositional fluctuations.The formalism is not essentially different from Eq.(6) although other notations are commonly used. For a binary liquid, Bhatia and Thornton [37] have introduced the scattering functions SNN, Scc and SNC where N stands for total number density and С for compositional fluctuations. With these scattering functions, the coherent elastic scattering is

( d n ) c o h = (Б )2 S n n ( Q ) + 2 S n c ( Q ) ( b A “ b B>b + SCC(Q) (bA- b B)2 (7)

Again, a “null-matrix alloy” with b = 0 gives the compositional fluctuations directly. Ruppersberg [38] has used the isotope 7Li with a negative scattering length (see Fig.3) to measure SCC(Q) in 7Li-28 at.% Ag. Figure 7 shows the short- range order in the liquid state at 300°C. More recent measurements at 400°C and 500°C do not reveal any major changes of the scattering pattern [39]. Systems with segregation tendency have also been studied, e.g. Bi-Sb [40], and more work is in progress (Li-Na [41,42], Al-Sn [43] etc.). In these alloys, neutrons also offer the possibility of following directly the process of mixing at different temperatures.

As clusters lead to an increase of scattering at small Q, small-angle scattering studies may be necessary in some of these segregation systems. This particular technique is the basis for the results discussed in the next section.


“EXTENDED” STRUCTURE

If we restrict ourselves to small Q values in a scattering experiment, the spatial resolution may no longer be adequate for a complete determination of all atomic positions. In fact, this detailed information is not absolutely necessary or not of primary interest for a large variety of problems. The term “extended” structure is to characterize this situation where we seek a more global information on the distribution or arrangement of scattering centres. Typical problems of this nature are: the shape, global structure and mutual arrangement of large molecules in solution and in the bulk (biological macromolecules, fibres, synthetic polymers, liquid crystals) of precipitates in liquid, crystalline and amorphous substances, and the distribution of magnetization in magnets and superconductors. If the desired spatial resolution is, e.g. ô ^ 20 Â, the Q range to be investigated can be limited to Q ^ 0.3 Â-1. For Cu-Ka X-rays, the corresponding full scattering angle would be 20max = 4.2°, and 16.5° for neutrons of 6 Â wave-length. For such small angles, special theoretical and experimental requirements have led to the development of particular techniques which constitute the field of small-angle scattering (SAS). In comparison with X-ray SAS, neutron SAS is still a relatively new method but among all neutron scattering techniques it is undoubtedly the one which is most relevant and promising for applied research. Instrumentation and application of neutron SAS have been reviewed by Schmatz et al. [ 11,44], and descriptions of instruments are available for high [45], medium [11] and low flux [46] installations.

As the discrete arrangement of scattering centres can be disregarded, the scattering length per site can be replaced by an (appropriately averaged) scattering length density p(r), and the observable small-angle cross-section is

da _ 1 d£2 ~ N0 / P (r) - p exp(i Q r )d3r

2з;' ' (8)

where V is the sample volume and p is the scattering length density averaged over a volume larger than the resolution volume of the instrument (determined by the minimum observable Q value). For a given scattering length distribution, the cross-section can be calculated analytically or numerically. However, it is in general impossible to evaluate p(r| from measured values of da/d£2.

In the case of Np widely separated identical particles of volume Vp each and of homogeneous scattering length density pp, imbedded in a homogeneous matrix with scattering length density pm, very simple expressions are obtained for da/df2:

da Vp2 Np ^— ( Q ) = ^ ( p p - p m) 2 IFp(Q)l (9)

194 K O S T O R Z

Fp(Q) is the single-particle form factor. For spherical particles or a random orientational distribution of non-spherical particles, the isotropic Guinier approximation yields

da^ 7 (Q) oc exp(-R^Q2/3) (10)

where RG is the average radius of gyration. This approximation is valid for small Q, (Rg Q 1) depending on the shape of the particles. In cases of orientational correlations between particles, the Guinier approximation instill valid but may yield different inertial distances for different directions of Q. More theoretical details can be found in the X-ray literature [47—49], in particular form factor calculations for various particle shapes and approximations of Eqs (8) and (9).

There is a wide range of applications of neutron SAS, e.g. in the study of polymers, macromolecules, fibres and membranes, catalysts, colloids, minerals, metals and alloys, glasses, etc. Only a few characteristic examples are presented here.

In inorganic materials “extended” inhomogeneities control many important properties, e.g. the mechanical strength of alloys, critical currents in hard superconductors, coercive forces in magnetic materials, optical properties of glasses, etc. With neutron SAS, the decomposition in alloys, compounds and glasses, clustering of point defects and spins, dislocation arrangements, pores and microcracks, voids and displacement cascades, critical fluctuations, interfaces and surfaces etc., can be investigated. In crystalline substances, double Bragg scattering can easily be avoided by working with wave-lengths above the Bragg cut-off.

The binary system Al-Zn has been the subject of many neutron SAS experiments [14, 50—55] in recent years, as it presents an interesting case for the study of different decomposition mechanisms (starting from “homogeneous”Al-rich solid solutions). Whereas the decomposition (formation of coherent zones) proceeds by nucléation and growth at elevated temperatures, e.g. at T < 150°C for 6.8 at.% Zn, all experimental techniques seem to indicate that spinodal decomposition [56] takes place at lower aging temperatures (T < 80°C). For this composition of 6.8 at.% Zn, magnetic susceptibility, electrical resistivity and X-ray SAS measurements have been combined [57-59], and a temperature of Ts = (129 ± 2)°C was proposed for the transition from one mechanism to the other. One of the criteria suggested for the existence of spinodal decomposition [58] was the appearance of a SAS “ring”, i.e. a maximum of SAS intensity at Q Ф 0 as shown in Fig.8 for an alloy of the same concentration aged at room temperature [50]. In this case, a peak of the neutron SAS at Qmax = 5.6 X 10~2 Â-1 is clearly revealed and can be attributed to interparticle interference of a system of spherical particles of a radius R = 30 Â. Figure 9 shows that after aging at 133°C


FIG.8. Neutron SAS intensity as a function of scattering vector Qfor Al-6.8 at.% Zn aged for several weeks at room temperature (from Ray nal et al. [5 0 p .

T 1 1 1 1 1 1 1 A I-6.8 a t.e/oZn

200

rg =i3iXaged for 34.8 h at 133°C "

¿I 100 сЭa so

t1 2b

RUN 34762/2 JULY 1 976I 1 1 _i----------1------- i i i

1 2 3

— Q2 ,10'* Г 2

FIG.9. Neutron SAS intensity (logarithmic scale) as a function of the square of the scattering vector, Q2 (“Guinier plot"), for Al-6.8 at.% Zn aged at 133 С for 35 h. Note that the singleparticle scattering function for any homogeneous particle cannot produce a plateau below Q2 £5X 10~s Â '2 (from Laslaz et al. [5 3 p .

an indication of a peak is also visible [53], but the value of Qmax is much smaller (~7 X IO-3 Â"1 ) and could in fact not be observed with common X-ray scattering instruments. It was concluded [53] that the existence of a SAS peak cannot be claimed to give evidence for the operation of any particular decomposition mechanism but simply represents the more or less pronounced interference of an assembly of particles. Furthermore, transmission electron microscopy performed on samples of the same composition indicates [60] that at the temperature of 129°C, no abrupt change of the precipitate morphology occurs.

196 K O S T O R Z

FIG. 10. Zone radius R as a function of aging time at room temperature for an Al-12 at.% Zn alloy quenched from 310°C (open circles) and 380°C (full circles) as determined from neutron SAS (and diffuse scattering) measurements (from Allen et al. [5 2 p .

From the slope of a Guinier plot, as shown in Fig.9, the radius of gyration can be determined if there is a well-defined particle size, and if interference effects do not modify too much the slope at lower Q. For the case shown, one finds Rg = 130 Â, whereas the results shown in Fig.8 yield RG = 23 Â. The zones formed at room temperature are spherical but the larger zones found after long aging times at 133°C are platelets of an intermediate phase a'R, as is known from electron microscopy (e.g. [60]).

The analysis of SAS curves as a function of aging time can be useful in the study of the kinetic aspects of decomposition, its dependence on annealing temperature, quench rate, aging temperature, impurity concentration, etc.Figure 10 [52] shows an example for the change of zone radius R as a function of aging time at room temperature for Al-12 at.% Zn alloys quenched from 310 and 380°C. The aging was interrupted at the indicated times, and measurements were performed in a liquid-helium cryostat. Although it is not possible to decide whether there is an incubation time for the growth of zones or not, the change of growth rate for the sample quenched from 310°C after 80—100 min of aging is quite drastic and can be attributed to the elimination of quenched- in excess vacancies [61].

An integration of Eq.(9) over all Q space allows one to determine the scattering length density of matrix and precipitate (see, e.g. Gerold [62]). As other scattering phenomena intervene for larger Q, an analytical extrapolation has to be used to complete the integration for Q -*■ °°, and this is sometimes difficult. Also, absolute values for the cross-sections and careful calibrations are necessary. Despite these limitations, the integrated intensity can often yield the composition of matrix and precipitates in binary systems, and it is possible to establish stable and metastable miscibility gaps. X-ray and neutron SAS results have recently been combined [63] to determine the metastable miscibility gap for ternary


л 0 h

— ► Average radius R , Â

FIG.ll. Average radius of y precipitates in Inconel 700 turbine blades at different distances ‘ from the blade base (23, 43, 63 and 83 mm) after different service times under normal and elevated temperature conditions (from Pizzi et al. [67]).

Al-rich Al-Mg-Zn alloys aged at room temperature [64]. An alternative way for such studies in ternary systems would be the variation of the isotopic composition of alloying elements as the total error could be reduced by using only one type of radiation.

Decomposition and reversion have been studied in Al-Mg [65] where the contrast is favourable. In Al-rich Al-Si, both low contrast and low solubility of Si lead to small scattering cross-sections, and surface irregularities have to date masked any possible zone scattering from the bulk [66].

The large sample volume for neutron SAS allows one to use this method nondestructively, and it is even conceivable to apply neutron SAS in routine testing operations. Pizzi and Walther [67] have studied these possibilities in detail using a multidetector SAS instrument at a small reactor [46]. In Fig. 11 [68], neutron SAS results on the radius of y' precipitates in turbine blades made from the alloy Inconel 700 (Ni, Co, Al, etc.) are shown for different positions along the blade axis. Two sets of blades, one used at normal temperature, the other at a more elevated temperature, hâve been examined after various service times. It can be seen that there is a considerable increase of the average precipitate radius with increasing service time in the middle of the blades, whereas precipitates in the base and the tip of the blades remain essentially unchanged. Operation at a higher temperature leads to more rapid growth of precipitates and degradation of mechanical properties. It is suggested [68] to use neutron SAS for a nondestructive evaluation of residual life-times of such components.

198 KO S T O R Z

Other nickel alloys and steels were also studied [68] after thermal treatment, creep and fatigue. In magnetic materials, multiple refraction by domains of different magnetization has been observed [68]. This effect could be used to estimate domain dimensions and to follow changes thereof. (In this context it is briefly mentioned that neutron topography [69] and simultaneous measurements of symmetry-related magnetic reflections using the Laue technique [70] provide alternatives for the observation of magnetic domains.)

Cavities, pores, voids etc. give rise to rather strong SAS (as their scattering length density is zero). An increase of SAS intensity near creep rupture surfaces was attributed to the formation of microcracks [68]. Neutron SAS may be useful in fracture studies, e.g. by monitoring the formation of a microstructure leading to rupture during fatigue and creep. Dislocations also contribute to SAS, and Schmatz [71 ] has reviewed this field in detail. The enhancement of dislocation scattering by a coupling of the local magnetization vector to the elastic stress field of dislocations has been the basis for systematic investigations on the SAS scattering in deformed Ni [72] and Fe [73] single crystals near magnetic saturation. The scattering from Fe (deformed to the end of stage I, single glide) can be attributed to dislocation groups in the primary slip plane [73].

Voids in A1 crystals after fast neutron irradiation have been studied by neutron SAS by Mook [74] and by Hendricks et al. [75]. It was found [75] that the single crystals (six of them, irradiated to doses between 0.3 and 2.0 X 1021 fast neutrons/cm2, were studied) showed “isotropic” SAS at small Q (i.e. the scattered intensity depends on the magnitude of Q, but not on its direction), whereas considerable anisotropy was observed for larger Q values as can be seen in Fig. 12. The scattering patterns are interpreted by truncated octahedral voids (there are thus {111} and {001} faces). For anisotropic scattering patterns, a two-dimensional position-sensitive detector as used, for example, at the Institut Laue-Langevin [45], is most appropriate as it allows a complete contour map to be obtained for a given crystal orientation. Figure 13 shows lines of equal intensity for a single crystal of stoichiometric ß'-NiAl containing facetted voids. In these crystals, voids form from thermal vacancies quenched-in from higher temperature. The details of void growth and shape are currently being studied as a function of aging time and aging temperature [76].

Other fast neutron irradiation effects have been investigated with neutron SAS, e.g. in GaAs [77], and in different types of graphite [78]. Most graphites contain a significant volume fraction of pores, and the porosity may change as a function of fast neutron dose. As the pore size distribution is very broad and other large defects may also contribute, the SAS curves do not follow any simple analytical relationship. It is nevertheless possible to follow the evolution of certain ranges of pore sizes as a function of fast neutron dose, and the authors find [78] that there is a decrease of pores with RG 100 Â and an increase.of


d l l d f i

qCT1) q(a_1)

FIG. 12. Neutron small-angle scattering curves for an aluminium single crystal irradiated with ~1.7 X 1021 fast neutrons/cm2 at 55°C. Detectors D2 and D3 are linear position-sensitive, detectors placed 7 cm below (D2) and 5 cm above (Û3) the beam centre line at different distances L.------- D2, D3 : L = 12 m

D2 : L = 5 m• D3 : L — 5 mД D2 : L = 2 mo D3:L = 2mdX/dQ, is the macroscopic differential cross-section. As the voids are facetted but no appropriate symmetry axis is parallel to the detectors, the two sets of data are not identical (from Hendricks etal. [7 5 p .

those with Rg ^ 25 Â upon irradiation. The defect structures of different types of graphite seem to become more similar as a result of irradiation. Similar empirical studies are in progress on irradiated metals [79] and steel samples (surveillance samples) of reactor pressure vessels [80]. In the latter case, a considerable change of the SAS intensity after exposure to 2 X 1019 neutrons/cm2 intensity was found for a weld containing minute amounts of copper whereas the bulk material did not show any measurable changes under similar conditions.

To conclude this brief review of applied SAS in inorganic materials, one very applied example from the field of soil science is presented. The rate of diffusion of water in clay minerals is of great practical importance. A programme has been started [81 ] aiming at an understanding of the properties of water.in

2 0 0 KO S T O R Z

T----- 1---- 1---- 1-----1----- 1---- 1-----1-----1-----1----- 1-----1---- 1-----1---- г

. 18 6 4 8 <j) A= 6 .5 X D=2.4m

,----1___i__ i__ i___i___i__ и__i___i___i___i___i__ i___i__ ■6 4 cm

FIG. 13. Contour lines on the two-dimensional position-sensitive detector of the neutron small-angle scattering instrument D11 at the Institut Laue-Langevin, Grenoble. The sample is a ß'-NiAl single crystal quenched from 1600°С and aged for 22 h at 400°C. The incident beam of a wave-length \ = 6.5 A is parallel to a (110) direction in the cubic single crystal. The crystallographic symmetry of the voids contained in the sample is revealed by the anisotropy of the scattering pattern at larger scattering vectors (from Epperson et al. [7 6 ]j.

R (cm)

FIG.14. Scattered intensity in Debye-Scherrer cone for Na Montmorillonite prepared at 78% relative humidity, as a function of cone radius on the plane multidetector: (a) no compression, (b) 1 compression, (c) 2 compressions (from Thomas et al. [81]J.

T H E R M A L N E U T R O N SCATTERING 2 0 1

FIG.15.- Models of polymer conformation in the bulk. A single tagged polymer molecule is shown in an untagged environment, (a) Gaussian coil, (b) ball, (c) meander concept (from Schelten etal. [85]J.

clay. Clay samples are prepared from sols in water, with platelet diameters of0.3 to 0.5 Atm and so-called small material. The water may then be removed by different techniques, e.g. by compressing the sol above a semi-permeable and porous plate. Neutron SAS has been used to look at the coherent scattering from the small-particle fraction. Without compression, there is almost no Bragg scattering, as shown by the absence of a well-defined diffraction ring in Fig. 14.One and two compressions lead to a “Debye-Scherrer ring” with marked “poles”. The compression has apparently reduced the randomness of the platelet arrangement but a large amount of “polycrystalline” material remains.

Results in other fields like glasses and liquid alloys, liquid crystals, superconductors, magnetic systems, etc. have recently been summarized [14, 44, 54, 82]. Among the current applied studies, the continuing work on Cu-rich precipitates in Се (Co, Cu)5 compounds (high coercive force) [83] and on silicate glasses containing titania [84] should be mentioned.

In polymer science, neutrons offer particular possibilities for the bulk states as it is possible to dissolve some deuterated (protonated) chains in a protonated (deuterated) matrix and to determine the chain configuration (conformation) from neutron SAS experiments, subtracting the scattering of the pure matrix. The Flory model predicts that a polymer chain in the non-crystalline bulk should have a Gaussian coil shape with the chain segments oriented at random (see Fig. 15), as is the case for polymers dissolved in a 0 solvent (i.e. in an ideal solution where the second virial coefficient is zero). These predictions have been confirmed completely for several polymers, e.g. polystyrene [85, 86] in the amorphous bulk and polymethyl methacrylate (PMMA) [87]. Figure 16 illustrates the method which is based on the Zimm expansion of the scattering function as a function of concentration leading to

---- = cK'|F(Q)|2 (1/M + 2A2c + ... Г 1 (11)

2 0 2 K O S T O R Z

ДО

Mff*)

30

20

10

ОÛ2(Â” 2 ) + 0,1clgcm'3 ) — f

_ J ______________ I0 5 10 15 1-10'*) 20

FIG. 16. Zimm plot for protonated PMMA dispersed in deuterated PMMA (from Schelten et al. [87]). See text for details.

where с is the concentration (in weight %) of tagged molecules, K' is a constant containing the contrast due to |bD—bH|, M is the molecular weight, and A2 is the second virial coefficient. As |F(Q)|-2 can be replaced by

for Q2Rq ^ 1, a plot of cK'(da/di2)_1 as a function of Q for different с yields the radius of gyration RG by extrapolating the slope to С = 0, and the second virial coefficient A2 can be obtained from the slope of the ordinate values extrapolated to Q = 0. In Fig. 16, this slope is zero. Extrapolation to Q = 0 and с = 0 must yield — if measurements are done on an absolute scale - the molecular weight of the tagged molecules, which provides an independent check on the assumption of isolated tagged molecules. Complications are frequently encountered in crystalline polymers (see, e.g. King et al. [88] for a discussion).

Interesting applied problems such as conformation in stretched polymers and polymer networks, and phase separation effects in polymer mixtures are currently being studied. Picot et al. [89] have found that the anisotropy of SAS patterns of stretched polymers (from a two-dimensional pattern, RG can be determined for directions parallel and perpendicular to the stretch direction), measured on a sample containing deuterated polystyrene was much less than expected from an

(1 2 )


affine deformation of the molecules. This indicates some relaxation on the molecular level, and it is hoped that the rate of relaxation can be studied directly by neutron SAS. More details on these rapidly progressing topics can be found in forthcoming review articles [90, 91 ].

In the field of biology, there is considerable interest in solving the structure of many large molecules and complex assemblies of molecules as in proteins, viruses, ribosomes, chromosomes, etc. Even if these substances can be obtained in crystalline form, the amount of data in a large-angle diffraction experiment may sometimes be prohibitively large. But in many cases, crystallization has not yet been achieved and the objects must be studied in solution. Neutron SAS offers two useful approaches, and their applications have recently been reviewed by Jacrot [92].

The first method is based on a variation of the contrast between solvent and molecules [93, 94] adopted from light and X-ray scattering and applicable, of course, to the study of polymers as well [95]. Generalizing Eq.(9), we can write for the SAS intensity I(Q), for dilute solutions,

where the integration is over the particle volume (or the part inaccessible to the solvent) and the brackets indicate an orientational average for non-spherical particles as we assume orientational disorder. The important difference from Eq.(9) is that scattering length variations in the large particles under discussion are admitted and can be revealed by SAS. One can write

where pp is the average scattering length density of the particle as before and pF(? ) is the local deviation from the average value. As the integral of pp(r*) over the particle volume must be zero, the extrapolation of SAS curves to Q = 0 yields (per particle)

If ps is varied (by H20/D 20 mixtures) around pp, a plot of the type shown in Fig. 17 [96] yields the volume Vp of the scattering particle. Vp is the dry volume as it corresponds to the volume inaccessible to the solvent. As the H20/D 20 ratio is varied in the solvent, the value of pp can also change because of H-D exchange. In equilibrium, the number of exchanged protons in the particle shoul be proportional to the D20 concentration in the solvent, and a straight line will

(13)

P(^> = Pp + PF(" ) (14)

1(0) = (pp - P s) 2 Vp (15)

204 K O S T O R Z

ps (l0 cm A

FIG.17. Square root of SAS intensity extrapolated to Q — 0 asa function of D20 concentration in H-iPlDiO for the 50 S subunit of ribosome (RNA + 34 proteins). Based on Eq.(15), a dry volume of 1.5 X 106 Â 3 is obtained (from Stuhrmann et al. [96],/.

still result, but Vp can only be determined if the fraction of exchanged protons is known.

More interesting information can be obtained from the low Q portion of the SAS curve where the Guinier approximation, Eq.(10), is valid. In the presence of scattering length fluctuations рр(г*), the apparent radius of gyration is

RG = R G V + ^ J T2Pv(?)d3? - ~ ( J r pF( F V i V (16)

v p v p

where

Ki = (Pp—Ps)Vp (17)

and RGy is the usual mechanical radius of gyration of a particle with homogeneous scattering length density. The second term in Eq.(16) is always present if scattering length fluctuations exist. It introduces a linear dependence of RG on (pp - ps)-1 . The third term takes into account that the centre of mass of the particle and the centre of scattering length distribution may not be identical. Their separation will be contrast dependent. Stuhrmann and Fuess [97] have been able to observe this term in hen egg-white lysozyme. The results are shown in Fig. 18. The separation is small in this case (2 Â for pp- ps = Ю10 cm-2). It is somewhat easier to determine the separation of two parts of a particle which have


Vp (l0~10 cm2)

FIG. 18. Variation o f the square o f the apparent radius o f gyration as a function o flfp = pp — ps, according to Eq.(16), for hen egg-white lysozyme (from Stuhrman and Fuess [97]).

FIG .19. Triangulation method applied to a complex particle in which a component can be deuterated (o) or protonated. The result o f the operation shown in the diagram yields the distance R 11 (drawing from Jacrot [92]).

drastically different scattering length densities, as demonstrated by Duval et al.[95] for a diblock copolymer of deuterated and protonated polystyrene in solution (D/H-cyclohexane). Careful separation of the different scattering terms over a wide Q range is necessary for testing models for the shape of large particles, e.g. proteins (recently reviewed by Stuhrmann [98]).

For very large assemblies, e.g. the ribosomes mentioned above, selective deuteration, the second important method in neutron SAS, is suitable. In these complex objects, the immediate goal is to understand the mutual arrangement of RNA and proteins. Since it is possible to reassemble the ribosome subunits from their components, it is easy to introduce deuterated proteins at identifyable positions. If one selects two components 1, 2 of the object which are either protonated or deuterated, the triangulation method [99, 100] sketched in Fig. 19 yields

Ipair = I (ld, 2d) + I(lp, 2p) - I (ld, 2p) - I (lp, 2d) (18)

206 K O S T O R Z

fiСDо

'e 0 E -0.2

0.2

2; 0S 0.2С

(b)

(C)

0.1

q (a ” ')

FIG .20. Triangulation experiment on the 30 S ribosomal subunit (RNA and 21 proteins). The intensity difference according to Eq.(18j is shown for three different pairs o f proteins:

(a) pair S2-S5 (distance 105 A)(b) pair Sg-Sg (distance 35 A)(c) pair SyS7 (distance 115 A)(from Engelman et al. [101]/.

where I is the intensity for the different combinations of protonation and deuteration and Ipair represents interference effects between 1 and 2. If both co m p o n en ts can be ap p rox im ated by p o in ts (w ith in th e exp erim en ta l re so lu tio n ),

where R12 is the distance between 1 and 2.Figure 20 shows an example from Engelman et al. [ 101 ] on the other ribosome

subunit, 30S. Much more work on proteins, chromatin, viruses, enzymes, etc. is discussed in the review by Jacrot [92] from which the above summary is drawn and where further references can be found.

Fibrous proteins, e.g. collagen [8, 102], muscles, and biological membranes and membrane components [103-105], represent somewhat more organized scattering systems than solutions of macromolecules, and low-angle diffraction peaks corresponding to the packing arrangement of fibrils and lamellar structures are analysed to determine molecular conformations.

Kinetic studies with neutrons on H/D exchange [106], on the incorporation of water and other species into biological substances and systems [92] are in an exploratory stage, and deserve further attention.

sin(QR12)QR12

(19)


So far, we have only covered elastic scattering phenomena. As many practical properties of substances depend on structural features, this aspect of neutron scattering relates more directly to applied problems. Inelastic scattering of neutrons reveals dynamical properties of matter, either on individual scatterers or individual groups, mostly by incoherent scattering, or on collective excitations (phonons and magnons), mostly by coherent scattering.

Apart from the possibility of measuring elastic constants of substances for which other methods fail, by extrapolating phonon dispersion curves to long phonon wave-lengths, scattering by phonons offers very detailed information on dynamical properties of crystalline solids which may be important, e.g. for testing models for phase transformations, especially of the displacive type [107—109]. Much interest is currently devoted to the study of layer-type crystals (see, e.g., Stirling et al. [110]) which may have some practical importance. Phonons in metal-hydrogen (deuterium) systems [111, 112] and in crystals containing radiation-produced defects [113, 114] have been studied and provide some fundamental insight into symmetry and coupling of defects in a lattice.

However, much ггюге practical information can be obtained from incoherent scattering studies related to the motion of individual atoms or molecules. The incoherent scattering cross-section is proportional to the Fourier transform of the self-correlation function Gs(r*,t) where t is time,

D Y N A M IC S

For simple translational diffusion controlled by a rate equation, the Chudley- Elliott model [115, 116] yields for Sinc (Q,cj) a sum of Lorentzians centred at energy transfer ftco = 0. The frequency width of this quasielastic scattering depends on the mean rest time and the site geometry in a solid. For small Q, Sinc can be expressed by a single Lorentzian.

Sine(Q>^) = — — “ exp(-Q2<u2» со +

(2 1 )

and

Г = Q2D (22)

where D is the diffusion coefficient.

208 K O S T O R Z

Т(’ С)

Ю3/ Temperature (к

FIG .21. Diffusion coefficient for hydrogen in NbH as a function o f reciprocal temperature, for dilute and near critical concentrations. Whereas the Gorski-effect results D G show critical slowing down because o f the thermodynamical factor, the neutron results show the diffusion coefficient in an equilibrium situation. I f DG is “reduced” to the same situation [123], there is good agreement between the neutron results and the reduced value D ( f r o m Springer and Richter [ 118],/.

For diffusion in the solid state, high-resolution instruments are necessary as D > 10~5 cm2/s is required for standard time-of-flight instruments. With the backscattering technique [117], D values down to 10~7 cm2/s are accessible. A high incoherent cross-section is nevertheless a prerequisite, and hydrogen has so far been studied most extensively [118, 119] although other nuclei have been used too (e.g. Ag in Agi [120], a superionic conductor with Ag as fast diffuser,Na self-diffusion near the melting point [121]).

As an example for hydrogen diffusion in the solid state, Fig.21 shows results for two different hydrogen concentrations in Nb [122] in comparison with results from Gorski-effect measurements [123]. More recent experiments with neutron quasielastic scattering [124] confirmed the change of activation energy near 250°K in NbH0. on which is probably related to a transition from classical jump diffusion to tunnelling motion and was first observed with the Gorski effect [125].


Another study concerns the trapping of hydrogen by nitrogen interstitials [126] which can be rationalized by a two-step random walk model involving trapped and free states for the proton with different life times [127].

First results have been obtained on the diffusion of hydrogen on surfaces of a nickel catalyst. At 150°C, the hydrogen surface diffusion coefficient for a0.25 monolayer was found to be ~0.8 X 10~7 cm2/s [128]. More experiments are planned. High energy transfer neutron spectroscopy has been used on similar material (Raney nickel) to study the vibrational properties of adsorbed hyrogen [129]. Hydrogen adsorbed on graphite-potassium intercalation compounds is currently being studied with the same techniques [130].

Whereas the above diffusion studies on hydrogen in the crystalline bulk or surface state are tedious and require high resolution, diffusive motion in the liquid state is easily measurable as long as the cross-sections are large enough. Diffusion coefficients of hydrogen in liquid Li have recently been determined by Sköld [131]. This information would be very difficult to obtain directly by other methods. In molecular systems, the dynamics can be decomposed into centre-of-mass motion,1.e. translational diffusion as above, and reorientation around the centre of mass.In addition, there are internal (vibrational) degrees of freedom. The latter can usually be separated quite easily but there is no general way of separating quasielastic scattering due to translational and rotational motion. The analysis of quasielastic scattering of liquid polymers, liquid crystals, etc. is therefore often based on simplified models, but well-planned experiments can also help to eliminate particular models. Review articles on polymers [132, 90], liquid crystals [133], and macromolecules [134] give complete accounts of these activities.

CONCLUSION

To review all the applications of neutron scattering in a single article is a formidable taSk, and the author apologizes for the lack of balance, profoundness, perspective and completeness that will certainly be discovered by the critical reader. It can only be hoped that some of the examples presented here have served their purpose of illustrating the possibilities of neutron scattering in applied research.

ACKNOWLEDGEMENTS

Thanks are due to many colleagues at I.L.L. for clarifying discussions and to Prof. B. Krebs, Bielefeld, for Fig.4.

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CHRICHTO N, R.R., Proc. Natl. Acad. Sei. USA 73 (1976) 2379.

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Brookhaven Symposia in B iology, N o .27, US Dept, o f Commerce, Springfield, Va.

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D E V E L O P M E N T S I N I O N I M P L A N T A T I O N

D.W. PALMERSchool of Mathematical and Physical Sciences,University of Sussex,Brighton, United Kingdom

Abstract

DEVELOPMENTS IN IO N IM P LA N T A T IO N .

Ion im planta tion is the deposition o f im p u rity atoms in to solid's fo r the purpose o f

changing or studying the properties o f the solids, this deposition being accomplished by

bombardment w ith accelerated ion beams. The paper outlines the physical phenomena that

have been the subjects o f experim ental and theoretical research in investigating the in teraction

o f energetic ions w ith solids, and describes and discusses the im portan t and recent applications

in respect o f the ion im plan ta tion o f metals and alloys, o f semiconductors and o f insulating

materials. The continuing expansion and progress in the applications o f ion im planta tion owe

much to the very close connections, as emphasized in the paper, between the many basic and applied aspects.

1. INTRODUCTION

“Ion implantation” is the term used to denote the irradiation of solid materials with fast atoms and molecules (of energies ~ 103—107 eV) for the purpose of changing or investigating the properties of the solids. The word “ion” in the name refers to the fact that the acceleration of the atoms or molecules is most conveniently accomplished by electric fields; the species to be implanted is thus usually formed into a beam in an ion accelerator comprising an ion source and the electric or electromagnetic acceleration and focussing systems. Equipment of the Cockroft-Walton, transformer or Van de Graaff types involving a high potential for the acceleration, or of the cyclotron type, have all been employed successfully to produce ion beams for implantation purposes. During the last ten years ion implantation has developed as an important field of academic and applied science, and its development can be considered as being one of continual interplay and innovation between basic resèarch and an increasing variety of applications. It is this continuing connection, the variety of investigative techniques from solid-state, atomic and nuclear physics, chemistry, electronics and materials science, and the wide range of technological aspects that give ion implantation its great interest and value.

The aim of the paper is to review recent developments in ion implantation and to emphasize the very close connections, or indeed the absence of distinction

215

216 P A L M E R

in many cases, between the basic research and the applied aspects. The paper considers the experimental and theoretical investigations concerning the interactions of energetic ions with solids, and describes and discusses the important applications in respect of the ion implantation of metals and alloys, of semiconductors and of insulating materials.

2. GENERAL CONSIDERATIONS AND BACKGROUND

Many papers concerned with all aspects of ion implantation can be found in the proceedings of two series of international conferences. The emphasis in the conference series “Atomic Collisions in Solids” (of which the four most recent, [1—4], were held in 1969-1975) is upon all aspects of the dynamic behaviour (kinetic energy E > kT) of the ion as it traverses the solid (of temperature T), and includes consideration of the processes by which the ion loses energy, the atomic and electronic excitations in the solid, and the formation of radiation damage defects. The second conference series, [5—13], deals especially with the effects due to the presence and behaviour of the implanted atoms and the radiation damage defects after the ion has effectively come to rest (E = kT) in the solid; this is the realm of solid-state physics and chemistry, of materials science and of modern physical electronics. The dynamic, collision processes are by their nature (kinetic energy of ion > atomic binding energy in the solid) somewhat insensitive to the detailed structure of any particular solid, and it has been found possible (see Sections 3 and 4) to formulate general theories of the processes which give reasonable or good agreement with experimental results in most cases. On the contrary, the non-dynamic ion-implahtation effects depend strongly on the natures of the implanted atom and of the solid, and no completely general rules can be applicable; the experimental studies of the ion implantation effects of many different ions in different elements and compounds have in fact demonstrated how relatively meagre is our broad understanding of the properties of impurities in solids and of their interactions with lattice defects.

In addition to the two conference series [1 -13] mentioned in the previous paragraph, various books [14-22] and review articles [23-26] have covered many aspects of ion implantation. A previous review article [26] by the present writer discussed ion implantation from a practical point of view, and described the equipment and techniques needed for developing experimental work in this field of academic and applied study. Two fairly recent bibliographies [27, 28] have given extensive lists of references to ion implantation work.

ION IMPLANTATION 217

3. RATES OF ENERGY LOSS AND THE SPATIAL DISTRIBUTIONS OFIMPLANTED ATOMS

3.1. Distinction between energy loss rates due to electronic excitation and nuclear-nuclear collisions

The processes by which energetic ions lose their energy in traversing solids have been considered in various celebrated papers over many years. A recent review by Dearnaley [29] contains a concise but very clear description of all important aspects of this topic. The various treatments consider that when an energetic ion enters a solid it interacts with and can transfer energy to the electrons in the solid, and can also lose energy to the atoms by ion-atom forces arising from the screened-coulomb interaction potential between the nuclei of the ion and each of the target atoms. Lindhard [30] suggested that these two kinds of process, “electronic” and “nuclear”, could be considered as independent contributions to the total rate of energy loss (-dE/dR)t0tal with path distance R through the solids, and thus that

The justification for this assumption is that the internuclear interactions are on average more violent but rarer collision events [31 ] than the ion-electron collisions and that the internuclear electronic screening is affected by excitation of only those electrons which are in orbitals at distances from each nucleus less than the internuclear separation, and not by the excitation of electrons at greater radii such as the valence electrons etc. (Excitation of non-inner shell electrons is almost always the major component of the electronic contribution to the total rate of energy loss.) It has been suggested by Thompson and Neilson [32] that under certain conditions, where there is a large probability for resonant inner- electron-shell excitation of target atoms, the screened-coulomb internuclear potential may be so significantly changed that the assumption of independent electronic and nuclear-collision energy loss rates may not be valid. However, recent experimental data [33—37], giving the projected ranges of various heavy ions in silicon, aluminium and aluminium oxide at ion energies where the dominant contribution to the energy loss rate is that of the screened-coulomb internuclear collisions, show smooth changes of projected range with ion energy and only small deviations from a common range-energy curve, in reduced coordinates pp and e (see Section 3.2), and give no evidence for the breakdown of the assumption of the separability of the electronic and nuclear energy loss contributions.

( (1 )

218 P A L M E R

3.2.1. Rates o f energy loss in the LSS theory

The term “ion implantation” contains the word “ion” because in practice it is most convenient of course to accelerate atoms when they are electrically charged. Furthermore, the arrival of electric charge on the target solid is a straightforward way of measuring the dose of implanted atoms. In fact whether the species to be implanted arrives at the surface of the solid in a charged or neutral state is likely usually to have very little effect upon its behaviour in the solid or upon its effect on the solid. This is because the species will, in a time of ~ 10-16 s (i.e. an electron orbital time), tend to take up an equilibrium charge state dependent mainly on its velocity but to some extent also on the electrons in the solid. Thus, for the ion energies usually used in implantation work the equilibrium charge state will be attained in a distance (e.g. of a few Â or a few tens of Â) small compared with the total range of the implanting atom. Ions moving at low velocities through solids will tend to capture electrons and become neutral atoms, and Bohr and Lindhard [38] in 1954 suggested that for atoms of atomic number Zx moving through a solid the critical velocity in respect of the charge-state could be considered as about Zf/3 v0 (where v0 is the Bohr velocity с /137), since this is the approximate value of the mean orbital electron velocity in the Thomas-Fermi statistical model of an atom [39a].

With this in mind and using the procedure of treating separately the electronic and nuclear energy loss contributions, Lindhard, Scharff and Schi^tt [39] in 1963 proposed a comprehensive theory (“LSS”) for the energy loss rates of fast ions incident upon amorphous solids, valid for ion velocities up to about Z?/3v0. This upper validity limit corresponds to the velocities of 2.5-MeV B+ ions, 15-MeV Na+ ions and 200-MeV Kr+ ions [29] and the regime of expected validity therefore includes the majority of uses and applications of ion implantation to the present date. It is certainly no exaggeration to say that the very considerable development of the subject and applications of ion implantation in the period 1967—77 would not have been possible without the availability of the LSS theory of 1963. Although (or even, perhaps, because) subsequent experimental results have shown some inaccuracies in the theory in respect of both the electronic and nuclear collision contributions, the existence of the theory has provided an extremely valuable stimulus to experimental and theoretical work.

The electronic contribution to the energy loss rate in the LSS theory is based on the work of Lindhard and Scharff [40] of 1961. Here, as also for considering the nuclear energy-loss contribution, the ion-target atom potential V(r) is assumed to be screened-coulomb of the Thomas-Fermi (TF) type:

3 .2 . T h e L S S th e o ry o f io n e n erg y -lo sses an d ran ges

(2 a )


where Ъх and Z2 are the atomic numbers of the projectile ion and target atoms respectively, ajF is the Thomas-Fermi screening distance given by

aTF = 0.885 a0/(Z2/3 + Zf'3)* (2b)

(where a0 is the Bohr radius equal to 0.529 Â), and ф0 (r/атр) is the Thomas- Fermi screening function (tabulated values are given in [39a]). The theory [40] treats the electrons as an electron gas of density and distribution calculated from

•the TF potential and gives the value of (~dE/dR)e as proportional to the ion velocity v for the condition v < Z2/3v0. The result is

8Z}/6 / Zi Z2e2 , ,

l f , T F V v. ( z ; /at z j ' 3> ̂ <3)

where N is the number of atoms per unit volume in the solid. In terms of normalized energy and distance parameters e and p, respectively, given [40, 41] by

e = E-

and

M2 \ / / Zi Z2e2 j 32.5 A2E (keV)M j + M j / / V aTF / (A1+A2)-Z1Z2-(Z?/3 + Z^3)̂

, M.M2 \ , 2 166.8 A ^ ^ g /cm 2)p ' ' M Ж aT F /-д + л.'»г./ 72/3 + 72/34 ( )

where E is the energy of the projectile, and M! (and At) and M2 (and A2) are the masses (and atomic mass numbers) of projectile and target atom, respectively, the electronic energy loss rate can be expressed as

~ 0 \ ° kei (5)

1 Z\n Z\n (M! + M2)3/2 where к a 3.396 m|- ' щГ, щ,Г

where me is the electron mass. If the masses Mj and M2 in this expression are taken to be in atomic mass units then the factor 3.396 m| becomes equal to 0.0795. The value of к falls between 0.1 and 1.5 for most ion solid combinations.

Values of (de/dp)e as a function of are shown in Fig. 1 for к = 0.15 and1 .5 .

2 2 0 P A L M E R

rVl

FIG.l. The rate of ion energy loss (de/dp)n by elastic internuclear collisions (solid line) and two possible forms, 0.15 e5 and 1.5 e5, of the rate of ion energy loss by electronic excitation effects, according to the LSS theory [39]. The parameters e and p are the reduced energy and distance values of expression (4). An energy-independent (de/dp)n would be obtained for an r~2 internuclear potential (broken horizontal line). From ЗсЫфП [45].

The calculation of the nuclear collision contribution to the rate of energy loss in the LSS theory [39] is again based on the assumption of a Thomas-Fermi interaction potential, and uses the basic formula

* m

' „ ■ N / T f e ) ' dT=N I T l ~ j - d T (7)

where dan/dT is the differential collision cross-section with respect to energy transfer T to a target atom and Tm is the maximum energy transferable (i.e. in a head-on collision). It turns out that a single universal curve, proposed to be valid for all ion-solid combinations, can be found giving the variation of (de/dp)n with 6. This universal curve is shown in Fig. 1.

It is seen from Fig. 1 that the nuclear stopping contribution dominates at low energies and the electronic contribution at high energies. The cross-over energy is, for example, at 17 keV for B+ ions in silicon and at 140 keV for P+ ions in silicon [16]. At much higher energies (v <; Zj/3 v0) the method of Lindhard

ION IMPLANTATION 2 2 1

and Scharff [40] for the electronic contribution is not applicable, and for v > Z2/3v0 the ion travels through the solid as a bare nucleus. For the latter condition the Bethe-Bloch expression [42, 43] for (dE/dR)e becomes valid; this causes (dE/dR)e to decrease with increasing ion energy.

3.2.2. Ranges and spatial distributions o f implanted atoms

From the point of view of applications, the primary concern is the depth distribution of the implanted atoms when they have come to rest in the solid. This need has stimulated the development of calculational methods to provide theoretical depth distributions from the LSS energy loss data. The total path length Rtot of the ion in the solid can be found from the expression

Lindhard et al. [39] showed universal p(e) range-energy curves obtained for different values of the electronic к parameter of expression (6); from these curves theoretical Rt0t(E) can be found by using expression (4). However, practical ion implantation requires knowledge of the projected range Rp parallel to the incidence direction and the perpendicular or lateral range R .̂ Because the ion trajectory in the solid is zig-zag rather than straight, Rp Ф Rt0t and R ̂Ф 0. Estimates of the ratio Rp/Rtot [16, 39, 41, 44] allow approximate values of Rp to be obtained. By using standard statistical formulae, Lindhard et al. [39] also estimated the spatial longitudinal spread of the implanted atoms by assuming in a first approximation that the distribution was of gaussian form defined by the mean range and the standard deviation (the “straggling”) of the range distribution, these then being converted to estimates of the corresponding projected range parameters, <Rp> and (ARp>.

However, in response to the need especially in semiconductor technology for greater accuracy in the knowledge of the spatial distribution of implanted atoms, more recent theoretical work has aimed to calculate directly the distribution with respect to the projected range coordinate. A convenient and effective method [45—49] is to calculate successive moments of the projected distance distributions (respectively, the mean range, straggling, skewness, kurtosis etc). The distributions themselves can be constructed from these moments by use of mathematical expressions of the Gram-Charlier or Edgeworth forms [50], or, if the skewness is not large, of the Mylroie-Gibbons (joined half-gaussian) form [51 ].

E

(8)

0

2 2 2 P A L M E R

FIG.2. The results of an LSS calculation of the mean projected range (Rp>, the standard deviation (ДRp) of the projected range and the lateral spreading standard deviation <AXi>

for nB* implanted into silicon. From Furukawa and Matsumura [54].

Tables of calculated values of the first, second and third projected distance moments for 20-1000 keV implantation of boron, phosphorus, arsenic and antimony ions into silicon have been given by Mylroie and Gibbons [51], and similar calculated data for many ions of energies 10— 1000 keV in various solids have been published by Gibbons et al. [52]. Such tables are of great value in practical applications of ion implantation. It should however be emphasized that it has been found (see Section 3.2.3) that the use of Thomas-Fermi interaction potentials and of the associated electron density distributions gives energy loss and range data which differ to some extent from experimental results, and thus that the calculated TF data should be treated with some caution. This matter is considered briefly in Section 3.2.3. The projected distance tables of Mylroie and Gibbons [51] and of Gibbons et al. [52] mentioned above in fact make use of experimentally based electronic-energy-loss rates, where possible, instead of LSS values. It is also to be emphasized that the methods of calculation of these range statistics assume that there is no crystallographic atomic structure in the target solids (i.e. that the target is amorphous).

The same general method outlined in the previous paragraphs can be applied to find theoretical values for the extent of lateral spreading of implanted ions, and this has been done for example by Furukawa et al. [53, 54] for several ions in silicon. Figure 2 shows the results of their calculations for boron ions incident upon silicon. It can be seen that the lateral spreading is expected to be larger


than the projected range straggling and to be a significant fraction of the mean projected range. In respect of the use of ion implantation in semiconductor technology for the fabrication of auto-registered FET devices (see Section 7.2), lateral spreading of the ions to regions inside the geometrical shadow of the mask can have deleterious effects on device characteristics, and lateral spreading as shown in Fig. 2 certainly needs to be taken into account. The tables of Gibbons et al. [52] also give theoretical values for the lateral standard deviations for all the ion-solid combinations considered. Comparison of the Furukawa et al. [54] and Gibbons et al. [52] data, where this is possible, shows that the two sets of calculations give very similar mean projected range values, fairly similar values for the standard deviation of the projected range, but rather different values for the lateral standard deviations.

3.2.3. Validity o f the LSS energy-loss calculation

The LSS theory deals with energy losses by both electronic processes and screened nuclear collisions. For each of these kinds of energy loss the LSS theory predicts smooth changes of the rates of energy loss with respect to the atomic numbers or masses of the implanting ion and the target solid. For the energy region where the electronic energy loss rate (~dE/dRp)e is dominant its dependence on Z1 (the atomic number of the projectile) at constant Z2 (i.e. in a certain element as target) is usually considered, in the context of the LSS theory, at a particular projectile velocity. Hvelplund and Fastrup [55] found that for various ions of the same velocity traversing amorphous carbon the electronic energy loss rate was an oscillatory function of Ъх, and that the smooth theoretical curve of the Lindhard-Scharff theory [39, 40] was a good mean of the observed oscillatory variation. Cheshire et al. [56] subsequently showed that the use of Hartree-Fock-Slater (instead of Thomas-Fermi) atomic wave functions in a Firsov- type [58] treatment of the energy loss to electrons gives theoretical results in very good agreement with the experimental data [55]; this indicates the effect of the shell-structure of the moving atom, the atoms of smaller size having lower electronic energy loss rates. Similar Zj oscillations in the electronic stopping regime have been found for other target solids.

Oscillations of electronic energy loss rate as a function of Z2 at constant Zx are also found. Recently, Land and Brennan [57] have shown that the same kind of Firsov-type treatment, using HFS wave-functions as were employed by Cheshire et al. [56] for considering the Ъх oscillation, gives very good agreement with the oscillatory energy-loss rates observed as a function of Z2 when 800-keV nitrogen ions are incident on various target elements. In both calculations [56] and [57], ground-state HFS wave-functions for singly-ionised projectiles and neutral target atoms were used.

224 P A L M E R

To complete this consideration of the electronic energy loss contribution it should be noted that the linear dependence on ion velocity predicted by both the basic Lindhard-Scharff [40] (expression (3)) and Firsov [58] treatments is not (e.g. [55, 59—61 ]) found to be in general agreement with experimental results.In the Firsov theory as used by Cheshire et al. [56] and by Land and Brennan [57] the linear dependence on ion velocity arises because of the rather arbitrary mathematical separation of the electron velocity due to the ion motion from the electron velocities within ion and atoms; it may be that better agreement of theory with experiment could be obtained if composite electron velocities could be used within a single integral in the calculation.

We now consider briefly the comparison of the nuclear energy loss data of the LSS theory with experimental results. Some recent experimental studies of the depth distributions of various heavy ions (Zt = 54—83; energies = 20 -1 0 0 keV) in silicon and aluminium [33—37] show that in the nuclear stopping region the actual projected ranges are systematically greater (by about 30%) than those predicted by the LSS theory. Thus, the LSS theory overestimates the nuclear energy-loss rate by about 30%. In contrast to electronic stopping, these nuclear stopping data seem to show no evidence for significant Z¡ oscillations. It is implied by Combasson et al. [37] that the Thomas-Fermi screened Coulomb potential (used in the LSS treatment) may not be accurate for these ion-atom collisions; they show that the measured projected ranges lie between those calculated using the Thomas-Fermi potential and the Lenz-Jensen potential [39a]. They also show that the relative range straggling (i.e. <ARp>/(Rp)) is rather insensitive to the choice of potential. From measurements of the nuclear energy loss rates for several light ions in silicon, Grahmann and Kalbitzer [62] have suggested that here also the LSS theory overestimates the loss rates; in their work the general difference between theoretical and experimental values was found to be much greater than 30%. However, Lee and Mayer [92] have presented graphical comparisons which show good agreement in the experimental and theoretical mean projected ranges and range straggling data for 11B+, 31P+ and 7SAs+ of energies 20 keV to 1 MeV implanted into silicon (for boron in silicon the theoretical calculations use experimental electronic energy loss rates); it seems that for these not-very-heavy ions any error in the LSS nuclear energy loss rate is not significant when taken for the whole path of the ion over which the ion’s energy changes from its initial value to zero.

3.3. Energy losses and ranges of very light ions

The LSS theory [39] of the energy loss rates of ions in solids deals with electronic-stopping in the low-velocity regime, and so is not applicable for considering the energy losses and ranges of very light ions (protons, deuterons and helium ions), of moderate or high energies. For protons and helium ions,


however, empirical or semi-empirical energy-loss and range data are available [63, 64] in tabular form for various elemental target materials. Recent experimental data for particular elements include that for 1.5 to 60 keV protons in silicon [65], and for 250 keV to 2.5 MeV protons and 4He ions in hydrogen, nitrogen, oxygen and erbium [66]. In the latter work [66] it is shown that the three-parameter theoretical formula of Brice [67] for electronic stopping (obtained by use of a modified Firsov [58] treatment) can be accurately fitted to the experimental results. The ranges of very light ions are much larger than those of moderate or heavy ions of the same energy because of the smallness of the nuclear stopping contribution.

4. DISPLACEMENT OF ATOMS DURING ION IMPLANTATION

It is well known that lattice defects in crystalline solids can strongly affect some properties of the solid, and theoretical solid-state physics can attempt to give quantitative explanations of these effects. Thus, it was clear from the beginning of the use of ion implantation that the ion-target-atom internuclear collisions (see Section 3) that transfer enough energy to displace target atoms from their usual sites could lead to large changes in the properties of the solid which might mask the required doping effect of the implanted atoms. It is therefore of importance to be able to make theoretical estimates of the depth, ion and target dependence of the lattice defects produced during ion implantation. (General discussions of radiation damage effects and of the properties of lattice defects can be found in Ref. [26].)

The most convenient approach in considering the irradiation damage effects of ion implantation is to say that the defect concentration as a function of projected distance Rp into the target should be proportional to (— dE/dRp)n, the rate of ion energy loss by nuclear collisions at the depth Rp [49, 68, 69]. If it is then assumed that it is possible to define, for each target material, a mean or effective displacement energy Ed, which is the average energy needed to be imparted in ion-atom internuclear collisions for production'of an interstitial defect and a lattice vacancy, the initial (i.e. pre-annealing) density nd of displaced atoms per unit volume should be expressed as

where Nj is the number flux of the incident ions. This expression (9) is based on theoretical ideas of Kinchin and Pease [70] modified slightly by Sigmund [71 ], and on consideration [49, 68, 69] of the collision processes themselves, often

(9)

in. this context called “atomic” or “elastic” collision processes. Expression (9)

226 P A L M E R

is expected to be only an approximation to the number of displaced atoms formed in any particular case because it ignores all the details of the recoil-energy spectrum of the displaced atoms, the likely anisotropy of the energy for displacement of an atom, the initiation of atomic collision sequences by recoiling atoms, the recombination of interstitials and vacancies and the formation of defect complexes. Nevertheless, when the spatial distribution of the energy into internuclear collisions, (—dE/dRp)n, calculated [47, 49, 68, 72] by the method of moments (see Section 3.2.2) is used in expression (9), the resulting depth distribution of lattice disorder is found to agree quite well in shape and depth scale with experimentally measured disorder distributions [49, 68]. Tabulations such as [72] of the rates of ion energy loss into nuclear collision processes for various ions, energies and target materials are thus of great value in ion implantation studies. It is difficult however to calculate absolute values for nj in expression (9) because Ed is rarely known and because of the complexity of the defect processes as mentioned above. During electron irradiation the minimum energy transfer to an atom to displace it from a lattice site to an interstitial site is found to be ~ 10—25 eV for most crystalline solids; the values of Ed are expected in many materials to be significantly greater than the corresponding minimum electron-irradiation values.

Graphs and tabulations of the depth distribution of energy transfer into electronic excitation processes, calculated by the moment method, are also available [49, 72] for use in considering ion-implantation-related processes, such as the production of defects in insulators and possibly ionization-enhanced diffusion in insulators and semiconductors, which may be dependent on local ionisation densities. It has recently been emphasized [73] that the large ionisation density along the track of an ion moving through a covalently-bonded solid (such as the elemental and III-V compound semiconductors) may, by bond-breaking, significantly reduce the displacement energy value E¿ to below the threshold value E<j found by electron-irradiation experiments. This would mean that Ed could not be taken in expression (9) as a constant, independent of ion- type, ion-energy and depth into the implanted solid.

5. PROPERTY CHANGES INDUCED BY ION IMPLANTATION: GENERAL ASPECTS

Before describing (in Sections 6, 7 and 8) the detailed applications of ion- implantation to metals, semiconductors and insulators, it is convenient in this section of the paper to discuss and list in a general way those properties of solids that have been found to be alterable by ion-implantation. Important features of such property changes are as follows:

(a) Because of the limited ranges of the implanted ions the effects will be confined to regions of the solid near the implanted surface. For ions of medium


to high mass number the depth affected will be only about 0.01 — 1 pm for the ion energies usually used, unless channelling [1—4, 18] of the ions is employed to increase the ranges [74]. In practice, the use of channelling is not really convenient since it requires the solid being implanted to be crystallographically oriented with respect to the ion beam to an accuracy of better or much better than about 0.5°, and is somewhat difficult to accomplish in connection with scanning [75] of the ion beam across the solid surface for the purpose of achieving uniform implantation of the solid over reasonably large areas. Furthermore, for channelling to be effective the solid needs to be a single-crystal; although this is usually so for implanted semiconductors it is rarely so for technological metals and of course not so for glasses. Thus, the appropriate applications of ion implantation are those for which changing the near-surface properties of the solid can produce beneficial effects. When very light ions (especially protons, deuterons and helium ions) are used (Section 3.3), ranges of 1 -5 pm or greater can be achieved at moderate ion energies of ~ 0 .1-1 MeV (Table la in [26]).(b) The greatest change of property in the as-implanted solid is likely to occur at a depth equal to the most probable projected range of the implanted atoms or to that of the maximum concentration of radiation damage defects, depending on whether the property change is due to the presence of the implanted atom itself or of the damage. It turns out that because both are dependent strongly on the nuclear energy loss rate the projected range and damage distribution are often not greatly different in shape, but that the peak damage concentration is somewhat closer to the surface than the peak implanted-atom concentration (see for example [75a] in connection with proton implantation of silicon). Heat treatment of the implanted solid will be needed to annéal the radiation damage if this damage causes deleterious property changes. The annealing of the disorder may itself then produce beneficial effects. Thus, crystalline silicon can be made am orp hous b y la ttic e d efe c ts fo rm ed during b o ro n im p la n ta tio n ; during subsequ ent

heating at 600—650°C the implanted silicon layer recrystallises and at the same time almost 100% of the boron atoms become substitutional (and therefore electrically active) in the silicon lattice. On the contrary, if it is the lattice disorder rather than the implanted element that is producing the required property change then annealing of the defects is to be prevented; one can imagine the possibilityof solids, such as potentially superconducting metals of certain kinds (see Section 6.4), being implanted at low temperature, and maintained at low temperature throughout their working life, in order to preserve the required lattice defects.(c) The advantages of the use of ion implantation over other methods of introducing impurities into solids can be summarized by saying that ion implantation is a versatile, non-specific process allowing virtually any element to be introduced into any solid in accurately controllable quantities, and also to specified depth distributions by use of a sequence of ion doses at various energies

228 P A L M E R

TABLE I. SUMMARY OF PROPERTY CHANGES AND APPLICATIONS ASSOCIATED WITH ION IMPLANTATION

Category

o f

solid

Property changes and applications References

to

reviews

Metals General [1 7 ,2 4 -2 6 , 7 6 -7 9 ]

Electrochem ical and chemical properties such

as corrosion resistance and catalysis

[7 9 -8 1 ]

Hardness, fr ic tio n a l and wear properties [7 6 ,7 9 ,8 2 ]

Electrical resistance See Section 6.3.

Superconductivity [83, 84]

Sim ulation o f neutron damage and void fo rm ation [8 5 -8 9 ]

Im plan ta tion m etallurgy (new phases,

precip ita tion , gas-bubble fo rm a tion , enhanced

d iffus ion etc.)

[77, 144, 145]

Semiconductors

General [16, 1 7 ,2 1 ,2 3 ,2 4 ,

2 6 ,9 0 -9 4 ]

E lectrical properties o f silicon [16 , 1 7 ,2 1 ,9 0 ,9 4 ]

Silicon electronic devices and circuits [9 1 -1 0 2 ]

Properties and devices o f diamond [1 0 9 -1 1 0 ]

Electrical properties and devices o f germanium See Section 7.3

Electrical and op tica l properties o f I I I -V

compounds and device applications

[1 1 1 -1 1 5 ]

Electrical and optica l properties o f I I-V I

compounds and o f SiC, and device applications

[111, 112, 114]

Glasses Refractive index changes [116, 117,'186, 204]

andother

insulators

Electro-optica l in fo rm ation storage [117, 194]

Properties o f and in fo rm a tion storage by

magnetic bubbles

[118, 186, 187]

[75]. Provided that the temperature of the solid is never so high that diffusion of the implanted atom can occur then equilibrium solubilities can be exceeded without subsequent precipitate formation. For the production of semiconductor electronic devices and integrated circuit units the fact that the semiconductor need not usually be heated to very high temperatures can prevent many impurity contamination and semiconductor decomposition problems associated with doping by high temperature diffusion; even if for special reasons thermal diffusion


is needed, ion implantation can very effectively be employed to provide an accurately known quantity of implanted diffusant in the solid before the diffusion treatment.

This section concludes by giving in Table I a summary of property changes and applications associated with ion implantation in various kinds of solid. Subsequent sections of the paper review the recent developments in detail.

6. PROPERTY CHANGES AND APPLICATIONS FOR METALS

6.1. Electrochemical and chemical properties of metals including corrosion

The effects of ion-implantation on the chemical and electrochemical properties of metals have been recently reviewed by Dearnaley [79, 80] and by Grant [81 ]. It is clear that such properties can be affected by the implantation of foreign atoms, since these properties are determined by the elemental constitution of the metal and since the chemical or electrochemical reagent attacks the metal via its surface. The starting point for investigations of this kind is the knowledge of the behaviour of metal alloys formed by conventional metallurgical processes, and one can consider forming such alloys by implantation into a pure metal of ions of the other components of the alloy; this might for example be a cheaper way of producing an alloy surface for particular applications and components, especially as only small concentrations (<| 1%) of certain impurities may be required. Dearnaley [79, 80] has emphasized how the ease of incorporating various kinds of impurity element by implantation can allow experimental studies leading to improved understanding of corrosion behaviour; he has also emphasized that ion implantation can enable surface alloys of good corrosion properties to be formed without changing the chosen mechanical strength properties of the bulk metal.

The chemical property of metals and alloys that has, because of its technological importance, been studied most so far in respect of ion implantation is the oxidation rate. Dearnaley et al. [79, 80, 119, 120] have investigated the thermal dry oxidation of the technologically important materials, titanium,18/8/1 stainless steel, zirconium and copper, after implantation of various ions at room temperature to depths of near 0.1 ¡um (ion energies up to 500 keV).It was found that both reductions and enhancements of oxidation rate occurred. Implantation of the less electronegative atoms calcium and europium reduced the oxidation rate of titanium while implants of the more electronegative atoms, ytterbium,bismuth, indium and aluminium, reduced the oxidation rate of the stainless steel. The implantation of argon, expected to be chemically inert, produced little effect on the oxidation of the titanium thus suggesting that radiation damage effects of the implantation were not significantly influencing

230 P A L M E R

dose (io n s /cm ^)

FIG.3. The reduction in the rate of oxidation of copper (99.9%), in dry oxygen at 200°С and a pressure of 1 atm, as a result of implantation with B* ions; the effect seemed independent of ion energy for energies of 50-390 ke V. From Naguib et al. [121 ].

the oxidation. The use of inert gas implantation is often of great help in this way to distinguish doping and damage effects of ion implantation. For zirconium, Dearnaley et al. found that the oxidation rate could be decreased by implantation to particular doses (~ 3 X 1015 ions/cm2) of iron and nickel; they suggest that the smaller size of these atoms compared with that of zirconium itself reduces the mechanical stress within the Zr02 and thus decreases in the oxide the number of cracks, pores and grain boundaries which allow the oxygen to penetrate.

Interesting results on the effects of implantation of boron, carbon, nitrogen and neon ions (to a mean depth of about 700 Â) in copper have recently been presented by Naguib et al. [121 ]. They studied oxidation of the copper in dry oxygen at atmospheric pressure and 200°C and found that B+ doses of 1016- 1 0 17/cm2 were very effective in reducing the oxidation rate; their results for this case are shown in Fig. 3. They found that the oxidation rate was less reduced by the neon and nitrogen implants and increased by the carbon implantation. In contrast to the suggestions of Dearnaley et al. [79, 80, 119, 120] for titanium, Naguib et al. interpret their data as indicating that implantation-induced radiátion damage in copper is detrimental to the beneficial effect of the chemical doping of the implanted atoms. -

Concerning electrochemical effects, Ashworth et al. [81, 122] have studied the aqueous corrosion of iron implanted with several ions including chromium


and iron itself. Their results show that the chromium-implanted layer behaves in respect of oxidation resistance in the same way as a bulk iron-chromium alloy of the same composition; the iron implantation did not significantly change the oxidation behaviour.

Dramatic effects of the implantation of 400-keV platinum ions to doses of 1015—1016/cm2 into tungsten and tungstic oxide have been found by Grenness et al. [123]; they report that such implanted surfaces, when used as cathodes during the electrolysis of water, can behave like platinum itself, and they point out the possible financial advantages of using platinum-implanted instead of platinum-alloy electrodes in electrolytic and fuel cells.

6.2. Hardness, frictional and wear properties of metals

It is clear that the hardness, friction and wear of metals and alloy surfaces are properties which are dependent upon the chemical and microstructural composition of the near-surface region of the material, and as such are likely to be altered by ion bombardment and implantation. Over many, many years surfaces with required properties, especially hardness, have been produced by conventional metallographic methods, the nitriding of steel being an important example of this. What ion implantation can do first is to allow studies of the mechanical properties of surfaces having compositions not previously obtained by traditional methods with the possibility then that large-scale non-implantation processes can be developed for manufacture of new surface alloys that are found to have valuable properties. Secondly ion-implantation itself can be usable on a production-line scale, such as for the surface treatment of small and crucial components of a larger mechanical assembly.

The theoretical aspects of how ion-implantation can affect surface mechanical properties have been recently carefully considered by Hartley [82] in an article which also reviews experimental results for implantation effects in metals. He points out that both hardness and friction are measures of the resistance of the surface to plastic deformation, and thus increase with yield stress. Ion implantation can introduce lattice defects and interstitial impurity atoms (such as boron, carbon and nitrogen in iron) which will hinder dislocation glide and therefore increase the yield stress (thus both damage and doping may contribute). At high implantation doses new alloys can be formed, and then the hardness and friction will be those of the new surface material. Kanaya et al. [123] and Gabovich et al. [125] have produced significant (e.g. at least 30%) increases in hardness in steel surfaces by implantation of nitrogen to doses of ~ 1017 ions/cm2. It is worth noting that these implantations were carried out using low-energy ions (up to only about 25 keV) and that this is an example of the fact that expensive high-energy ion accelerators are very often not needed for implantation purposes; indeed for a given dose, implantation at lower energies will produce a greater volume concentration of the implanted species.

232 P A L M E R

In other studies of steel, Pavlov et al. [126] have related the increases in hardness and friction coefficient obtained by argon bombardment (40 keV, up to 1018 Ar+/cm2). Hartley et al. [127, 128] have reported experimental results showing both increases and decreases in friction coefficient brought about by ion implantation.

Of greater concern than friction, which with use of efficient liquid lubricators is not usually a problem, is the question of mechanical wear of lubricated surfaces. Most studies of this kind have been conducted at the AERE, Harwell by Hartley and Dearnaley. Hartley [82] suggests that ion implantation can increase the wear resistance by facilitating the smoothing of the surfaces brought about by the rubbing action between the moving parts; when the surfaces are smooth the load is taken more by the lubricant film, and the wear is reduced; the enhancement of smoothing may be due to the hardening of the surface (e.g. by implantation of light atoms) which causes rough contáct points to break off more easily, or to the production of lateral stress in the surface (e.g. by implantation of large atoms). Figure 4 (from [82]) shows the very great improvement in the wear resistance of a steel surface as a result of implantation of 30-keV nitrogen ions.

Investigation of the effects of ion implantation on mechanical wear are to be considered as being mainly still in the research rather than the applications stage. But in view of the technological importance of this matter and the encouraging results mentioned above one can believe that this is a field of work of very great potential value.

6.3. Electrical resistivity

A number of investigations of the changes in the electrical resistivities of metals due to ion bombardment have been made in connection with studies of lattice defect production and of defect properties [26, 129]; lattice defects produced by ion-target nuclear collisions, as described in Section 4, scatter the conduction electrons in the metal or alloy and the electrical resistance is increased. There have, however, been several published reports of electrical resistivity changes due to the doping (rather than the damaging) effects of ion bombardment. Of particular interest is the work by Deery et al. [130] and Wilson et al. [131] who have made measurements of the resistivities and the thermal coefficients of resistivity of thin tantalum films (containing oxygen) as functions of implantation doses of argon, oxygen and nitrogen ions. The technological interest of these studies is that tantalum films can be used as resistors in integrated circuits (especially for use at high temperatures) and there is the possibility of optimizing the sheet resistivity of the films and of minimizing the thermal coefficient of resistivity by forming a film of the appropriate composition of Ta with Ta20 5, Ta2N or TaN. The measurements showed [130, 131] that with nitrogen implantation thermal coefficients of resistivity


FIG.4. The effect of 30-ke V N* implantation on the wear rate of En 40B steel. From Hartley [82].

of less than 10“4 per °C could be achieved, this being considerably better than that for conventionally produced tantalum oxide or tantalum nitride resistors.

6.4. Superconductivity

The subject of changing the electrical superconducting properties of metals and alloys by ion implantation has beön reviewed recently in full articles by Meyer [83] and by Stritzker [84]. The proceedings of conferences held at Albuquerque [10] and at Warwick [12] contain additional research papers on this subject. The technological aim of course is to produce a stable or metastable alloy which stays superconducting to well above 20 К (liquid hydrogen temperature) and if possible to above 80 K. Among pure metals the superconducting/ non-superconducting transition temperature Tc is highest (at 9.2 K) for niobium, and among alloys the highest Tc so far found is 23 К for Nb3Ge.

The Bardeen-Cooper-Schrieffer (BCS) theory [132] relates the occurrence of superconductivity to interaction between electrons and phonons, and the value of Tc is greater when the density of electronic states at the Fermi energy is larger, when the average phonon frequency is smaller, and when there is more efficient electron-phonon coupling. Incorporation of impurities into a metallic solid (e.g. by implantation) may alter all three of these parameters in either direction and the transition temperature Tc may increase or decrease. Implantation as a doping process simultaneously introduces lattice disorder, and the disordered solid may well have a “softer” phonon spectrum (i.e. lower phonon frequencies) because of the presence of lattice vacancies, this in itself tending to raise Tc. However, the disordering may simultaneously reduce the density of electronic states, and so produce the opposite effect upon Tc ; for example, Poate et al. [133] have shown that 2-MeV He+-ion irradiation to doses of about 1017 ions/cm2 near room temperature can reduce the Tc values of Nb-Ge films (of composition

234 P A L M E R

FIG.5. The superconduction transition temperatures T c of evaporated oxygen-free (<* 1%) molybdenum films implanted with N*, P+, As+, A u + and Sb * ions to various concentrations. From Meyer [137].

approximately that of Nb3Ge) from their initial values of 8 -2 2 К down to about 4 K, and they attribute the reduction in Tc to the presence of micro-strains in the irradiated samples.

A further fundamental problem involved in trying to increase Tc by softening the phonon spectrum is that the lattice structure consequently becomes less stable and is likely spontaneously to change to a more stable atomic arrangement of lower Tc. However, firstly, the thermodynamically unstable phase might be maintainable if kept at low temperatures, and secondly, as suggested by Stritzker [84], if a valuable new high-temperature superconductor is made, it might be possible to find a way to stabilize its lattice structure.

Let us now consider briefly examples of studies where significant increases in Tc have been brought about by ion implantation. Among these is work by Stritzker et al. [84, 134, 135] on palladium alloys. Although palladium itself is not superconductive it has been known since 1972 [136] that Pd-H alloys are superconductive if the H/Pd atomic ratio is greater than 0.8. Stritzker and Buckel [134] showed that Tc could be raised to 8.8 К by implanting H so as to give a H/Pd ratio of 1.0; furthermore, they showed [134, 135] that implantation of deuterium into palladium, so as to produce an alloy having a D/Pd ratio of 1.0, raised Tc to 10.7 K. In consideration of their results [135] with implants of other light impurities they suggest that the effectiveness of an impurity in palladium depends on its being interstitial and also able to donate electrons to the palladium 4d state, and on there being strong free-electron/phonon coupling


via the coulomb potential of the impurity; they believe that the greater Tc value for deuterium implantation compared with that for implantation of hydrogen may be due to differences in the phonon spectra.

Further results where ion implantation raises the superconducting transition temperature concern molybdenum. Figure 5 (from Meyer [137]) shows the effect of implantation of N, P, As, Sb and Au at 4K upon the value of Tc for molybdenum (initial value for pure Mo, 0.9 K). It is seen that large increases in Tc resulted from the N, P and As implants. No increase in Tc was however produced by Ne, Xe or Al implantations, and it is clear that the considerable effects with N, P and As are impurity-specific and certainly not due just to disordering of the lattice. Meyer suggests however that the enhanced Tc values may be due to the presence of defect complexes involving the N, P or As atoms.In this wide study of implantation into molybdenum Meyer shows that if the molybdenum layer contains more than 1% of oxygen then Ne+ and Xe+ bombardments increase Tc, and he attributes this effect to the formation of defect-oxygen complexes. It is important to note that high-Tc properties of both the implanted pure and oxygen-containing molybdenum layers were retained even when the samples had been heated to 200°C.

The examples mentioned in this section show how ion-implantation, by its versatility in allowing the fabrication of very many kinds of alloy with a wide range of compositions, is leading to a better understanding of superconductivity in metal alloys and can produce very significant increases in critical superconducting temperatures. This seems to be a valuable field of basic and applied research, likely to lead to important technological advances.

6.5. Simulation of neutron radiation damage and void formation

As previously discussed in Section 4, ion implantation is always accompanied by the production of lattice defects in the implanted solids by collisions between the incident projectile and the atoms of the solid. This is part of the general field of irradiation damage studies [26, 139] and for metals and alloys many papers on recent investigations can be found in the proceedings of a conference held in 1975 in Gatlinburg [138]. However, because of its technological importance, the use of ion beam irradiation to simulate neutron irradiation deserves special mention in this paper. In particular, the formation at high temperatures of voids (large vacancy clusters), which can lead to macroscopic swelling of structural and fuel components by neutron irradiation in fast-breeder reactors, has been studied by means of irradiation with energetic ion beams; because of the high atomic displacement rates under ion irradiation, void formation effects that might take several years to develop in the fast-breeder reactor (despite the fast neutron flux being ~ 1016 neutrons/cm2 -s) can be studied in a matter of hours using ion beam irradiations. This use of ion beams for void studies has been discussed in various review articles [85—89 and 139, 140].

236 P A L M E R

10

g0

1 10 LU

0.1

1

DOSE (dpa)

FIG. 6. The experimentally-deduced volume swellings of types 316 and 321 stainless steels resulting from bombardment with electrons and various ions at 600°C; the bombardment doses are given in terms of calculated displacements per atom (dpaj. The samples had been preimplanted with helium to the atomic concentrations indicated. From Hudson [141].

In a recent paper [ 141 ] Hudson has reported measurements by transmission electron microscopy of void formation in 316 and 321 stainless steels under nickel ion bombardment and has compared the results with those obtained in other work for irradiation with charged-particle beams. Some of the results are shown here in Fig. 6, where the experimentally deduced swelling are plotted as functions of the theoretically estimated numbers of displacements per atom (dpa). Because very considerable and continuous annealing of defects occurs during the bombardments, each atom can be displaced many times and the dpa number is much greater than unity. As in the results of Fig. 6, in many experiments the metal


sample being studied is pre-implanted with helium ions to concentrations of 10 -6—10_s (atom/atom) of helium so as to represent that which would be formed by (n, a) reactions during actual neutron irradiation; in many materials the helium can play a rôle in the nucléation of voids. In order to use data such as those of Fig. 6 for prediction of neutron irradiation effects it is necessary to have a theoretical basis for comparing the damaging rates of ~ 10 '3 —10~2 dpa/s, that can be obtained by ion irradiation, with the much lower rates of — 10-6 — 10-s dpa/s given by the fast neutron reactor fluxes. The first basic theoretical criterion [142] is that similar effects should be obtained if the ion beam and neutron irradiations are conducted with the samples at different temperatures such that the ratio of atomic displacement rate to vacancy diffusivity is the same for each. This means that the sample temperatures during ion irradiation experiments must be somewhat higher, for example by 150—200°C, than those of components in actual reactors in order to simulate void production by the reactor neutron irradiation. A further criterion for comparison of ion and neutron irradiations is whether the recoil energy spectra of knock-on target atoms are similar in the two cases. Hudson [ 141 ] points out that the void-induced swelling rates in the 316 and 321 stainless steels are almost the same for irradiations with nickel ions (of 5 MeV or 46.5 MeV) and fast neutrons when the expected ion/neutron temperature shift is taken into account; this result seems to be in agreement with conclusions that can be drawn from calculations of Marwick [143] concerning the recoil energy spectra for different kinds of irradiating species.

6.6. Implantation metallurgy (new phases, precipitation, gas bubbles blistering,enhanced diffusion)

This Section 6 concerned with ion implantation effects in metals and alloys is terminated by mentioning some general aspects which have been called “implantation metallurgy”. The same refers to the fact that alloys of new compositions and micro-structures can be produced by implantation, and that these new alloys can have special metallurgical characteristics and properties.Some of the special properties have been discussed in previous parts of this paper, but in a recent review article [77] Picraux has suggested that questions concerning the substitutional or interstitial nature of the implanted atoms, and their diffusion, enhanced diffusion, solubility and precipitation, need particular attention from a metallurgical point of view. The subject of the behaviour (migration, trapping, precipitation into bubbles etc.) of atoms of gaseous elements implanted into solids can be considered also as part of the same general field of study, and this has attracted much interest recently in connection with the formation of gas bubbles and the likely consequential “blistering” of the metal walls of the gas plasma chambers of possible future thermo-nuclear reactors; the subject of blistering and bubble formation has been recently reviewed by Roth [144]. Various aspects of

238 P A L M E R

implantation metallurgy including gas bubble formation were previously considered by Nelson [145].

One of the major aspects discussed in the articles by Nelson [145] and Picraux [77] is that of the formation of new phases and precipitates. Picraux notes that the inherent intimate mixing of the implanted atoms with those of the matrix (the separation often being less than 10 Â) means that diffusion-limited reactions can occur in times more than 104 shorter than would be needed for standard metallurgical alloying methods; new phases stable only at low temperatures may thus be producable by ion-implantation. He notes also that metastable substitutional alloys, for example of tungsten in copper, can be produced under conditions at low temperatures where the implanted tungsten can, perhaps as a result of collision cascades, move into substitutional lattice sites but not be able to diffuse far enough to form tungsten precipitates.

Of very great importance therefore is the possibility of forming and using at room temperature, alloys, of thermodynamically unstable compositions and structures, which are effectively stable because of the insignificant mobilities of • the component atoms. Vogel [146] has shown that implantation of carbon ions (of 38 keV energy) into iron at room temperature produces the metastable martensitic iron-carbon phase of tetragonal symmetry, and that heating at about 330°C or above was required before the usual iron carbide phase of carbon steels began to form as precipitate particles.

As examples of enhanced diffusion two pieces of work concerned with the implantation of nickel ions are now mentioned. Enhanced diffusion in this connection is the increased diffusion rate of an impurity (or isotope) in a solid resulting from the presence of irradiation-induced lattice defects. Most commonly the effect occurs when the extra lattice vacancies aid the diffusion of substitutional impurities. In work at Harwell, Turner et al. [147] measured the concentration/ depth profiles of 40-keV Ni+ ions implanted into polycrystalline titanium at temperatures of about 300—600°C. .(The profiles were determined from the energy spectra of back-scattered 1.5-3.5 MeV He+ ions.) They found that for high implantation dose-rates (¡> 40 ¿uA/cm2) the nickel profiles extended to depths (of 0.5—1 цт) which were an order of magnitude greater than expected from the combined effects of LSS implantation range and ordinary thermal diffusion, and they suggested that the results could be compatible with the occurrence of radiation-enhanced diffusion. The studies by van Wyk and Smith [148] also used 40-keV Ni+ ions, but implanted at 60—70 ßA/cm2 into copper and silver targets at temperatures between 76 and 673 K. Their measurements showed that the amounts of implanted nickel retained in the targets were greater at all temperatures than the amounts expected on the basis of LSS depth profiles together with continuous nickel-ion-induced erosion (sputtering) of the target surfaces. They concluded that very significant radiation-enhanced diffusion had occurred, and they emphasized the importance of such effects, in the applications of ion-implantation to metals, for increasing both the concentrations and depths of the implanted atoms.


7.1. General aspects

Over the past ten years it has been the potential and actual doping of semiconductors, particularly silicon, by means of ion beams that has given the major encouragement to the development of the whole field of ion implantation to the techniques, the theory and the equipment. Various books and review articles devoted to the general consideration of ion implantation of semiconductors have been published [16, 17, 21, 23, 24, 26, 90—94] and many more reviews discuss the different kinds of semiconductor (see Table I); in addition, there are many hundreds of individual research papers on and pertinent to this subject.The present article can attempt only to outline important considerations and developments and to review sonie recent significant results and aspects.

Of very great importance for this field of physical science and technology were the two factors that the basic principles of the doping of semiconductors (during crystal growth and by thermal diffusion), for incorporating electrically active substitutional acceptor and donor impurities, were well known, and that the flourishing semiconductor industry was finding that these established techniques were nevertheless not sufficiently controllable and versatile for many requirements. Ion implantation of silicon is now widely used on a commercial scale as one of the essential processes in the fabrication of silicon integrated-circuits and of some discrete silicon devices; it is described without hesitation in recent physical electronics text books as a standard and very convenient device fabrication process. In contrast, the use of ion implantation for doping III—V compounds such as gallium arsenide and other semiconductor compounds is still at present in the research or research and development stages. The following sections consider the ion implantation of the various semiconductor materials; the emphasis is placed upon the electrical and optical properties since these have the greatest technological importance at the present time.

7.2. Electrical properties and devices of silicon

The use of ion implantation to change the electrical properties of silicon has been considered in a number of reviews [16, 17, 21, 90, 94], ,and the way in which this enables silicon electronic device structures to be fabricated has also been well described and discussed [91-102]. It is worth noting especially a fairly recent and excellent article by Lee and Mayer [92] which gives a comprehensive account of why this method of doping silicon is now so valuable, and of its application for making field-effect and bipolar transistors; the article shows also in graphical form compiled experimental data for the projected ranges and projected range stragglings of 10—1000 keV n B+, 31P+ and 7SAs+ ions incident on silicon.

7 . P R O P E R T Y C H A N G E S A N D A P P L IC A T IO N S F O R S E M IC O N D U C T O R S

2 4 0 P A L M E R

For silicon, the basic background result is that ions of group III (such as especially boron) of the periodic table and of group V (such as phosphorus and arsenic) implanted into silicon take up substitutional lattice sites and become electrically active as acceptors (giving p-type silicon) and donors (giving n-type silicon), respectively, when the implanted material has been thermally annealed at a temperature of about 600°C . It has been found that if the ion implantation dose used produces enough radiation damage that the implanted layer becomes amorphous, then a subsequent anneal at 5 5 0 -6 0 0 °C gives layer recrystallization leading to a large proportion of the implanted dopant atoms taking up electrically active substitutional sites. Heating at up to 1000°C can be used if necessary to produce almost 1 0 0 % substitutional fractions, and to anneal most lattice defects so as to produce the larger electron and hole mobilities characteristic of bulk single-crystal silicon.

Nicholas [149] has recently presented a simplified theoretical method for predicting the sheet resistances (in ohms per square) of silicon surfaces implanted with boron. It is assumed that all the boron atoms are electrically active and have a gaussian spatial distribution, and these correspondingly imply anneals at 9 0 0 -1 0 0 0 °C at which temperatures no diffusion should occur. He finds that the sheet resistance R should be given by the expression

R = 1.0 X 10 11 D“0-7 s-0 '3 ohms per square

where D is the implanted boron ion dose per cm2, s is the projected standard deviation of the implant in cm, and the numerical constant 1.0 X 10 11 is deduced from experimental data. He shows that this expression gives values of R in good agreement (better than ± 1 0 %) with experimental results for doses of ~ 10 13—101S B+/cm 2. The expression for R can be employed for predicting the results of implantation through silicon dioxide or other insulating layers by using appropriate values of s. Figure 7, from the same paper, shows the expression for R in the form of a nomogram valid for bare silicon, the value of s being set by the boron ion energy. It can be seen that the sheet resistance obtainable is fairly insensitive to the implanting energy. Presumably the same kind of theoretical treatment could be used for other implants in silicon provided that the assumption of 1 0 0 % electrical activity is a useful one.

As mentioned previously, it is metal-oxide-semiconductor (MOS) technology for field-effect transistors (FET ) that has very much benefitted from the application of ion implantation. To a large extent this is due to the geometrical planar structure of field-effect devices and of integrated circuits containing them. This structure is admirably suited to the large area coverage but smallish penetration of ion beam doping. An important example, considered first, is the use of ion implantation in MOSFET fabrication to give accurate alignment of the edges of source and drain electrodes with the edges of the metal gate electrode; this is

IO N IM P L A N T A T IO N 2 4 1

(П/D)-т-50

- - 1 0 0

- - 2 0 0

- -500

- - IK

- - 2 К

- -5k

- -10k

- -20k

- -50k

Dose /cm2

-1016б’2

-10155"2

-101452

-101352

-10125 2 ■

-1011

Energy

(keV)

30-4.50 100*200

F IG . 7. A nom ogram fo r boron im plantation o f silicon relating sheet resistance obtainable

after annealing at 9 0 0 - 9 5 0 ° С to the n B + dose and energy used. F ro m N ich o la s [149].

accomplished by initial formation of the gate by evaporation followed by implantation to produce the other electrodes, the gate structure acting as a mask (“auto-registration”). Gate-source and gate-drain capacitances are thus minimized. The accuracy of the electrode alignment is however limited by lateral spreading of the implanted ions (Section 3 .2 .2). A further very valuable implantation technique is used to reduce the threshold voltage Vth needed on the gate of an enhancement-mode FET to give type-inversion of the substrate beneath the gate so as to switch on the device; the majority of such enhancement devices are of the p-channel kind where the substrate is n-type and where Vth Is negative. Boron ion doses in the substrate reduce its n-type character near the silicon surface and Vth is reduced in magnitude. The reduction of Vth is proportional to the boron dose, at a rate of about 1 V per 3 X 10 11 B+/cm 2 [97]. Since the initial, prereduction magnitude of Vth is usually 2 - 3 V a very significant reduction, with many circuit benefits, is obtained with modest boron doses.

Considering doping of silicon more generally, the great advantage of ion implantation over diffusion used by itself is that the ion charge incident on the semiconductor during implantation can be measured accurately by standard electronic equipment. Thus, the amount of dopant can be well controlled, and the reproducibility and reliability of the devices and circuits are very much enhanced. This is so important that low-energy implants (up to ~ 50 keV) are often used to provide sources of diffusant, the implant being carried out at room temperature and the “drive-in” diffusion at 1100—1200°C. For some purposes even the initial, silicon substrates, as cut from grown crystals or as deposited, have too much resistivity variability, and it is then advantageous to dope the substrates themselves sufficiently by ion implantation to make the initial variability insignificant.

2 4 2 P A L M E R

F IG . 8. A com posite enhancem ent an d depletion m ode M O S F E T structure in a silicon fast

logic circuit fabricated by electron-beam lithography and b y boron and ph osphorus im plantations;

the device has 1-ßm gate widths. F rom F a n g et al. [150].

F IG .9 . The d op in g profile o f a silicon B A R I T T m icrowave-oscillator diode made b y im plantations

o f 6 0 0 -k e V 1 X 1 0 12 B */ c m 2 and o f 3 0 -k e V 1 X 1 0 ls A s +/cm 2 into a 6.5 ohm -cm n-type epitaxial

layer on a 1 0 ~3 ohm -cm n-type substrate. F ro m E k n o y a n et al. [152].

As examples of these techniques for silicon we now note briefly two particular device structures each formed by a sequence of implantations.Figure 8 (from Ref. [150]) shows a combination of n-channel enhancement-mode and depletion-mode MOSFETS on a lightly doped p-type silicon substrate; the drain and source extension electrodes were made by the 5 X 101S cm -2 phosphorus dose, and the other phosphorus and boron implants served to give precise enhance-


ment and depletion device-chäracteristics. In this structure (Fig. 8 ) the drain-to- source separations (gate widths) were only 1 jum, this being achieved by using electron-beam lithography to produce the silicon dioxide/metal gate masking pattern. The use of very small gate widths 1 jum is essential in integrated circuits so as to produce high-speed devices and to aid the achievement of high devicepacking density per unit area. In a forward-looking paper, Keyes [151] has recently discussed silicon integrated circuit miniaturisation including various aspects of lithographic processes as a prelude to the ion implantation and diffusion. He notes that 30 000 components can now be fabricated upon each silicon chip of size about 5 mm X 5 mm, and that ion-beam lithography may be a way of further decreasing the size of each individual component.

Figure 9 (from Eknoyan et al. [152]) shows the depth structure of a silicon BARITT (barrier-injection and transit-time) device formed by successive implantations of boron and arsenic into a lightly n-type epitaxial layer (v) on a highly n-type substrate (n++). (BARITT devices are used as the active components of microwave oscillators giving rather low power but also lower noise than for example IMP ATT oscillators.) Eknoyan et al. found that the efficiency (microwave power out per DC power in) of the structure of Fig. 9 was about 5% which they state to be the highest ever reported for any BARITT device; they associate the high efficiency with the special n+-intrinsic-p-^-n multi-layer structure which can be very conveniently made by ion-implantation.

This section has aimed to outline the important and more recent aspects of ion implantation of silicon. This is now an established technique in industrial silicon-device technology, and the situation has come about because of the inherent accuracy and versatility (including the possibility of doping through thin passivating oxide layers) of ion implantation doping in the making of transistors, capacitors, resistors and other components especially in integrated circuit form. The editor of Solid State Technology has written [153] that “ion implantation is superior to other techniques because it allows precise control of charges and depth distribution of dopant profile as well as excellent reproducibility” . Morehead and Crowder [93] note that ion implantation machines are available that allow 400 silicon slices to be loaded at a time and implanted in two hours to a uniformity of 1 %.

7.3. Electrical properties and devices of diamond and germanium

This section considers ion implantation effects in diamond and germanium, which constitute the semiconducting solids immediately before and after silicon in group IV of the periodic table of elements. As for silicon, the atoms of groups III and V are expected to be acceptors and donors respectively when in substitutional sites. Compared with the situation for silicon, the present practical applications of implanted diamond and germanium are very few, and the number of individual research papers is also fairly small.

2 4 4 P A L M E R

Ion implantation of diamond has been considered in two fairly recent review articles [109, 110]; a previous research paper [ 154] reviewed the results of various investigations to 1970. An early experimental difficulty in studying diamond was the presence of impurities and defects in natural material; synthetic samples of good quality and size are now available. Semiconducting diamond of p-type conductivity and blue colour exists both as natural crystals (“lib” variety) where the acceptor impurity is believed to be boron [155], and as synthetic crystals grown with boron-doping; the acceptor ionisation energy is 0 .37 eV [155].Synthetic diamond grown with aluminium content is only slightly conducting and material containing nitrogen (Group V) is usually highly insulating.

Davidson et al. [ 154] studied the properties of diamond implanted with boron, phosphorus and nitrogen ions at room temperature. They concluded that the observed implantation-induced changes in electrical conductivity were probably due to radiation damage effects; heating at 950°C annealed the lattice disorder produced by phosphorus implantation but they observed no phosphorus- associated conductivity. Clark and Mitchell [156] pointed out the strong possibility that graphite or amorphous carbon layers of reasonable conductivity could be produced by ion bombardment, especially in experiments involving thermal annealing treatments, and Brosious et al. [157] indeed showed the presence in ion-implanted diamond of amorphous carbon, most of which could be recrystallized back to the diamond structure by careful annealing at 1400°C. However, Vavilov [ 1 1 0 ] has reviewed various sets of results giving evidence that elements of groups III and V can, under suitable conditions, be implanted to give electrically active acceptors and donors. He reports that the p-type conductivity of boron- implanted diamond increased with thermal annealing (presumably due to defect annealing and incorporation of the boron into active sites) and then remained constant even with heating at 1200°C ; hole mobilities as high as 4 0 0 —700 cm2 ‘V~lÉs- 1

at 300 К could be obtained with doses of ~ 5 X 10 14 B+/cm 2 followed by annealing at 1350°C, and the acceptor activation energy was ~ 0.3 eV. Layers exhibiting stable n-type conduction could be made by hot (600°C ) implantation of phosphorus, and p-n rectifying junctions could be formed by successive boron (with anneal) and hot phosphorus implants. Rectifying junctions could be formed also by successive boron and hot (800°C ) antimony implantations followed by an anneal at 1400°C (see Fig. 10); the layers implanted with antimony at 800°C and annealed at 1400° С were found however to have very low carrier mobilities (less than 1 cm2 • V “1 -s_1) and the conduction was ascribed by Vavilov and co-workers to a charge-hopping mechanism.

In principle, because of its large energy band gap (about 5.4 eV), diamond could be of value as a high-ternperature electronic device material; the research and development needed to hope to reach the present state of the technological use of silicon would however be enormous. In recent work, Blanchard et al. [158] have shown that available experimental electronic energy loss rates for boron ions


«2

-3 -2 -1

1 1

I 1

V t|

t--->7 2 3 U(V)— -1

F IG . 10. The current-voltage characteristics o f a p-n diode fo rm ed in d iam ond b y room -

temperature im plantation o f boron and 800° С im plantation o f antim ony, before and after

subsequent annealing at 1 4 0 0 ° C. F ro m Vavilov [110].

in carbon used in LSS/moment calculations (see Section 3 .2 .2) give depth distribution profiles in reasonable agreement with their experimental data for 4 0 —250 keV boron implanted into diamond.

In the case of germanium there have been a few detailed implantation studies, and applications concerning the fabrication of gamma-ray and particle counters have been described. Meyer [159] reported on investigations of the implantation of 27 elements (from groups III, V and others) and showed that annealed (300°C ) B+-, In+- and Ga+-implanted layers in n-type germanium were strongly p-type. He found that Li+ implanted at 20°C into p-type germanium gave a low resistivity n-type layer without annealing treatments; implants of group V elements required heating at 3 0 0 —600°C to remove acceptor-type lattice defects before n-type activity was produced. Gallium implantation at12 keV and boron implantation at 15 keV have been used respectively by Dearnaley et al. [104] and by Ponpon et al. [106] to produce p+ layers for germanium radiation detectors. Germanium has an advantage over silicon for detecting gamma rays and high-energy charged particles because of its higher attenuation coefficient and stopping power respectively for such radiation.

Detailed studies of the electrical properties and damage states of implanted germanium have been described for implants of B+, Ga+, P+ and As+ [105], of B+ [106] and of Al+ [160], for B+ concerning radiation-enhanced diffusion during implantation [161], for B+, C+, N+ and P+ [162] and for B+ and C+ [108]. The general picture of the results for the group V elements is that implantations of phosphorus and arsenic in germanium lead to n-type activity upon thermal annealing at 4 0 0 —500°C , and that nitrogen implantation gives some slight n-type conductivity upon annealing at 6 0 0 -7 0 0 °C . For implants of gallium and aluminium (of group III).annealing at 300—500°C produces strong p-type activity.

In contrast to gallium and aluminium, the implantation of boron into germanium at 20° С produces strong p-type activity which seems to be due to

2 4 6 P A L M E R

F IG . l l . The sheet resistances o f the p-type surface layers o f n-type germ anium im planted at

20° С w ith 6 0 -k e V 12 C + (filled circles and squares) or with 6 0 -k e V nß+ (triangles) to doses o f

1.0 X 1 0 is ions/cm 2, before and after successive 30-m in thermal annealing treatments. F ro m

M a cD o n a ld and Palm er [108].

electrical activity of the implanted atoms without the need for thermal annealing treatment [105, 106, 108, 162]. Figure 11, from Ref. [108], shows the very different sheet resistances obtained by 60-keV C+ and B+ implantations of germanium at room temperature (without subsequent heating) for very similar implantation dose-rates. Both implanting ions produced p-type layers; but since these ions would create almost identical initial concentrations of lattice defects, the high conductivity of the B+-implanted layer must have been due to considerable electrical activity of the implanted boron atoms (presumably on substitutional sites), together with significant defect annealing during the implant. Nuclear depolarization measurements involving the j3-decay of 12 В have indeed indicated [163] that a substantial fraction of boron atoms implanted into germanium can become immediately substitutional in the lattice.

Various aspects of the fabrication of ion-implanted germanium transistors have been considered by Schmid et al. [107].

7.4. Electrical and optical properties of compound semiconductors and theirdevice applications

The term “ compound semiconductors” refers to a variety of compounds having electronic energy gaps of several tenths of eV to several eV, of which the most well known are binary III—V and II—VI materials such as, respectively,GaAs and CdS, and the IV—IV compound SiC. Ion implantation results for


compound semiconductors have been reviewed in various articles [ 1 1 1 — 115]; the recent valuable reviews by Hemment in 1975 [114] and Donnelly in 1976[115] concentrate on GaAs, which is of much technological importance, and contain many references. The conference and symposium proceedings [12, 13,164, 165] include additional research papers relating to ion implantation in GaAs and the fabrication of devices by this means.

Just as for the elemental semiconductors, impurity atoms in the compound materials can produce p-type or n-type electrical conductivity if they take up substitutional sites in the crystalline lattice. Thus, in III—V compounds, atoms of group II elements, such.as zinc and cadmium, can occupy the sites of the group III element and give p-type activity by becoming acceptors. Similarly, group VI impurity atoms, such as sulphur and selenium, can occupy group V positions and become donors. The methods by which effective doping can be achieved in GaAs by ion-implantation are now known [114, 115], although the physical reasons why they work are certainly not understood in detail. It has been found that p-type electrical activities approaching 1 0 0 % for lowish doses (up to ~ 1014 ions/cm2) can be achieved in GaAs by implantation of Be+, Zn+ or Cd+ ions at room temperature followed by thermal annealing of the implanted material at 800—900°C. For high ion doses some diffusion of the implanted element usually occurs during the annealing treatment, the diffusion rate probably being affected (enhanced or reduced) by the presence of implantation-induced lattice defects. The production of n-type conductivity in GaAs by implantation of the group VI ions S+, Se+ or Te+ requires implantation at 2 0 0 —400°C , followed also by an annealing at 8 0 0 —900°C ; it is thought that the hot implantation requirement is related to a known radiation-damage annealing stage at about 150°C. As an example of n-type doping, Fig. 12 (from [166]) shows the measured sheet electron concentration as a function of the temperature of implantation of 10 14 S+/cm 2 into GaAs preceding an anneal at 825°C ; for implant temperatures of 200 to about 450°C an electrical activity of about 40% was obtained. For lowish doses of Se+ and Te+, activities of up to 80% can be produced [115].Some diffusion of the group V implants usually occurs during the annealing treatment. Implantations of the group IV elements silicon [167, 168] and tin [169] into GaAs also produce n-type electrical conduction, and this donor activity implies that the silicon and tin must replace gallium in the lattice. The Si+ implantation method of forming n-type layers is advantageous in that room-temperature implantation (following by anneal at 8 0 0 —900°C ) leads to high electrical activity; i.e. the more difficult, hot implantation is not needed.

Since all of these implantations of GaAs require heat treatments at 8 0 0 —900°C to anneal lattice disorder and to produce the highest possible electrical activities and carrier mobilities, an encapsulant layer is needed around the GaAs to prevent its decomposition and the out-diffusion of the implanted element; it has been . found (see discussion and references in [114] and [115]) that Si3N4 is very

2 4 8 P A L M E R

F IG . 12. The effect o f the temperature at w hich 1 -M e V su lphur ions are im planted into G aA s

(annealed subsequently at 8 2 5 ° C) up on the sheet electron concentration N s o f the im planted

surface. F ro m Davies et al. [166].

effective for this purpose. Welch et al. in 1974 [170] described how the use of a layer of Si3N4 allowed successful annealing of S+-implanted GaAs at 900°C and the fabrication of an n-channel depletion-mode, the Schottky-barrier field-effect transistor (metal-semiconductor FET or MESFET) having excellent characteristics.

It is clear that GaAs devices incorporating ion-implanted regions will form a very important part of future semiconductor technology. Firstly, the electron mobility in n-GaAs is very much larger than that in n-Si, and this gives the possibility of n-channel F E T ’s working to higher frequencies as amplifiers, and with shorter switching times in logic circuits. Hunsperger and Hirsch in 1975 [171] described the fabrication by ion-implantation and the characteristics of an n-channel depletion-mode GaAs MESFET, with a 2 jum gate length, for microwave amplification up to 20 GHz. In 1976, Liechti [172] of the Hewlett-Packard Corporation described n-channel GaAs MESFET structures formed by ion

¡ implantation and having switching times of only 50—100 ps; he notes that all the required GaAs processing techniques including ion implantation are now available for GaAs fast-logic technology. Secondly, it is likely that ion-implantation will be valuable in fabrication of light-emitting diodes and lasers of GaAs and of other binary and ternary III—V compounds [173—176]. Thirdly, there are the very important microwave-producing devices such as the IMP ATT diode;Donelly [115] discusses and gives references to several ion-implanted forms of


F IG . 13. The structure o f a P t-G aA s Schottky-barrier IM P A T T diode in w hich a very efficient

L o -H i-L o n-type d op in g profile is obtained b y S i * im plantation; the annular active region is then

defined b y m asking the device by means o f the ring-shaped back contact du ring a series o f

5 0 - 1 4 0 0 k e V proton irradiations. F ro m M u rp h y et al. [177].

this. Figure 13, from Murphy et al. in 1976 [177], shows a GaAs Schottky-barrier IMP ATT diode of novel and ingenious annular design where proton irradiation, masked by the platinum-ring Schottky layer, was used finally to produce a high- resistivity [114, 115, 178] inactive region, after Si+ implantation from the other side had been employed to form a high-efficiency n-type doping profile ( “Lo-Hi-Lo”). The special annular shape improves heat dissipation in the device during operation as a microwave generator, and Murphy et al. report 7 .4 W power output at 3 GHz with the very good conversion efficiency of 35%.

The conclusion to be drawn from the ion-implantation results for GaAs reviewed in the previous paragraphs is that this is an important and fast-developing field of semiconductor technology. Donelly in 1976 [115] expressed the belief that this technology should soon start to move from the research laboratory to the production line.

Finally in this section, let us not dismiss compound semiconductors other than GaAs without further mention. General discussion and references for ion implantation in these materials can be found in the articles by Allen [111] and Stephen [112]. The compounds such as SiC, GaP, GaN, ZnO, ZnS, ZnSe, ZnTe and CdS, with electronic band gaps greater than about 2 eV, are of special interest in connection with photoluminescence and electroluminescence.

Luminescence properties and lattice disorder related to ion implantation in ZnS, ZnSe and CdS were the subjects of six research papers at the Yorktown Heights Conference [9], and sharp cathodoluminescence lines due to implanted ytterbium in ZnS and several other II—VI compounds have been observed by Bryant and Fewster [179]. Bontemps et al. in 1974 [180] studied (by the channelling method) the lattice disorder produced in ZnTe by Ar+ implantation and suggest that the saturation of the disorder before an amorphous state is

2 5 0 P A L M E R

reached is due to the strongly ionic nature of ZnTe. For the same compound, Pautrat et al. in 1976 [181 ] showed that ion implantation can produce an electrically compensated insulating region over a depth of several micrometres related to the long-range migration of defects; on the basis of an observed anomalously large penetration of 65 Zn implanted into ZnTe, it has been suggested [182] that these defects are interstitial zinc atoms. In earlier work [183],. chlorine-implanted ZnTe annealed at 5 20° С was found to have n-type electrical activity, and n-p junctions and electroluminescence could be observed in implanted p-ZnTe. In ZnO, implantations of P+ and V+ ions have been observed [184] to give n-type conductivity, not due to lattice disorder but possibly due to impurity-band conduction. The doping of. II—VI compounds by implantation was investigated initially on the grounds that high-temperature growth and diffusion methods of incorporating impurities always led in these ionic materials to electrical compensation by the formation of charged lattice defects; at the present time effective, practical ion-implantation of these materials seems also to be hindered by the interactions of defects during the implantation itself and during the annealing treatments.

As a valuable recent example of the use of ion-implantation to change the luminescence properties of III—V compounds we can note that Pankov and Hutchby in 1976 [185], in a study of the photoluminescence emission spectra of the wide-band-gap material GaN after implantation with each of thirty-five elements and anneal at 1050°C, found that the elements Mg, Zn, Cd, Ca, As, Hg and Ag produce characteristic emission bands between 1.5 and 3 .4 eV. This could certainly be of value in the fabrication of light-emitting diodes to give outputs of various colours (e.g. the zinc emission band is blue).

8 . PROPERTY CHANGES AND APPLICATIONS FOR GLASSES ANDOTHER INSULATORS

The properties of glasses and other insulators depend, as do those of metals and semiconductors considered in Sections 6 and 7, upon their compositions and structures, and it is to be expected therefore that ion implantation can alter these properties. The experimental studies that have been made of ion implantation effects in these materials have been those where near-surface changes in properties or special surface patterns of implanted impurities could have value for device purposes. Three particular kinds of investigation have concerned changes in the refractive index in glasses, electro-optic information storage possibilities and lattice expansion effects in magnetic bubble garnets, again in connection with information storage. Each of these is briefly considered here.

Experimental investigations of how ion implantation can affect the refractive index of glassy materials have almost all concerned silica glass S i02. The subject


has been discussed in reviews by Brown in 1971 [186], Dearnaley et al. in 1973[116] and more recently by Townsend in 1975 [117]. In summary, the findings are that the refractive index of the S i0 2 can be increased by about 1% or more by ion-implantation but that the change is due to an irradiation-damage-induced compaction (increase of density) of the glass and is not ion-specific; the refractive index returns to its pre-implantation value upon thermal annealing. There has been much consideration of the possible use of the implantation-produced refractive index changes for fabricating optical light-guides [117, 186, 188, 189, 204], but the optical absorption associated with implantation-induced defects corresponds to a power loss of ~ 1 dB/ с т , whereas optical communication guides require losses of ^ 5 dB/km [196]. Investigations o f the disorder, chemical and other properties of implanted silica glass have also been made [1 9 0 -1 9 2 ] .Mattern et al. in 1976 [191 ] suggested that S i0 2 could be useful as a protective layer for structural components subject to gaseous ion implantation in thermonuclear fusion reactors, since it seems that S i02, unlike other glasses, oxides and metals, would not flake or blister under such conditions.

The fabrication of electro-optic information storage devices (i.e. “memories”) by ion implantation of insulators has been considered in the review article by Townsend [117]. He notes that both the electronic and optical properties of impurity centres can be important in giving information-storage possibilities, the writing in and reading out method being by a scanned electron or light beam; ion implantation doping of the insulator has the advantage of allowing the formation of an optimum depth profile of the impurity near the surface and of a fine-mesh impurity distribution pattern in the plane of the surface. He cites LiN b03, B aT i03, MgO and ZnO as being among insulators of interest. The storage of information in alkali halides by use of the anisotropic polarization- dependence of optical absorption by certain colour centre defects has been considered for a number of years (e.g. Schneider et al. in 1970 [193]). A recent paper by Magee and Lehmann [194] has described studies of the M-centre information storage properties of NaF implanted with Li+, B+, C+, 0 + or F + ions. Polarized light (which orients the M-centres) in a spatially-modulated pattern provided the information for the NaF, and erasure of the information could be accomplished by illumination with unpolarized light. It was found that NaF implanted with the lightest ions (Li+ and B+) had memory storage and retrieval properties, and that of these the Li+-implanted material was the more effective medium, allowing high-contrast, non-fading (in 2\ years) storage of detailed holographic images. It seems that this whole area of electro-optical pattern and information storage, aided by the use of ion-implantation, could be of much future technological importance.

Finally, in this section we note the use of ion-implantation in connection with the properties of magnetic-bubble materials (Brown in 1971 [186], North and Wolfe in 1972 [118, 195] and Dearnaley et al. in 1973 [187]). Magnetic bubbles

2 5 2 P A L M E R

are cylindrical magnetic domains that can be formed in thin layers of the cubic ferrimagnetic insulators called iron garnets which have the general formula R 3F e 50 12 where R is an element of the lanthanum series or yttrium, and where some of the iron may be replaced by gadolinium or aluminium. It has been found that the lateral stress produced in an implanted surface region of an iron garnet crystal as a result of lattice defects induced by the implantation can, through magnetostrictive effects, change the direction of easy magnetization; this can allow magnetic bubble domains to be formed with axes perpendicular to the surface by the influence of smallish magnetic fields. The absence or presence of a bubble in a particular region can be used for binary information storage; field- induced bubble motion can in principle allow sequential read-out. Ion implantation can be used also [118, 195] to suppress “hard bubbles” which are magnetic bubble domains that require fields of up to ~ 100 Oe for their formation instead of the usual value of about 75 Oe. It remains to be seen whether this technique of information storage can be developed to compete with the established and still progressing metal-insulator-semiconductor device methods.

9. LATTICE LOCATIONS OF IMPLANTED ATOMS

It is the lattice location of an implanted impurity that very often determines the effect that the impurity has upon the properties of the material, and the factors that influence the lattice site and the use of ion-channelling and other methods for determining the site are important aspects of ion-implantation. However, lack of space prevents a detailed discussion of this topic here and only references to relevant publications can be given. Suffice it then to say that review descriptions of the theoretical and experimental aspects of these can be found in a number of books (Mayer et al. [ 16], Townsend et al. [20], Carter and Grant [21]) and in recent articles (Davies [197], general features of the channelling method; de Waard and Feldman [198], channelling and hyperfine-interaction methods and results for metals; Picraux [199], the channelling method and results for metals and semi-conductors). Subsequent individual research papers include those of Beloshitsky et al. [200] on detailed studies of boron in silicon, Borders and Poate [201] concerning various implanted impurities in copper, silver, nickel and palladium, Ligeon and Guivarc’h [65] on hydrogen in silicon, Bugeat et al. [202] on hydrogen in aluminium, and Nojiri et al. [203] for boron-12innickelby use of a hyperfine-interaction nuclear technique. The papers by Ligeon et al.[65, 202] describe the very interesting use, in a channelling experiment, of the ‘ H (U B, a) reaction induced by a n B+ beam to determine the lattice location of hydrogen.

IO N IM P L A N T A TIO N 2 5 3

This paper has aimed to give a comprehensive outline of the field of research known as ion implantation and to describe the very great variety of technological applications actually in current use or being investigated. For metals there are ion implantation effects of much potential technological importance in connection with surface hardness and wear, catalysis, corrosion resistance and super-conductivity, and the use of ion beams in the simulation of neutron irradiation damage is now well established. Ion implantation is employed as a production-line process in silicon integrated-circuit technology, and there are strong indications that it will soon be used commercially in the fabrication of field-effect and microwave devices from GaAs and related compound semiconductors; effects in other semiconducting materials are being investigated. It seems likely that ion-implantation will also be of definite value for making pattem-storage devices in insulating optical materials. An essential background to all such studies is formed by the LSS-based calculations of the spatial distributions of implanted ions and of the energy depositions into elastic collision and electronic excitation processes, these however still being the subjects of current experimental and theoretical study.

Ion implantation relates to many aspects of physics, chemistry, materials science and electronics; it continues to be an important, exciting and still developing field of basic and applied research.

ACKNOWLEDGEMENTS

The author would like to thank Dr. J.C. Combasson for valuable discussions in connection with various ion-range concepts, and also to express his gratitude to Miss L. Loveday for her careful typing of the paper.

1 0 . C O N C L U S I O N S

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( 1 9 7 6 ) 7 .159] M E Y E R , O., IE E E NS - 15 (1 9 6 8 ) 232 .160] ITOH, T ., OHDOMARI, I. , Jap. J . Appl. Phys. 10 (1 9 7 1 ) 1002.161] GUSEVA, M.I., MANSUROVA, A.N., Radiat. Eff . 20 ( 1 9 7 3 ) 207.162] MACDONALD, P .J . , D. Phil. Thesis, University o f Sussex, U K (1 9 7 4 ) .163] MCDONALD, R .E . , McNAB, Т .К., Phys. Rev. 13 ( 1 9 7 6 ) 39.164] Proc. Sixth Int. Symp. on Gallium Arsenide and Related Compounds, Edinburgh, 1976

(HILSUM, C., Ed.), Institute o f Physics, Bristol and London, Conf. Series 33a (1 9 7 7 ) .165] Proc. Sixth Int. Symp. on Gallium Arsenide and Related Compounds, St. Louis, 1976

(EASTMAN, L .F . , Ed.), Institute of Physics, London and Bristol, Conf. Series 33b(1 9 7 7 ) .

166] DAVIES, D.E., ROOSILD, S., LOWE, L. , Solid State Electronics 18 (1 9 7 5 ) 733 .167] SANSBURY, .LD., GIBBONS, J.F., Radiat. Eff. 6 (1970) 269.168] H A R RIS, J .S . , in Ref. [8] p. 157.169] WOODCOCK, J .M ., SHANNON, J .M., C LA RK , D.J . , Solid State Electronics 18

(1 9 7 5 ) 267.170] WELCH, B.M., EISEN, F.H ., HIGGINS, J .A . , J . Appl. Phys. 4 5 ( 1 9 7 4 ) 3685 .171] HUN SPERGER, R.G., HIRSCH, N„ Solid State Electronics 18 ( 1 9 7 5 ) 349.172] LIECHTI, C.A., in Ref. [164 ] p. 227.173] ONO, Y . , SAITO, K., SH IRA KI, Y . , SHIMADA, T., Jap. J . Appl. Phys. 14 ( 1 9 7 5 ) 1489.174] ITOH, Т . , O ANA, Y . , Appl. Phys. Lett . 2 4 ( 1 9 7 4 ) 320 .175] C H A TT E R JEE , P.K., VAID YANATHAN, K.V., M cLEVIGE, W.V., STREETM A N , B.G.,

Appl. Phys. Lett . 27 ( 1 9 7 6 ) 567.176] BARNOSKI, M.K., H U N SPERGER, R .G., L EE , A., Appl. Phys. Lett . 24 ( 1 9 7 4 ) 627 .177] MURPHY, R .A., in Ref. [1 6 5 ] p. 210 .178] DAVIES, D.E., K EN N ED Y, J .K . , HAW LEY, J . J . , in Ref. [12 ] p. 81.179] B RY A N T, F . J . , FE W S T E R , R.H., Radiat. E ff . 2 0 ( 1 9 7 3 ) 239.180] BONTEMPS, A., LIGEON, E. , DANIELOU, R ., Radiat. Eff . 22 (1 9 7 4 ) 195.181] PAUTRAT, J .L . , BEN SAH EL, D., KA TIRCIO GLU , B „ P F IS T E R , J .C., R E V O IL , L.,

Radiat. E ff . 3 0 ( 1 9 7 6 ) 107.182] LIGEON, E., PAU TRA T, J .L . , BOURIANT, M„ Phys. Lett . , A 59 (1 9 7 6 ) 307 .183] MARINE, J . , in Ref. [7] p. 153.184] THOMAS, B.W., WALSH, D „ J . Phys., D (London) 6 ( 1 9 7 3 ) 6 1 2 , and personal

communication.

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190191192193194

1951961971981992 0 0

2 0 1

2 0 2

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PANKOVE, J . I . , HUTCHBY, J .A . , J . Appl. Phys. 4 7 ( 1 9 7 6 ) 5387 .BROWN, W.L., in Ref. [8] p. 430 .D E A R N A LE Y , G., et al., in Ref. [17 ] p. 720 .B A Y L E Y , A .R., Radiat. E ff . 18 ( 1 9 7 3 ) 111.NAMBA, S., A RITO M E, H., NISHIMURA, T. , MASUDA, K., TOYO D A , K.,J . Vac. Sei. Technol. 10 (1 9 7 3 ) 93 6 .E E R N IS S E , E.P., J . Appl. Phys. 45 (1 9 7 4 ) 167.M ATTERN , P.L., THOMAS, G .J . , B A U E R , W., J . Vac. Sei. Technol. 13 ( 1 9 7 6 ) 430. WEBB, A.P., HOUGHTON, A.J . , TOWNSEND, P.D., Radiat. E ff . 30 ( 1 9 7 6 ) 177. SCHNEIDER, I. , M ARRON E, M., K A B L E R , M.N., Appl. Optics 9 ( 1 9 7 0 ) 1163. MAGEE, T . J . , LEHMANN, M., in Ref. [12 ] p. 112.W OLFE, R „ NORTH, J .C ., Bell Syst. Tech. J . 51 (1 9 7 2 ) 1436.Electronic Engineering (UK), May (1 9 7 7 ) 5.DAVIES, J .A . , in Ref. [18] p. 391 .DE WAARD, H„ FELDM AN, L.C., in Ref. [10 ] p. 317 .PICRAUX, S .T., in Ref. [22] p. 229 .BEL O SH IT SK Y , V .V ., DIKII , N.P., KUMAKHOV, M.A., M A TY ASH, P.P., SKAKUN, N.A., Radiat. E ff . 25 ( 1 9 7 5 ) 167.B O R D E R S , J .A . , POATE, J .M ., Phys. Rev., В 13 ( 1 9 7 6 ) 96 9 .BUGEAT, J .P . , CHAMI, A.C., LIGEON, E., Phys. Lett . , A 58 ( 1 9 7 6 ) 127.N O JIR I, Y ., HAMAGAKI, H., SUGIMOTO, K., Phys. Lett ., A 6 0 ( 1 9 7 7 ) 77. TOWNSEND, P.D., J . Phys., E (London) 10 ( 1 9 7 7 ) 197.

TRACK FORMATION

Principles and applications

M. MONNINCNRS, Laboratoire de Physique Corpusculaire,Université de Clermont,Clermont-Ferrand, Aubière,France

Abstract

TRACK FORMATION: PRINCIPLES AND APPLICATIONS.The principles and technical aspects o f track formation in insulating solids are first

described. The characteristics o f dialectic track detection are discussed from the technical point of view: the nature of the detectors, the chemical treatment, the sensitivity and the environmental conditions of use. The applications are reviewed. The principle o f each type of applied research is described and then the applications are listed. When used as a detector, nuclear tracks can provide valuable information in a number o f fields: element content determination and wrapping, imaging, radiation dosimetry, environmental studies, technological uses and miscellaneous other applications. The track-formation process can also be used for making well-def defined holes; this method allows other applications which are also described. Finally, some possible future applications are mentioned.

1. INTRODUCTION

The continuous damage trail created by fission fragments in a dielectric solid was first observed by E.C.H. Silk and R.S. Barnes, and the etching enlargement technique was first described by D.A. Young. But their papers remained comparatively unnoticed and it was only the remarkable work of P.B. Price,R.L. Fleischer and R.M. Walker that put the now well-known track-formation processes in the spotlight. Their long list of discoveries began in 1962. They showed that the tracks could be etched and enlarged so as to become visible under the optical microscope and that most insulating solids exhibited this property; they discovered the sensitivity energy threshold and later the fossil tracks in uranium-bearing natural rocks; they proposed models for the track-formation mechanism. At the early stage of development of this technique they described how what was already known as “solid-state nuclear detectors” could benefit a lot of research areas including nuclear physics, solid-state physics, cosmic-ray studies, extraterrestrial material studies, geology and so on. Not only did Price,

2 6 1

2 6 2 M ONNIN

Fleischer and Walker carry out pioneering work, but they also created a broad new discipline. Their work was followed by many laboratories all over the world and now hundreds of research teams are involved in track studies.

A comprehensive review of the subject entitled Nuclear Tracks in Solids was published in 1975 by Fleischer, Price and Walker [1] and contains most of the data available to-day. In this paper, the opportunities offered by the track technique are outlined and some of the most recent literature references are given.

2. TRACK FORMATION: PRINCIPLES

2 .1. The basic fact

“The passage of heavily ionizing particles through most insulating solids creates narrow paths of intense damage on an atomic scale. These damage tracks may be revealed and made visible in an ordinary optical microscope by treatment with a properly chosen chemical reagent that rapidly and preferentially attacks the damaged material. It less rapidly removes the surrounding undamaged matrix in such a manner as to enlarge the etched holes that mark and characterize the sites o f the original, individualdamaged region” [1]. In the following text, the term “ tracks” will always refer to such a revealed channel etched along the path of a heavy ionizing particle.

2.2. Heavy ions interaction with matter

The density of damage created along the trajectory of any charged particle can be roughly correlated with the so-called “ linear energy transfer” (LET). The charged particles can be classified into two main groups: the low LET particles (electrons, muons, pions, elementaries, and the high LET particles (ions). The low LET particles are unable to create sufficient damage to give rise to a continuous etched track. Therefore, we will only deal with ions ranging from alpha particles to uranium ions, with a possible but infrequent extension to protons.

. When an ion is travelling through a solid it primarily interacts with the electrons belonging to the atoms or molecules within the medium. In this process, electrons are either excited to a higher energy level or ejected from their parent body and then act themselves in the same manner, i.e. by interacting with other electrons (delta rays). In the process, the ionizing particle alternatively loses some of its own electrons and captures electrons from the medium. This purely Coulombian interaction is responsible for the slowing down of the moving particles, for the fact that an effective average charge Z* can be attributed, to a given ion travelling in a solid of known composition and, finally, for the damages induced in the bombarded medium. .

T R A C K F O R M A T IO N 2 6 3

The rate of energy loss as well as the effective charge can be calculated from various mathematical functions (see, for example, Refs [2—6]). For practical purposes, energy loss rate and range can be found in tables such as Northcliff s [7] and Benton’s [8 ]. From these data, one notices that the energy transfer from ions to a medium is more than a thousand times higher than for electrons, even for the lightest ion (alpha particles). Furthermore, recent calculations [ 8 ,9 ] , already corroborated by experiments, have shown that the energy released to the medium is confined within a very narrow region around the ion path; in addition, this energy is deposited in a very short period of time. Accordingly, not only is the energy density extremely high (thousands of J/g) but the power impulse is tremendous (of the order of 100 GW/g). Therefore, it is not surprising that fast moving ions will induce a lot of phenomena which will result ultimately in a trail of high-density specific damages. Various mechanisms have been proposed to explain the ways by which these damages are created both in organic and inorganic media and, similarly, the chemical etching itself has been thoroughly investigated. A complete review of the current state of the problem can be found in Ref. [ 1 ].

L e t u s j u s t r e m e m b e r t h a t t h e p a s s a g e o f a m o v i n g i o n t h r o u g h a n i n s u l a t i n g

s o l i d r e s u l t s in a h i g h c o n c e n t r a t i o n o f d a m a g e a l o n g t h e i o n t r a j e c t o r y a n d t h a t

t h i s d a m a g e t r a i l c a n b e c h e m i c a l l y e t c h e d a n d e n l a r g e d s o a s t o b e c o m e v i s i b l e

u n d e r a n o p t i c a l m i c r o s c o p e ( s e e F i g s 1 a n d 2 ) .

We will now look at the practical characteristics of the insulating solids that exhibit preferential etching after ion penetration. These sensitive media are also called “solid-state nuclear track detectors” (SSNTD) or “dielectric track detectors” .

2.3. Characteristics of solid-state track detectors

2.3.1. Nature

One will notice first that the track detectors belong to the organic world as well as to the inorganic one. High polymers and plastics, minerals and glasses do exhibit these dual properties. Nevertheless, they have to be dielectric media to store the damages properly and a lower limit of 2 0 0 0 Í2 • cm is a minimum value for their electrical resistivity. The nature of the most commonly used detectors, together with their registration sensitivity (see later), are given in Table I.

2 .3 .2 . Etching techniques

The etching procedure for a solid that properly records and stores tracks can be defined by:

(a) the composition and concentration of the chemical reagent,(b) the temperature of the etching bath, and(c) the duration of the chemical,attack.

2 6 4 M ONNIN

F IG . l . Nuclear-track form ation by an ion stopp ing w ithin the detector (left) and b y an ion

travelling through the detector (right).

(a) E nergy deposition by the ion.

(b) and (с ) Track form ation during etching.

(d) F in a l shape o f the track.


F I G .2 . I r o n - i o n t r a c k s in p o l y c a r b o n a t e .

2 6 6 M ONNIN

T A B L E I . R E G I S T R A T I O N S E N S I T I V I T Y [ 1 ]

Organic detectors

D etector Atom ic com positionLeast ionizing ion seen

Amber c 2h 3o 2 Full-energy fission fragments

Phenoplaste c 7H6o

Polyethylene CH2 Fission fragments

Polystyrene CH

Polyvinylacetochloride C6H90 2C1 42 MeV 32S

Polyvinylchloride- polyvinylidene chloride copolymer

C2H3C1 + C2H2C12 42 MeV 32S

Polyethylene terephthalate (Cronar, Melinex)

c sH4o 2

Polyimide С 11H4O4N 2 36 MeV 160 .

lonom eric polyethylene (Surlyn)

36 MeV 160

Bisphenol A-polycarbonate (Lexan, M akrofol)

C 16H1 4 0 3 0 .3 MeV 4He

Polyoxy methylene (Delrin)

с н 2о 28 M eV “ B

Polypropylene CH2 1 MeV 4He

Polyvinylchloride C2H3C1

Polymethylm ethacrylate(Plexiglas)

C5H80 2 3 MeV 4He

Cellulose acetate butyrate c B H ieo 7

Cellulose triacetate (Cellit, Triafol-T, Kodacel TA -401 unplasticized)

C 3H40 2

Cellulose nitrate (Daicell)

C 6H8o 9N2 0.55 M eV 'H


T A B L E I ( C o n t . )

Organic detectors

Hypersthene Mg 1.5 F e0.s Si2O6 100 MeV 56Fe

Olivine M gFeSi0 4

Labradorite Na2Ca3Al8S ii2 Оад

Zircon ZrSK>4

. Bronzite Mgi .7 F e 0,3 SÍ2O6

Enstatite M gSi03

Diopside CaMg(Si0 3)2 170 MeV S6Fe

Augite CaMg3F e 3Al2 SÍ4 0 i9 170 MeV s6Fe

Oligoclase N a^ aA leSi^ O ^ 4 MeV 28Si

Bytow nite NaCa4Al9Siu O40 4 MeV 28Si

Orthoclase K A lSi30 8 100 M eV ^ A r

Quartz S i0 2 100 MeV Âr

Phlogopite mica KMg2 Al2SÍ3O 10(OH )2

Muscovite mica KA l3S i3O 10(OH )2 2 MeV 20Ne

Silica glass S i0 2 16 MeV адАг

Flint glass 1 8 S i0 2 :4 P b 0 :1 .5 N a 20 :K 20 2 - 4 MeV 20Ne

Tektite glass 2 2 S i0 2 : 2A120 3 : FeO(Obsidian similar)

Soda lime glass 2 3 S i0 2 : 5Na2 0 :5 C a O : A120 3 20 MeV 20Ne

Phosphate glass 10P2 0 5 : 1 .6BaO : Ag20 : 2K 20 : 2A120 3

Usually, only a few chemical reagents are suitable for etching a particular detector. In contrast, temperature, time and concentration can be varied within a rather large range, according to the particular purpose of the experiment or the experimentor’s personal habits or preference. Table II gives the etching conditions for revealing fission fragment tracks in various detectors. For other particles, the etching conditions can be derived from this table after a few preliminary tests (according to the charge and speed of the particle one seeks to get tracks of) like increasing or decreasing the temperature, or concentration, or both, or by combining time, temperature and concentration accordingly.

2 6 8 M ONNIN

The best results are obtained with simple equipment. A temperature- controlled bath contains a vessel with the etching solution which is occasionally stirred. The detector is allowed to stay in the vessel in such a way that it is entirely surrounded by the etching solution. After etching, the detector is removed, carefully washed and dried. In some cases, when only a quick and/or approximate result is wanted, the procedure can be simplified to the extreme: heat up the reagent in a beaker, drop the detector in it, wait, remove the detector, wipe it and examine it under the microscope. This rusticity in the use of track detectors is not the least of their advantages, especially for practical applications.

2 .3 .3 . Sensitivity

Not all the detectors exhibit the same sensitivity to heavily ionizing radiation. Some of them are able to record fission fragments only, while others record alpha particles or even low-energy protons. Generally, plastic detectors are more sensitive than minerals. There has been a lot of argumentation among the scientific community to decide which criterion should be used to characterize the sensitivity of a particular recording solid. Actually, there is only one definite way to do it, that is to state the limit in energy (or speed) above which a particular ion is no longer recorded in the considered detector etched with a well-defined reagent concentration at a fixed temperature. And this should be done for all the possible ions. An alternative way is to determine the fastest and lightest ion recorded (Table I), but this is not very easy to use either because it does not allow one to decide a priori whether the particular ion one is interested in will be recorded or not. This is why we would like to go back to the old “dE/dx criterion” proposed at the early stage of development of this technique by Price, Fleischer and Walker. It states that if an ion A loses a certain amount of energy per unit of path, an ion В losing the same or more energy per unit of path will be recorded equally. Therefore, for each detector a (dE/dx)t limit value can be attributed as being the threshold under which one cannot get any track whatsoever (Fig.3). It must be pointed out that this criterion does not fit the data perfectly, particularly for high-energy ions up to the relativistic region. Nevertheless, for the sake of simplicity, it can be considered a useful tool.

2.4. Environmental effects

The solid-state nuclear track detectors are amongst the least sensitive detectors to environmental effects. However, they are not totally inert and it must be kept in mind that, sometimes, external constraint can affect their recording ability.

2 .4 .1 . Thermal effect

“By far the most pervasive and useful environmental effect on particle tracks has been the thermal alteration of tracks” [1 ]. This effect is exhibited by both


ENERGY / NUCLEON (MeV)

VELOCITY, v/c

F IG .3 . Sensitivity thresholds (after R e f.[l]).

organic and inorganic solids, even though it is much more striking in organic media than in minerals or glasses. In general, an irradiated detector stored at high temperature will lose some o f the information recorded in it. If the temperature is high enough, or if the storage time is long enough, this effect may even cause a total loss of information. Otherwise, if the storing conditions are not too severe, only a fraction of the recorded tracks will be lost, or only a fraction of their etchable length will be erased. This track annealing effect is summarized in Table III. Advantage can be taken of this effect in some particular cases. As a matter of fact, low Z particle tracks are more easily annealed than high Z particle tracks. Therefore if, for any reason, the two types of particle are recorded and if one is interested only in the heavy ones, the light particle tracks can be preferentially removed. The annealing properties can also be used as a technique for dating natural or man-made samples (see later).

2 .4 .2 . Gases

Most, if not all, of the inorganic detectors are not affected by the presence or absence of a gas. On the other hand, the characteristics of organic detectors may be altered, depending on their structure, if they are bombarded by ions in vacuum or in the presence o f gases, if they record tracks while oxygen is present or not, and so on. Extensive studies have been performed on this topic and

2 7 0 M ÖN N IN

TABLE II. ETCHING CONDITIONS OF FISSION FRAGMENT TRACKS FOR VARIOUS MATERIALS (after Ref. [1])

M aterial E tch in g cond itions

Feld spar (A lb ite , N aA lSi30 8)

Feld spar (A n o rth ite , C aA l2S i2 0 8 )

Feld spar (M icro clin e , O rth oclase ,

KA1 S i30 8)

F lu orite (C a F 2 )

A u tu n ite (C a (U 0 2 ) 2 P2 0 8 ■ 8H 2 0 )

M ica(B io t ite , K (M g ,F e ) 3 A lS i3 O 1 0 (O H )2)

M ica

(L ep id o lite ; Z innw aldite,

K 2 L i3 AI4 S i7 0 2 1 (O H ,F ) 3 )

M ica (M u scovite, K A l3 S i3 O 1 0 (O H )2 )

M ica (P h lo g o p ite ; L ep id om elan e,

KM g2 A l2 S iA o (O H )2 )

Q u artz ( S i 0 2)

Lead ph osph ate glass

P hosph ate glass

S i l ic a glass (fu s e d q u a rtz ;V y co r; L ib y an D esert Glass)

Sod a-lim e (m icro sco p e slide; cover slip; w indow glass)

C ellulose a ceta te (K o d a ce l; T r ia fo lT ; C ellit)

C ellulose a ceta te b u ty ra te

C ellulose n itra te (D iace ll;

N ixon-Bald w in)

C ellulose tria ce ta te (K o d acel

T A 4 0 1 , un plasticized ; B ay er T N )

H B p alT (P o ly ester, C 1 7 H 9 O 2 )

Io n o m eric p o ly e th y len e (S u rly n )

3 g NaOH: 4 g H2 0 , 85 m in, boiling

3 g NaOH : 4 g H2 0 , 14 m in, boiling

5 g КОН : 1 g H2 0 , 8 0 m in, 1 9 0 °С

9 8 % H2 S 0 4 , 10 m in , 2 3 °С

10% HCl, 1 0 - 3 0 sec, 2 3 °C

2 0 % H F , 1 - 2 m in, 2 3 °C

4 8 % H F , 3 - 7 0 sec, 2 3 °C

4 8 % H F, 1 0 - 4 0 m in, 2 3 °C

48 % H F , 1 - 5 m in, 2 3 °C

K O H (aq ), 3 h , 1 5 0 °C , or

4 8 % H F, 2 4 h, 2 3 °C

1 ml 70% H N 0 3 :3 ml H2 0 , 2 - 2 0 m in

4 8 % H F , 5 - 2 0 m in

4 8 % H F , 1 min

> H F , 5 sec (b e tte r : 5% H F , 2 m in) 24% H B F 4 : 5% H N 0 3 : 0 .5% a cetic acid , 1 h .

1 ml 15% NaClO : 2 m l 6 .2 5 N NaOH ,1 h , 4 0 °C25 g NaOH : 2 0 g КО Н : 4 .5 g K M n 0 4 : 9 0 g H 2 0 , 2 - 3 0 m in, 5 0 °C

6 .2 5 N NaOH, 12 m in, 7 0 °C

6 .2 5 N NaOH,. 2 - 4 h , 2 3 °C

1 ml 15% NaClO :2 m l 6 .2 5 N NaOH,

1 h , 4 0 °C

6 .2 5 N NaOH, 8 m in, 7 0 °C

10 g K 2 Cr2 0 7 : 35 m l 30% H2 SO4

1 h , 5 0 °C


TABLE II (Cont.)

M aterial E tchin g con d ition s

Polyam id e (H -film ) K M n 0 4 (2 5 % , aq ), 1.5 h , 1 0 0 °C 6 N NaOH solution

P oly im ide КМПО4 in H2 0

P oly carb o n ate (L e x a n ; M akrofo l; 6 .2 5 N NaOH, 2 0 m in, 5 0 °C

M erlon ; K im fo l) 6 .2 5 N NaOH + 0 .4 % B e n a x , 2 0 m in,

7 0 °C

P oly eth y len e terep h th a la te 6 .2 5 N NaOH, 10 m in, 7 0 ? C(M ylar, C h ron ar, M elin ex, T erp h an e) K M n 0 4 (2 5 % , aq ), 1 h , 5 5 °C

P olystyrene sat. K M n 0 4 , 2 .5 h , 8 5 °C

10 g K 2 Cr2 0 7 , 35 m l, 3 0 % H 2 S 0 43 h, 8 5 °C

S ilico n e-p o ly carb o n ate cop oly m er 6 .2 5 N NaOH, 2 0 m in, 5 0 °C

TABLE III. THERMAL EFFEC T [ 1 ]

M aterial1-hour annealing tem p eratu re (°C )

T o ta l fading . 50% track loss S ta rt o f track loss

A p atite 5 3 0 3 3 6 4 0 0

A ragonite (C a C 0 3) 15 0 - 13 0

C ellulose a ce ta te (C e llit-T ) 165 160 1 0 0

C ellulose n itra te 147 140 1 1 0

Feld spar (A n o rth ite ) 6 8 0 5 5 0 3 5 0

Feld spar (B y to w n ite ) 7 9 0 7 5 0 6 9 0

G lass (B o ro s ilica te , P y rex ) 3 8 0 3 0 2 - .

G lass (V 2 O s . 5P 2 O s ) - 95 -

M ica (M u scov ite) 5 4 0 5 1 0 4 5 0

P oly carb on ate 185 - -

P y ro xen e (D iop sid e) 8 8 0 8 5 0 '8 2 0

Q uartz ( S i 0 2 ) 10 5 0 - 1 0 0 0

2 7 2 M ONNIN

useful data may be found in the literature [1]. However, the general and more important features of this effect are that the absence of oxygen during or after irradiation decreases the detector sensitivity and sometimes even precludes any recording, whereas electron donor gases increase the sensitivity. The desensitization due to the lack of oxygen can be more or less avoided if the sample is stored in oxygen at a pressure of about 10 0 atmospheres.

2 .4 .3 . Radiation effect

Solid-state detectors are normally not sensitive to radiation other than to ions. In other words, one should not expect to see a track of electrons and it is not possible to use the detectors to detect “gamma” radiation, for instance.This is an extremely useful feature since it is possible to handle the detectors in the open daylight and to use them even in a high intensity radiation background. However, irradiation with ultraviolet and higher energy photons and with electrons or other particles can have significant and sometimes profound effects on the properties of track detectors. No general rule can be made and if one uses a particular detector under some of the above-mentioned conditions it is best to first make sure that the corresponding effect is acceptable [ 1 ].

2.5 . The reading of the tracks

The more widely used piece of equipment in track scanning is the optical microscope which allows the sample to be viewed either in transmitted or reflected light. Some care should be taken in preparing the sample; in particular it should be dried thoroughly. Various attempts have been made to enhance the contrast (see Ref.[ 1 ]). In a recent paper [ 11 ], it has been shown that by using the detector foil as a light guide the contrast can be increased and the unwanted background decreased.

The optical microscope allows one to count the tracks, locate them with great accuracy, measure their dimensions, etc., but its numerous advantages are somewhat counterbalanced by the fact that optical microscope scanning is a time- consuming, delicate and often tedious operation. This is why several techniques have been proposed that do not require the optical microscope [175]. These techniques are reviewed in Nuclear Tracks in Solids [1] which also contains descriptions of the very fruitful automatic spark scanning devices. A novel and interesting procedure has been devised: the energy spectrum of a monoenergetic particle beam transmitted through the detector is measured. The particles that have been transmitted at a place in the detector where a track is located merge with an appreciably higher energy than those transmitted through the .bulk detector. After proper calibration, the ratio of the high-energy-to-low-energy peaks can be expressed in terms of the number of tracks [ 168]. -


2.6. Available detectors

The numerous solids that can be used as detectors were not primarily considered for this purpose. Most of the minerals are of natural source with the exception of some artificial micas, quartz and diamonds. The same is true for the glasses and plastics which are available from private companies as slabs, sheets, or foils, but since they are produced for industrial use their properties are not always constant and laboratory tests should be performed on each batch before being used as a detector. To our knowledge, Kodak-Pathé (Vincennes, France) is the only company that especially devised and produces plastic sheets for track detection. The products are the now well-known L R -115 high-contrast alpha- particle-sensitive cellulose nitrate, the CA-80-15 and the neutron-sensitive foils.

2.7. Other techniques

Tracks can be obtained in plastics by other means than chemical etching.The grafting technique is under development. It consists of growing a polymer along the trail produced by particle damage. This different polymer is then dyed and the track is made visible [13—17].

The use of amorphous semiconductors has been proposed as a possible way of producing tracks [ 1 2 ], but so far no practical applications have been developed.

2.8. Summary of the track-formation characteristics

Solid-state nuclear track detectors are:

insulating solidsable to record ion trajectorieseasy to etchavailable in large quantities inexpensivemostly insensitive to radiation and light.

Their recording properties are as follows:

there is an energy threshold above which they record ions with a 1 0 0 % efficiency

they do undergo some thermal fading but it is far less severe than for any other known integrating detector

when chosen according to the planned experiment they can be used under extreme environmental conditions

their reading can be automatized.

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Right from the beginning, almost immediately after their initial discovery, solid-state track detectors (SSTD) have been widely used in an ever-increasing number of applications relevant to almost any field of scientific research and technology. Many opportunities have been offered by their particular properties and a considerable amount of work has been done in fundamental research. An extensive survey of the subject is given in Ref.[l ]. Here, interest is focused on the numerous applications in which an SSTD is considered to be a very simple and easy to use tool; that is to say when extremely careful etching conditions are not required, when multiple corrections are not necessary, when side effects are not o f prime importance, etc. Applications that involve particle charge or energy identification or both are omitted. With regard to applications, solid-state nuclear track detectors can be used as particle detectors as well as finely perforated sheets. In the first case, the sample to be studied emits heavily ionizing particles or modifies their initial number or energy spectrum, and the particles are then recorded by the detector which can be the sample itself. In the second case, the detector is used as a wall with a fixed number of “passages” or “holes” acting as a filter to let “something” pass through.

Here, instead of mentioning all the possible applications, information is given about the different techniques and some of the applications relevant to each technique are listed.

3.1. Element mapping

“ Solid state track detectors have unique capabilities for measuring the concentration and spatial distribution of certain elements. In principle, any isotope is capable of being studied if it emits heavy nuclear particles, either directly because of its natural radioactivity, or, more important, as a result of specific nuclear reactions when bombarded in an accelerator or nuclear reactor. Certain isotopes with large cross sections for specific reactions are, of course, more suitable than others. This area of research began with the measurement of uranium via the detection of fission fragments in samples irradiated with thermal neutrons. As time has gone on, however, more and more nuclides have been studied and the number of diverse problems that can be approached by track methods is enlarging rapidly” [ 1 ].

3 .1 .1 . Uranium determination

The uranium concentration of a given sample can be determined by means of track detectors by recording the number of fission events induced by a known flux of thermal neutrons. Since only 235U undergoes thermal neutron fission, the

3 . T R A C K F O R M A T I O N : A P P L I C A T I O N S


isotopic ratio of 235U to 238U has to be known. One usually assumes that it is equal to that of the natural abundance but, in some cases, it has to be determined independently. Absolute measurement of the thermal neutron flux is generally avoided because it is rather difficult to do and it can be a major source of error. Instead, one uses solids of known uranium concentration (usually glasses). Preferably, a glass with a uranium content close to that of the unknown sample is chosen. The standard glass is irradiated simultaneously with the studied sample so that they receive exactly the same number of thermal neutrons. Before irradiation an auxiliary detector is placed in close contact with the glass. The ratio of induced tracks in this detector to those measured coming from the sample itself allows the uranium content of the sample to be calculated. Two methods can be used. In the external method, an auxiliary detector is placed in close contact with the unknown sample and the standard glass. In the internal method, an auxiliary detector is placed against the standard glass but the sample itself is used as a detector in order to measure the number of fission events indùced within the sample. It must be pointed out that this relative measurement of uranium content can be greatly affected by the range of the fission fragments both in the sample and in the standard glass. If, for instance, the standard glass has an elementary composition with an average Z much lower than that of the unknown sample, the range of fission fragments within the standard glass can be much larger than their range within the sample. Consequently, even if the number of induced fission events is the same, the number of tracks recorded in the detector in contact with the glass will be larger than the number of fission fragments emerging from the sample. Therefore, when possible, the standard solid must have an average Z close to that of the unknown sample. With the external method, a uranium concentration as low as 50 ng-g' 1 can be measured, whereas with the internal method, measurements down to a 10 ~ 14 atom fraction can be made.Of course, when the internal method is used the unknown sample has to be heated sufficiently to remove any possible fossil track provided by spontaneous fission of 238U during the “life” of the sample.

The technique that has been schematically summarized can be used for measuring the uranium content of samples of uniform concentration. This is not likely to be true for most samples. Heterogeneity adds special complications in determining a bulk, average uranium concentration. Two possible ways of removing the heterogeneities are by dissolving the sample and measuring the uranium concentration in the dried solute product [18] or by crushing the sample to a scale that is much finer than the scale of the heterogeneities [19]. Another way is to move the sample away from the detector and to irradiate in vacuum [2 0 ]. If the sample is a liquid or a suspension, a uniform solid target must be made.This can be done either by evaporation or collection on a filter, for instance. It is also possible to add known amounts of uranium standards to several samples of the same origin. The track densities are measured for each case and then extrapolated to zero addition to get the initial uranium content of the sample.

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It is often very important to measure the detailed spatial uranium distribution. With dielectric track detectors it is easy to measure the uranium distribution on a micrometre scale [174]. Basically, the technique is always the same. A fission- fragment track detector is tightly applied to the flat surface of the sample in which one wants to determine the uranium distribution. This “pack” is subjected to thermal neutron irradiation. The external detector is then etched out and the uranium-rich regions of the sample are revealed. They appear like small sea urchins or tiny stars. There are, of course, many possible ways to map uranium but the principle of the technique is the same. The main difficulty is to relate accurately the fission stars seen on the detector and the actual position of uranium grains in the sample. If one decides to keep the detector and the sample together, thus assuring a good spatial resolution, the problem of etching arises. This problem is overcome by the use of a detector that is thinner than the fission-fragment range. Then it is possible to etch the detector from the outside, without separating it from the sample, but this is not an easy method. Most micromapping has been done by separating the detector from the material after irradiation. The etching is easier than with the previous method but locating the uranium-bearing regions is a little more complicated. In one way or another some sort of fiducial marks have to be made. For instance, a striking feature of the sample can be connected to a scratch or a grid engraved on the surface of the detector and a photograph of the montage taken; or, the sample itself produces a print of its structure on the surface detector [25].

3 . 1 . 2 . U r a n iu m m a p p in g

3.1 .3 . Fission-fragment mapping of other elements

Fission of elements other than uranium can be induced by fast-moving particles and high-energy gamma rays. The technique is generally the same as for uranium. The only added difficulty is to have a detector or a target thin enough to allow the impinging particle to reach the fissionable material. Thorium mapping is achieved by bombardment with 30-MeV alpha particles or 85-MeV protons [21,22]. Since uranium is associated with thorium, the uranium content is measured first by the thermal neutron method. Then, the fast-moving particle irradiation induces both uranium and thorium fission and the thorium content is eventually determined by subtraction. To measure the Th/U ratio in samples with a low uranium content, fast neutrons can be used too [23, 24]. Other elements which undergo fission when hit by fast-moving particles are also likely to be mapped. These include Pb and Bi, which could be measured at the ng/g level, and Au when bombarded by energetic mesons; photo-fission of U, Th and Bi can be induced; elements as light as Ho can be split by energetic oxygen ions [1].


The lightest ions are alpha particles and protons. They can be recorded, at low energies, by solid-state track detectors. Consequently, any element that emits these particles, either naturally or because o f an n,a-induced nuclear reaction, can be mapped by techniques similar to uranium mapping. As far as spontaneous emission is concerned, one deals with the “alpha autoradiography” technique.This technique was already used with nuclear emulsion but it has been greatly improved by using track detectors. Isotopes o f uranium, plutonium, thorium, lead (via the 208Pb (a, 2n) Po reaction), polonium and other heavy elements can be used as alpha emitters for autoradiography. Elements that are not naturally radioactive can be mapped too. Boron, for instance, has an n,a cross-section of 760 barns and it is the element that is the more easily mapped in this way. Boron can be detected to a level of /ng/g. The minimum detectable particle size is 10' 16 g and the short range of the alpha particle and Li fragment makes the spatial resolution extremely good (0 .3 jum). Other elements like 6Li and 170 can be determined by neutron irradiation and subsequent alpha-particle emission. Proton bombardment provides a way to measure 180 and 1SN, while I4N content can be measured by neutron irradiation leading to the emission of recorded protons.

3.2. Applications of element mapping

3.2.1. Fission-track dating

The first application of the measurement of the average content of uranium that should be mentioned is the dating of samples of various origin. If a sample by itself is a fission-fragment detector (or if it contains grains of such minerals) and if, in addition, it does contain some uranium, it can be dated with a good accuracy. Immediately after the “birth” of the sample (the cooling down of the melt for a rock, the solidifying process for a glass, the firing for a piece of pottery, etc.), the uranium contained in the sample starts to undergo spontaneous fission. Each fission fragment is recorded by the surrounding material and its track is stored in it. The spontaneous fission half-life of 238U is known (4.51 X 109years). Therefore, if both the number of spontaneous fission events that occurred since the birth and the uranium content are known, the age of the sample can be measured. Typically, the studied sample is first etched to reveal and count the fossil tracks and then the uranium content is measured by neutron irradiation. This technique has been applied to many different samples: rocks (terrestrial and extraterrestrial), glasses (natural and man-made), and various minerals. Of course, some care should be taken when handling and measuring the sample. Particularly the thermal history of the sample is of great importance

3 . 1 . 4 . A lp h a p a r t i c l e s a n d p r o t o n m a p p in g

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(for details see Ref. [1]). The fission-track dating method has proved extremely successful in solving problems in geophysics, geochronology and archaeology.It is impossible to list here all the innumerable applications. Among the more spectacular findings has been the information obtained from dating lunar samples, evidence for the spreading of the ocean bed and the knowledge acquired about archaeological fields like the one at the Olduvai gorge.

As soon as Apollo XI returned from the moon, the lunar samples were submitted to fission-track investigations and the age of the moon samples was successfully measured by the technique described here [ 1 ].

Spreading of the ocean bed can be measured by dating geological samples taken from the bottom of the ocean at various distances from the mid-ocean ridge. Since these samples were hot when they emerged at the surface of the submerged earth crust, they started recording tracks at this time only. Therefore, if there is any spreading of the ocean bed, the samples found at a greater distance from the median valley should exhibit greater ages. Fission-track dating provided clear results that fully conform with the ideas of oceán-bed spreading [ 1 , 2 6 —28].

The К-Ar age (1.75 X 106 years) of the archaeological field at the Olduvai gorge seemed surprising to the experts, but when the track dating of volcanic pumice from this location gave a cross-check of 2 .0 X 1 0 6 years, the site became considered as one of the best-dated of the anthropological sites that are too ancient for carbon dating [29].

3 .2 .2 . Biology

Most of the elements that can be measured or mapped are fairly ubiquitous and can be found in many biological samples. They can induce several transformations within the biological material (radiation effect) or they combine with molecular products of prime biological importance. Biologists were quick to recognize the usefulness of solid-state track detectors in these studies. In addition to the sensitivity of the detectors, their relative innocuity to biological media ' offered a further advantage.

The content of uranium has been measured in, for instance, blood cells [30], seaweeds and plants [31 ] and bones [32, 33 ], and the content of plutonium has been determined in bones and bioassays of various origins [34, 35]. Both uranium and plutonium distributions have been determined in bones [ 4 4 -4 7 ] , liver, kidney and animal blood that has previously been injected with these radionuclides [36]. Such studies indicate the processes by which harmful nuclides diffuse within the body and which organs are more likely to be affected in an accidental or chronic internal contamination.

Boron distribution has been determined in a variety of biological materials [37] including bone [38] and leaves [39]. It can be seen that boron is not homogeneously


distributed within the leaf and strong depletions of boron are observed along the veins of the leaf. Boron can also be used for labelling a compound of biological importance to follow its biochemical processes.

The problem of lithium determination in biological material has been discussed by Carpenter [40].

Oxygen-17 can be used as a tracer in, for instance, the preparation of labelled citric acid and in the study of oxygen uptake and transport in the brain.An animal is given 170-enriched air to inhale before it is killed and subsequent ,70 mapping of the brain sections shows the oxygen-rich regions in relation to the brain structure [ 1 ].

Similarly, nitrogen has been determined in several biological materials at the National Bureau of Standards [41—43] by means of the n,p reaction on 14N.

“The localization of deuterium and tritium via the D(T,n) reaction has been demonstrated by Geisler and his colleague ( [4 4 -4 6 ] in Ref.[l]) who have emphasized the potential biomedical applications of the method. In particular they have measured lymphocytes labelled with deuterated thymidine and have suggested the use of this method in human cancer detection” [ 1 ].

Koechel and Kalff [70] have used autoradiography to determine the productivity of phytoplankton species, and other biological applications have been suggested [71].

3.2 .3 . Geology and geochemistry

Apart from the remarkable studies on the lunar and meteoritic samples —. to which an entire paper could be devoted (see Ref. [ 1 ]), “ terrestrial” geology has benefited greatly from the track method. The average content of uranium has been measured in terrestrial samples like petroleum [48], in manganese nodules found at the bottom of the ocean [49], in sediments [50, 51], and in almost any imaginable rock. A particularly interesting example of uranium mapping is offered by the technique of plastic prints initiated by Kleeman and Lovering [25]. A plastic sheet is placed in contact with a polished section of the rock. After irradiation by thermal neutrons (fission fragment print) or after an exposure time long enough (alpha autoradiography), the detecting material gives a “print” that shows the repartition of uranium or thorium. This technique was also used by Chan tret [52, 53] to study radionuclide distribution in uranium ores.

The distribution of alpha-particle emitters in minerals and rocks (like Li) has been discussed [54] and the distribution of boron was determined [55] in diamond and shown to bear a relation to the colour and other optical properties as well as the electrical properties of the sample.

For more details on geological applications the reader is referred to Refs [ 5 6 -6 6 ] and Ref. [178].

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3.2.4. Environmental studies

Either naturally or as a result of man’s activities, radioactive nuclides are present in our biosphere. They are a potential hazard to human, animal and plant life. It might be of vital importance to know the size and abundance of the particles that contain radioactive nuclides present in the air and water.The size of plutonium oxide particles has been measured by means of track detectors [1]. Uranium as an air and water contaminant can be traced too [6 8 ,8 2 ] , and plutonium has been detected in various environmental samples [67 , 69].

3 .2 .5 . Technological applications

Some man-made materials contain elements that can be detected or mapped by the track technique. The measurement of their distribution by this technique has cast some light on the understanding of their behaviour and structure. For instance, boron can be found in various alloys and can affect the metallurgical properties; its distribution is of great importance in understanding steel composition [73]. Boron can be responsible for crack formation and swelling [72].Boron mapping shows that this element is preferentially distributed along crystal boundaries and in inclusions in silicon-iron steel [1 ]. Boron distribution has been rather extensively determined for alloys and steel structure studies [ 7 4 -7 6 , 173]. Similarly, some experiments have been performed to elucidate the diffusion of trace amounts of uranium in metallic crystals [1] and in steel [77] and to measure the thickness of the reaction zone in Nb-Sn superconducting wires [78].

Numerous applications of track etching have been developed to study the internal structure of materials [79]; this is particularly true for atomic energy technology. Solid-state track detectors have been used for controlling the thickness of electrodeposited radioactive sources [80], for testing reactor materials [81], for measuring the isotopic enrichment of reactor fuel [83] and for determining contamination of nuclear targets [84].

3.3. Imaging or internal structure probing

We will now discuss non-destructive techniques for studying the internal structure of a body. Basically, these techniques are based on the fact that when charged particles are travelling through a condensed medium, they undergo a lot of interactions with the constitutive electrons, atoms or molecules of the medium. As a result, the characteristics of a particle beam entering the medium will be drastically changed so that the characteristics of the out-going beam can be used as an indication of the internal structure. For instance, the particles can be scattered or preferentially absorbed by some of the constituents only, or their energy can be differentially decreased. Obviously, the probing particles are generated by an


external source and their initial energy must be high enough for at least some of them to travel through the sample and be detected at the other side. Two types of particles are used: neutrons and ions (alpha particles or heavier ions) that produce an image to be analysed [169].

3 .3 .1 . Neutron radiography

As neutrons are non-charged particles they cannot be detected directly by solid-state detectors. Therefore an active material, which can convert neutrons into detectable particles, is placed between the sample and the detector. When using thermal neutrons, one takes advantage of the n,a reaction on boron or of the n,f reaction on uranium because of their large cross-section. With fast neutrons, detectable charged particles can be induced by recoil reactions within the detector itself by the (n ,n ')3a reaction on carbon or by fission reactions of various nuclides such as 237Np or 232Th [1].

Neutron radiography is particularly useful when X-ray radiography is less suitable, for instance with samples made of light elements like organic materials. Berger [85] has made a survey of the work on track etch radiography so let it suffice here to state that neutron radiography has four special qualities. It can be performed in a high radiation environment; it has a high resolution, only limited by track length; radiographs can be taken with a low exposure to neutron beam and their sensitivity to thickness differences is high (~ 1%) [85, 8 6 , 168]. The practical applications of neutron radiography are numerous and are not limited to small-sample studies. In fact, G. Farny, at the CEN-Fontenay, France, has developed neutron radiography for inspecting irradiated reactor fuel elements on a large scale. Recently, a complete survey of neutron radiography was published in Atomic Energy Review (Vol. 15, No.2, 1977).

3 .3 .2 . Charged-particle radiography

“Whereas neutron radiography is sensitive to variations of certain chemical elements, proton and heavy radiography are sensitive to small changes in density or mass thickness along the beam. The general principle is to shoot a monoenergetic beam o f charged particles through the object to be imaged and to bring the beam to rest in a stack of plastic sheets” [1 ]. “Where the specimen is thicker or has a higher stopping power the particles stop earlier in the stack; where it is thinner or has a low stopping power, the particles stop in a layer of the stack which is further downstream” [87]. The principles of this technique were . developed by Tobias and Benton in 1972. Since then, they have added further improvements and developed it into a routine method [ 8 7 -8 9 ] . The detector can be “read out” by photographing each layer individually; it is also possible to synthesize the images with the aid of a computer. This technique is extremely

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powerful and makes it possible to detect very small abnormalities that would hardly be detected with X-ray techniques, particularly in soft tissue. For instance, mass-thickness changes as small as 0.03 g/cm 2 can be detected. However, it should be mentioned that this sensitivity is achieved with particles of charge greater than 6

for thick objects and greater than 2 for thin objects (less than 1 cm). The technique is clearly the most sensitive tool available at present for the measurement of internal density distribution of objects. The striking opportunities offered by the method for diagnostic radiology will undoubtedly add to the interest in building heavy-ion medical accelerators. Nevertheless, the greater availability of protons and alpha beams has urged scientists to study the possibilities of using such beams for radiography [90, 91 ].

Low-energy alpha particles from a radioactive source can be used to study thin metallurgical samples and to detect defects such as grain boundaries, twin boundaries and the accumulation of dislocations [1, 9 2 - 9 4 ] . The sample under study is placed between the ailpha-particle source and the detector. The energy of the particles is adjusted by means of an absorber-collimator so that non-scattered particles will just reach the detector and be recorded. On the other hand, scattering will reduce the flux detected. The resulting picture allows information to be obtained on the internal structure of the sample or measurements of activation, energies of metallurgical processes [96]. The method is not limited to alpha- particle probing and the study of heavy metals like platinum and tungsten is carried out with fission fragments [95].

When charged particles move between atomic planes or atomic rows of a crystallized sample, they can travel farther than in other directions. This phenomenon is known as “ channelling” . Consequently, if charged particles enter a sample with an energy a little less than needed to get out, only the channelled particles will emerge and be recorded on an adjacent track detector. With this technique one can measure crystal structure, symmetries and orientation [97, 98] in the same way as one would with an X-ray Laue pattern.

3.4. Radiation dosimetry

Some of the unique properties of solid-state track detectors make them extremely attractive for radiation dosimetry and it is not surprising that the first neutron fluence measurements were performed as far back as 1963 [99]. Since then the development of the method has been astonishing and hundreds of papers have been published on the subject. It is not possible to make a detailed survey of this field of application of solid-state track detectors here. Instead, the principles of the method are outlined and a few salient literature references are given.

Research in radiation dosimetry began in the United States of America and in Europe at about the same time [100—104] and was mainly centred on thermal neutron dosimetry. The scope of interest soon widened to include other particle dosimetry (for a review see, Ref.[105]).


Application of solid-state track detectors in radiation dosimetry extends from thermal and fast neutron fluence measurements to alpha-particle and heavy ion dosimetry. In contrast, beta-ray, gamma-ray, and X-ray doses cannot be measured by this technique.

3.4.1. Thermal neutrons

The method for thermal neutron dosimetry is based on a similar arrangement as described for the imaging technique. Some form of converter has to be used to induce the emission of detectable particles whose number would be in relation to the neutron flux. There are two possibilities, the n,f and the n,a reactions.

With fission fragment registration, the targets that are more likely to be used are natural uranium, 235U-enriched foils and 239Pu, but because of the chemical toxicity of plutonium it is preferable to use uranium converters. Besides, by varying the concentration of uranium of the target, an enormous range of neutron fluence can be measured (from 10 3 to 1022 n/cm2). The detectors them- ■ selves are chosen according to the environmental conditions, or the etching or reading technique one intends to use. They can be mica or glass sheets or plastic foils. Various possibilities are offered for the reading of the dosimeter. Optical and electron microscopes can be used but, more often, scientists have tried to replace manual counting by automatic or integrating devices. Most of these devices have been developed by groups concerned with radiation dosimetry because research in dosimetry will ultimately lead to application on a large scale and an increasing number of dosimeters have to be read repeatedly and in a short time.

The advantages of solid-state dosimeters are [ 1 ] :

No need for specialized electronic counting equipmentEase of processing and evaluationNo need for gram quantities of fissile materialsInsensitivity to ß-, X- and 7 -radiation which makes track counting in

nuclear emulsion impossible at high dosesNo fogging, fading or other storage problems at ambient temperaturesHuge range of doses amenable to studyEase of activation or inactivation by separating fissile and detector foils.

When the thermal neutron flux is small one has to look for alpha-induced particles. The 3840-barn and 950-barn cross-sections of the 10B(n,a) and 6Li(n,») reactions are significantly higher than the thermal neutron fission cross- section of 235U. These two elements are used in conjunction with an alpha- sensitive detector in order to measure low thermal neutron fluxes. The sensitivity that can be achieved is high (0 .013 track/neutron) [106] with a 0 .2 -cm-thick boron sheet. This high sensitivity was proven useful for several special applications

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besides practical dosimetry. For instance, low fluxes of thermal neutrons produced by cosmic rays over the earth’s surface can be measured [106] and the first direct measurement of the distribution of thermal neutrons with depth in the lunar soil was carried out in 1973 [107].

Special mention should be made of neutrons in the energy range of about 20 to 100 keV. In this energy interval, the cross-section for the n,a reaction on Li or В is too low to be useful and recoils of heavier nuclei in the detector (see later) would have too short a range to be detectable. It is then possible to detect proton recoil directly induced in a hydrogen-rich organic detector [108 , 109].

3 .4 .2 . Fast neutrons

Fast neutron dosimetry is a bit more complicated than thermal neutron dosimetry because one should get information not only on the neutron fluence but also on the energy spectrum of neutrons. Fission can be induced in various elements such as 232Th, 238U, 231Pa and 237Np. By using several such fissile materials with different thresholds for induced fission it is possible to measure fluence from approximately 1 0 7 to 1 0 19 neutrons per cm2 , and to obtain a rather good idea of their energy spectrum. The first particle personnel dosimeter on this principle was made by Baumgartner and Brackenbush [110].

Neptunium-237 is particularly attractive for neutron dosimetry. Its fission threshold is well defined at 0 .4 MeV, it has a high cross-section and at the same time a low sensitivity to high-energy gamma rays. It has been extensively studied [ 1 1 1 ] and its sensitivity has been as good as 6 X 10 "4 tracks/(neutron/cm 2 ).

With 232Th, the measurable dose range lies between 20 mrad and 20 krad [112]. This raises the question of converting neutron fluence to biological doses. There is no simple relation between the energy of neutrons and their efficiency for inducing damage in the human body. This underlines the importance of getting information on the neutron energy spectrum. Some attempts have been made to optimize this measurement [113, 114].

In addition to fission fragments in fissile materials, fast neutrons can be detected by observing the recoil nuclei that were directly hit by neutrons. Light nuclei have high elastic collision cross-sections and are the most favourable. They can be initially located within the detector itself (H ,C,N ,0) or they càn be contained in a separate screen in contact with the detector (He, Be) [115, 116]. The sensitivity to fast neutrons ranges from about 3 X 1 0 "6 to 2 X 10"s tracks/ neutron depending on the plastic and the neutron energy.

3 .4 .3 . Alpha particles

The major sources of potentially dangerous alpha particles are airborne radon gas and aerosol particles containing radon daughters, such as 218Po and 214Po.


Radon can be found almost everywhere, even at high altitudes in the atmosphere, though generally at trace levels. It is present at appreciable concentrations in the air in uranium mines where it is a serious hazard to miners and must be monitored. Because of the warm, humid atmosphere in the mines and the necessity for an opaque wrapping to prevent the 5.5-MeV alpha particles from being detected, nuclear track emulsions are unsatisfactory as dosimeters. On the other hand, alpha- sensitive plastic detectors do not have to meet these requirements and are very suitable for miners’ personal dosimeters [1, 117, 118]. Their sensitivity is sufficiently high, but some practical problems have still to be solved. For instance, dirt tends to deposit on the detector surface and alter its response.

Radon dosimeters have been used in mines as well as to study the distribution of radon in the atmosphere [119]. '

3 .4 .4 . Highly charged particles

Man may suffer from highly charged particle bombardment only when placed under special conditions. Highly energetic and charged particles are not present in our usual biosphere, but they can be found at high altitude (SST and space flights) in the cosmic rays, and on the earth around heavy ion accelerator facilities when subjecting patients to heavy ion therapy or diagnosis. Since heavy ions are extremely powerful projectiles that can induce permanent damage in cells it is of prime importance to be able to monitor them. This can be done for fluence as well as charge and energy measurement by means of plastic detectors [ 1 2 0 —1 2 2 ].

For recent data on dosimetry see Refs [123—133].

3.5. Miscellaneous applications of the charged-particle detectors

This section describes some more situations, not covered in the previous sections, in which solid-state track detectors can be profitably used.

3.5.1. Tracers

Small particles containing atoms detectable by track detectors can be used to trace the flow o f a fluid, whether it is a gas or a liquid. Again, fission fragments and alpha particles are the available alternatives. In the first case, 0 .5 -/im uranium oxide particles or any other fissile material like thorium, gold and platinum are suspended in the fluid flow. The fluid is then filtered, the filter is pressed against a detector and neutron or high-energy particle irradiated to produce fission events that can be recorded [134, 135]. The measured concentration in “tracer” is related to the fluid flow characteristics. Similarly, boron, lithium and 170 can be used with alpha-sensitive detectors [136]. It is also possible to code individually the tracer particles by specifying, for example, the uranium-to-boron ratio [1 ].

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The tracer technique has many attractive features; it can be used to study pollution and water networks, to follow atmospheric and sea stream behaviour, for environmental surveys, in medicine, etc.

3.5.2. Nuclear technology

In addition to the applications already mentioned, solid-state track detectors have been used in nuclear power plants and nuclear research centres. For instance, they have been used for accurate mapping of the neutron beam structure from a reactor [137, 170] and for measuring the reactors’ static and dynamic characteristics [171, 172] such as fission density distribution inside fuel elements, near the interface and along the reactor holes, as well as fractional burn-up [ 1 , 1 3 8 - 1 4 2 , 1 4 8 ] .

Another interesting application in connection with nuclear energy is the localization of uranium ore deposits. “The method depends on the fact that one member of the U decay chain, Rn, is a gas with a half-life of 3.8 days, which is long enough for radon to be transported to the earth’s surface from ores buried at considerable depths, provided the permeability of the overlying rock and soil is sufficiently high. Plastic cups, each containing a piece of cellulose nitrate, are inverted and buried at shallow depths where they record alpha decays from 222Rn and its daughters throughout a burial time of two to four weeks” [ 1 ].The points of high radon concentration are selected for exploratory drilling. This procedure has now located previously unknown ore bodies at depths of up to 300 ft and obtained clear signals from known deposits at depths of 450 ft [143—145].

3.5.3. Barometric measurements

Since the stopping power of a gas is a direct function of its pressure, the range of charged particles will also be a function of the pressure. The lower the pressure, the larger the range. Therefore, a radioactive source and a detector can be made into a pressure gauge. This idea was put to practical use by Gustafson et al. [146] who measured the maximum altitude of birds. They devised an altimeter made out of a polonium source and a cellulose acetate detector tilted at 45° to the particle from the source and found that homing pigeons fly at altitudes ranging from 300 to 1700 m and that swifts fly at a height of 1400 to 3600 m.

3.5.4. Polymer stability

The characteristics of track storage and etching have been used to determine the technical thermal stability of polymers [147].


“Many of the uses of the particle track derive from the simple fact that an etched particle track is a hole. By proper control of the spatial distribution of the track forming particles and of the etching conditions, structures can be produced that have unique, useful geometries” [ 1 ].

3.6.1. Filters

By exposing a thin detector to a collimated beam of particles that produce tracks across the entire thickness of the sheet and by subsequent etching, one can produce the required number of holes with a known geometry [149]. The diameter of the holes can be as small as 50 Â. Continuously adjustable holes can be obtained by perforating a stretchable plastic such as a silicon-polycarbonate film [150]. Filters are commercially available from the Nuclepore Corporation.The first applications of such filters were of biomedical interest. Seal [151] demonstrated that when malignant cells were present in blood they could be separated and recognized and that their number was related to the stage of development of the tumour. The same technique has been used to detect cancer cells in spinal fluid and lung material sampled by needle aspiration biopsy [152, 153]. Nuclepore filters have also been used in immunology [154] and in the study of metabolic interactions [155].

Environmental studies have also benefited from track-etch filters. Airborne particles, radioactive aerosols, cloud nuclei and fine particulate matter from suspension in ocean water can be sampled by this method [1, 156—158].

Even every-day commodities can be improved by Nuclepore filters. They can be used to clarify and stabilize wine and beer by removing the bacteria, sediment and yeast. In contrast to thermal sterilization this technique does not change the taste of the beverage [ 1 ].

Track-etch filters are also used for cleaning gases that are used for removing the dust from electronic components before encapsulation [ 1 ].

3 .6.2. Virus and bacteria counter

The DeBlois-Bean counter employs a single etched track to count and measure small particles in an electrolyte [159]. The single perforated membrane is placed between two electrodes. The resistance between the electrodes depends on the conducting path through the hole. When a charged and insulating particle enters the hole the resistance increases proportionately to the size of the particle, and the velocity of the particle moving through the hole is a measure of its charge. Thus, from a knowledge of the resistance change between the electrodes and its time behaviour it is possible to count, characterize and identify the particles.

3 . 6 , A p p l i c a t io n o f t r a c k d e t e c t o r s a s h o le - f i l l e d m a te r ia ls

2 8 8 M ONNIN

The method has been successfully applied to virus and bacteria counting [160].An added bonus of the technique is that individual pulses with harmonic oscillation are sometimes observed when elongated particles pass through the hole. From the distribution of amplitudes it is possible to infer the shape of such particles [161].

3 .6 .3 . Diverse applications

Since holes created by ion bombardment and etching can be filled with appropriate substances, new materials of technological interest can be produced.

Fleischer et al. [1] demonstrated that hot, liquid gallium can be forced into micrometresized holes in mica where it solidifies. Similarly, photosensitive silver chloride can be made to precipitate in such holes. Lead iodide fixed in the same manner can be transformed into superconducting lead by electromagnetic or electron irradiation at 180°C. Superconductivity can be studied in fine wires produced by electroplating tin into fine holes in etched mica. Wires 400 Â in diameter and 15 pm long can be produced from tin, zinc and indium.

Porous muscovite mica produced by track etching has been used as a model system to test theories of flow and ideas of the structure of water and of super- fluid flow [162, 163]. Some of the ideas that anomalous water exists as an unusual ice-like structure on the interior of fine capillaries have been refuted by convincing experiments on fine holes [ 164— 166].

It has also been shown that etched tracks filled with a metal by chemical reduction can serve as anchoring points for a continuous electroplated layer on top of a plastic object. Adhesion strengths of more than 50 kg/cm2 can be achieved [176].

CONCLUSIONS

A large number of practical applications has emerged from the development of solid-state nuclear track detectors. Some applications are routine methods (e.g. radiation dosimetry) and others are still laboratory techniques. The use of solid-state track detectors has made major contributions to our knowledge in many scientific disciplines.

Although it is dangerous to predict the future development of a method, a conservative extrapolation on utilization of nuclear-track formation in some tentative, possible, potential and not too exotic applications of solid-state track detectors is presented.

Since environmental factors can affect the registration characteristics of tracks, solid-state track detectors could perhaps be used to study or measure these factors. The rate of fading of registered tracks provides a method to determine temperature, vacuum can be measured from the loss of efficiency of registration, and pressure from travelling distances of detectable particles.


Obviously, the possible applications depend primarily on the ingenuity of the experimentator. One possible future for solid-track detectors is the generalization of their use for monitoring environmental conditions. Detectors can be activated without any power consumption and offer advantages for a long-term experiment or measurement. Such detectors migh be placed in the atmosphere or in waters (sea, river or lake) to measure the content of radioactive elements, to detect possible accidental contamination or to follow the evolution of a pollutant in the biosphere. ' This type of permanent control might be useful particularly near nuclear power plants or fuel-treatment factories.

Solid-state track detectors have been used in conjunction with tracer elements. Such applications will increase in number and importance. In this particular case we are in the position of having answers and being able to seek questions.

There is a good chance that the micro-hole filters will be of great assistance in solving problems in various fields of science and technology. As an example, consider the catalytic sheets made of large plastic foils with a catalytic agent inserted. This method would allow the separation of the catalytic agent from the reagents and final piroducts at low cost [177].

The actual surface area of a solid in which tracks have been etched is much larger than its apparent surface. One could take advantage of this surface property for catalytic effects in a similar way as zeolites are used. .

Plastics can be electroplated after irradiation and etching. By this method, large and cheap electrodes could be produced for the electrolysis of water or salt solutions.

By putting appropriate compounds into the holes or by using the grafting technique it might be possible to make fluorescent screens or digital display devices.

A single hole made in a piece of detector (like in the virus counter) might provide a way to obtain an extremely well-defined point source of light and several holes in a suitable geometrical distribution might lead to the formation of light-scattering screens.

As a far-reaching but possible technological development, one can think of successive deposition of several different chemicals (metals, semiconductors, isolators) along an etched long track (made by heavy ions). Extremely microsized electronic components could be manufactured in this manner.

Addresses of companies commercially involved in track detectors

D e t e c t o r s

Kodak-Pathé, 30 rue des Vignerons, 94300 Vincennes, France

2 9 0 M ONNIN

F i l t e r s

Nuclepore Corporation, 7035 Commerce Circle, Pleasanton, California, USA Nomura Micro Science Co, Ltd., Japan

U r a n i u m e x p l o r a t i o n

Terradex Corporation, 1900 Olympic Blvd, Walnut Creek, California 94596 , USA

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) CHAPUIS, A .M . , FRANCOIS, H . , GERARD-NICODEME, N . , C o n t r p le de l ' e n r i c h i s s e m e n t de l 'u r a n i u m m é t a l l i q u e à l ' a i d e de n i t r a t e de c e l l u l o s e , Rad. E f f e c t s , 5 ( 1 9 7 0 ) 91

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( 9 1 ) S0M0GYI, G . , SRIVASTAVA, D .S . Alpha r a d io g r a p h y w ith p l a s t i c t r a c k d e t e c t o r s , I n t . J . Appl. Rad. I s o t o p e s , 22 ( 1 9 7 1 ) 289

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( 9 3 ) JOUSSET, J . C . , MORY, J . , ¡QUILICO, J . J . , Etude des F i g u r e s de C a n a l i s a t i o n dans l e s C r i s t a u x , P r o c . I n t e r . C onf. N u cl . T ra c k R e g i s t r a t i o n in I n s u l a t i n g S o l i d s , C l e r m o n t -F e r r a n d , F r a n c e , IX -4 7 ( 1 9 6 9 )

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(9 6 ) DELSARTE, G . , JOUSSET, J . C . , MORY, J . , QUERE, Y . , D e c h a n n e l in g o f F a s t T r a n s m it t e d P a r t i c l e s by L a t t i c e D e f e c t s in Atomic C o l l i s i o n Phenomena i n S o l i d s ( 1 9 7 0 )

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(9 8 ) DELSARTE, G . , DESARMOT, G . , MORY, J . , D e t e r m in a t io n par c a n a l i - g r a p h i e de l ' o r i e n t a t i o n de m i c r o c r i s t a u x , P hys. S t a t . S o l . , 5.(1971) 683

(9 9 ) WALKER, R.M ., ' PRICE, P . B . , FLEISCHER, A v e r s a t i l e d i s p o s a b l e d o s i m e te r f o r slow and f a s t n e u t r o n s , A ppl. Phys. L e t t e r s , 3 ( 1 9 6 3 ) 28

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( 1 0 3 ) MEDVEDEZKY, M . , S0M0GYI, G . , F a s t n e u tro n f l u x measurement by means o f p l a s t i c s , Atomki Kozlem, 8 ( 1 9 6 6 ) 226

( 1 0 4 ) BECKER, K . , N u cle a r t r a c k r e g i s t r a t i o n in d o s im e te r g l a s s e s f o r n e u tro n d o s im e tr y in mixed r a d i a t i o n f i e l d s , H e a lth P h y s i c s , 12 ( 1 9 6 6 ) 769

( 1 0 5 ) BECKER, K . , D o s i m e t r i c A p p l i c a t i o n s o f T r a c k E tc h in g T o p i c s i n R a d i a t i o n D o s im e tr y , S u p p l . 1 , ( 1 9 7 2 ) 7 9 , Academic P r e s s , N.Y.

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1 0 8 ) VARNAGY, M . , C SIK AI, J . , SZEGED I , S . , NAGY, S . , O b s e r v a t io n o f p ro to n t r a c k s by a p l a s t i c d e t e c t o r , N u cl . I n s t . M ethod s, 8 9( 1 9 7 0 ) 27

1 0 9 ) STERN, R . A . , PRICE, P . B . , Charge and e n e r g y in f o r m a t i o n from heavy io n t r a c k s i n l e x a n , N ature Phys. S e i . , 2 4 0 ( 1 9 7 2 ) 82

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1 1 1 ) SOHRABI, M . , BECKER, K . , F a s t n e u tro n p e r s o n n e l m o n i to r in g by f i s s i o n f r a g m e n t r e g i s t r a t i o n from 2 5 7 Np, N u cl . I n s t , and M eth od s, 104 ( 1 9 7 2 ) 409

11 2 ) MONNIN, M . , ISABELLE, D . B . , Les d é t e c t e u r s s o l i d e s de t r a c e s e t l e u r s a p p l i c a t i o n s en b i o l o g i e , Ann. P hys. B i o l . e t Med, 4 , ( 1 9 7 0 ) ,9 5

1 1 3 ) BURGER, G . , GRUNAUER, F . , PARETZKE, H . , The A p p l i c a b i l i t y o f T ra c k D e t e c t o r s i n Neutron D o s im e tr y , Pap. S M - 1 4 3 .1 7 , P r o c .Symp. on New R a d i a t i o n D e t e c t o r s , I . A . E . A . , V ienn e ( 1 9 7 0 )

1 1 4 ) REMY, G . , RALAROSY, J . , T R IP IE R , J . , DEBEAUVAIS, M . , STEIN, R . , D o s i m e tr i e e t s p e c t r o m é t r i e a p p r o c h é e s de n e u tr o n s de f i s s i o n e t t h e r m o - n u c l é a i r e s à l ' a i d e de d é t e c t e u r s v i s u e l s p l a s t i q u e s ,Rad. E f f e c t s , 5 ( 1 9 7 0 ) 221

1 1 5 ) BECKER, K . , D i r e c t F a s t Neutron I n t e r a c t i o n s w i th P o ly m e r s ,ORNL-Rept. 4 4 4 6 - 2 2 6 ( 1 9 6 9 )

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1 1 8 ) FRANK, A . L . , BENTON, E . V . , A d i f f u s i o n chamber radon d o s im e te r f o r u s e in mine e n v ir o n m e n t , N u cl . I n s t . M eth od s, 109 ( 1 9 7 3 ) 537

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1 2 2 ) ALKOFER, O . C . , ENGE, W., HEINRICH, W., ROHRS, H . , P r e l i m i n a r y R e s u l t s o f Measurments o f Heavy P r i m a r ie s i n t h e R egio n o f S u p e r s o n i c T r a n s p o r t Using P l a s t i c S t a c k s , I n t . C o n f . on P r o t e c t i o n a g a i n s t A c c e l e r a t o r and Sp a ce R a d i a t i o n , Geneva ( 1 9 7 1 )

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1 2 4 ) SINGER, J . , TROUSIL, J . , PROUZA, Z . , T r a c k d e t e c t o r s i n p e r so n n e ln e u tro n d o s i m e t r y , R a d i o i s o t o p y , 1 7 , 3 ( 1 9 7 6 ) 371

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LIST OF PARTICIPANTS

H. Arriola

S. Bashkin

J.P. Blewett

N. Clark

R.L. Cohen

M. Gallardo

L. Haug

P. Hautojärvi

Centro de Estudios Nucleares, UNAM, Circuito Exterior, CU,México 20 DF

University o f Arizona,Tucson, AZ 8 5 7 2 1 ,United States of America

Department o f Physics,Brookhaven National Laboratory, Associated Universities Inc.,Upton, N J 11973,United States of America

Escuela de Física,Universidad de Costa Rica,Ciudad Universitaria “ Rodrigo Fació” Costa Rica

Bell Laboratories,6 0 0 Mountain Avenue,Murray Hill, NJ Ó7974,United States o f America

Universidad de Costa Rica,Ciudad Universitaria “ Rodrigo Facio” Costa Rica

Universidad de Costa Rica,Ciudad Universitaria “ Rodrigo Facio” Costa Rica

Helsinki University o f Technology, Department o f Technical Physics, S F -0 2 1 5 0 Espoo 15,Finland

3 0 0 L IS T O F P A R T IC IP A N T S

R. Jiménez Universidad de Costa Rica,Ciudad Universitaria “ Rodrigo Facio” , Costa Rica

G. Kostorz Institut Laue-Langevin, Avenue des Martyrs, 156X Centre de Tri, 3 8 0 4 2 Grenoble Cedex, France

J.A. Lubkowitz Chemistry Department,University o f Georgia,Athens, G A 3 0 6 0 2 ,United States of America andInstituto Venezolano de Investigaciones Científicas, Centro de Petróleo y Química,Apartado 1827,Caracas,Venezuela

M. Monnin Laboratoire de physique corpusculaire, Université de Clermont, Clermont-Ferrand,6 3 1 7 0 Aubière,France

L. Moya Universidad de Costa Rica,Ciudad Universitaria “ Rodrigo Facio” . Costa Rica

S.S. Nargolwalla Nuclear Applications Research Laboratory, SC IN TREX Ltd,222 Snidercroft Road,Concord, Ontario L 4K 1B5,Canada

D.W. Palmer The University o f Sussex School of Mathematical and Physical Sciences,

Falmer, Brighton BN1 9QH,United Kingdom

T.B. Pierce UKAEA Research Group,Instrumentation and Applied Physics Division, Building 148,AERE, Harwell,Oxfordshire OX 11 ORA,United Kingdom

L IS T O F P A R T IC IP A N T S

A. Salazar

E.A. Schweikert

V. Valkovic

G.W. Wertheim

Scientific Secretary

J . DolniCar

Universidad de Costa Rica,Ciudad Universitaria “ Rodrigo Facio” , Costa Rica

Texas A & M University,Dept, o f Chemistry,College Station, T X 7 7 8 4 3 ,United States o f America '

Rice University,Dept, o f Physics,Houston, T X 7 7 0 0 1 ,United States o f America andInstitut Ruder Boskovic,Zagreb,Yugoslavia

Bell Laboratories,6 0 0 Mountain Avenue,Murray Hill, NJ 0 7 9 7 4 ,United States o f America

Division o f Research and Laboratories, International Atomic Energy Agency, Kärntner Ring 11,P.O. Box 590,A-1011 Vienna,Austria

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