Alpha Capture on 18_O during Stellar Helium Burning

8/9/2019 Alpha Capture on 18_O during Stellar Helium Burning

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Forschungszentrum Karlsruhe

in der Helmholtz-Gemeinschaft

Wissenschaftliche Berichte

FZKA 6782

Alpha capture on 18O during stellar

He burning

Saed Dababneh

Institut fr Kernphysik

von der Fakultt fr Physik und Astronomie

der Ruprecht-Karls-Universitt Heidelberg

genehmigte Dissertation

Forschungszentrum Karlsruhe GmbH, Karlsruhe

2002


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Impressum der Print-Ausgabe:

Als Manuskript gedrucktFr diesen Bericht behalten wir uns alle Rechte vor

Forschungszentrum Karlsruhe GmbHPostfach 3640, 76021 Karlsruhe

Mitglied der Hermann von Helmholtz-GemeinschaftDeutscher Forschungszentren (HGF)

ISSN 0947-8620


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Abstract

Th e Ne(

, n ) re a ct io n is t h e d o m i n a n t n e u t r o n s o u r c e fo r n e u t r o n c a p t u r e n u -cleosyn th esis (

-process ) in m ass ive s ta rs a nd plays a lso a s ignifican t role for the

- p r oc es s in t h e r m a lly p u ls i n g AG B s t a r s . In t h e s e s c en a r io s Ne is produced

b y t h e r e a c t io n s e q u e n c e

N( -

)

F(

)

O( -

)

Ne. Wh ile th e first react ion iswell un derstood, - ca p t u r e o n O was affected by considerable uncertainties. Att h e t e m p e r a t u r e s o f s t e l l a r H e b u r n i n g t h e r e a c t i o n r a t e i s d e t e r m i n e d b y t w or e s on a n c es a t

- en e r g ie s o f 4 7 0 a n d 5 6 6 k e V. S in c e t h e s e r e s o n a n c es w er e n o ty e t s u c c e s s f u l l y m e a s u r e d , t h e r a t e s h a d t o b e b a s e d o n e s t i m a t e d r e s o n a n c es t r e n gt h s . In t h e p r e s e n t w or k t h e fi rs t d i r ec t m e a s u r e m e n t o f t h e s t r e n g th so f t h e s e e xt r e m e ly w e a k l ow e n e r gy r e s o n a n c es is r e p o r te d . Th e u s e o f a h i gh -efficiency segmented Ge detector in coincidence with BGO counters cover ing alarge solid angle led to a significantly improved experimental sensitivity, thus al-lowin g for th e clear identification of specific

-

- t ransi t ions. As a resul t , resonancestren gths of 1.0 0 .2 eV an d 0 .4 0 .2 eV could be obtained for the 56 6 a nd 470

keV resonances, respect ively. When compared to the previously repor ted upperlimits of

1 .7

eV, this resul t provides the basis for the re l iable determinat ionof th e react ion ra te d u r in g ste llar He-bu rn in g. Accordingly, these dat a a llow tor e d u c e t h e u n c e r t a in t i es i n t h e

- p r oc es s n e u t r o n b a l a n c e .

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Ei n f an g v o n a lp h a -Tei l ch e n a m O w h ren d d e s

s t el l a ren He- B ren n en s

Z u s a m m e n f a s s u n g

Die Ne(

, n ) R ea k t i on is t d ie H a u p t - Ne u t r o n e n q u e lle f r d ie E le m e n t s y n t h e s ed u r c h N e u t r o n e n e i n f a n g (

- P r o z e s s ) i n m a s s i v e n S t e r n e n u n d s p i e l t a u c h e i n ebedeutende Rolle beim - Pr o ze s s i n t h e r m i s c h p u ls i er e n d e n AG B S t er n e n . Indiesen Szen ar ien wird Ne w h r e n d d e s H e - Br e n n e n s d u r c h d ie R ea k t i on s k e t t e N(

-

) F(

) O( -

) Ne erzeugt . Whrend die erste Reaktion gut verstandenist , war der

-Einfang von O m it b e t r c h t lic h e n U n s i ch e r h e it e n b e h a ft e t . B e id e n Te m p e r a t u r e n im s t e lla r e n H e- Br e n n e n w ir d d ie R e a k t io n s r a t e vo n zw eiR e s o n a n z e n b e i - En e r gie n v on 4 7 0 u n d 5 6 6 k e V b e s t im m t . D a d ie s e Re s o-n a n z e n b is je t zt n ic h t e xp e r im e n t e ll e r fa s s t w er d e n k o n n t e n , b a s ie r e n d ie i n

-Prozess-Studien verwendeten Reaktionsraten ledigl ich auf Abschtzungen derR es o n a n zs t r k e n . In d e r v or lie ge n d e n Ar b e it w ir d b e r e in e d ir e k t e Mes s u n gd e r S t r k e n d ie s e r e x tr e m s c h w a c h e n R es o n a n ze n b e r ic h t e t . Mit H ilfe e in e sAu fb a u s a u s e in e m h o c h e ffi zie n t e n , s e gm e n t ie r t en G e D e t e k t or in Ko in z id e n zm i t B G O Z h l e r n , d i e e i n e n g r o e n R a u m w i n k e l a b d e c k e n , k o n n t e d i e e x p e r i -mentel le Empfindlichkei t so weit verbesser t werden, dass die fr den

-Einfangspezifischen

-

- b e r g n g e e in d e u t i g n a c h w eis b a r w a r e n . D a d u r c h k o n n t e n d i eR es o n a n ze n b e i 5 6 6 u n d 4 7 0 k e V e r s t m a l s ve r m e s s e n u n d d ie R es o n a n zs t r k e nmit 1.0

0 .2

eV, beziehungsweise 0.4

0 .2

eV, bestimmt werden. Im Vergleichzu den bisher igen Obergrenzen von

1 .7

eV ergibt sich jetzt die Mglichkeit,die Reaktionsraten im ste l laren He-Brennen zuver lssig festzulegen und so dieUnsicherhei ten b ez glich der Neutron enb ilan z im

-Prozess zu r edu zieren.

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C o n t e n t s

Lis t o f Table s v

Lis t o f Figu re s v ii

1 INTRODUCTION 1

1 .1 Pr ea m ble . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 . 2 Ne u t r o n s ou r c es fo r t h e s p r oces s . . . . . . . . . . . . . . . . . . . 2

1 .3 - ca p t u r e o n O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 EXPERIMENTAL TECHNIQUES AND DATA REDUCTION 5

2 .1 Gen er a l fea tu r es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 .1 .1 E xp er im en ta l s et u p . . . . . . . . . . . . . . . . . . . . . . . 5

2 . 1 .2 D a t a a c qu is it io n a n d d a t a a n a lys is s o ft wa r e (LiMD A) . . . . 8

2 .2 Gea n t4 Sim u la tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 12 .3 Ga m m a d et ect ion s ys tem . . . . . . . . . . . . . . . . . . . . . . . . 1 3

2 . 3 .1 Th e Clo ve r d e t ec t or : Fe a t u r e s a n d m o d e s o f o p er a t io n . . . 1 3

2 . 3 .2 C lo ve r d e t ec t ion effi cie n c y in d iffe r en t m od e s . . . . . . . . 1 7

2 .3 .3 Dop pler cor rect ion . . . . . . . . . . . . . . . . . . . . . . . . 2 1

2 .4 Ta r get p r ep a ra t ion a n d s t ab ilit y . . . . . . . . . . . . . . . . . . . . 2 4

2 .5 S econ d a ry cos m ic r a y s u p p res s ion . . . . . . . . . . . . . . . . . . 2 6

2 .6 E xp er im en t a l r u n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 7

3 MEAS UREMENT OF THE REFERENCE RES ONANCES 2 9

3 .1 Deca y m od es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 9

3 .2 Th e coin cid en ce t ech n iqu e . . . . . . . . . . . . . . . . . . . . . . . 3 43 .2 .1 Com p t on su p p r es s ion vs . coin c id en c es . . . . . . . . . . . . 3 4

3 .2 .2 Coin cid en ces a n d a d db a ck m od e . . . . . . . . . . . . . . . 3 8

3 .2 .3 F eed in g of t h e fir s t e xcit ed s t a te . . . . . . . . . . . . . . . . 3 9

4 RES ONANCES AT S TELLAR ENERGIES 4 3

4 .1 Pr eviou s a tt em p ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3

4 .2 Cu r r en t m ea s u r em en t of t h e wea k r es on a n ces . . . . . . . . . . . 4 4

4 .2 .1 Th e 5 66 k eV r es on a n ce . . . . . . . . . . . . . . . . . . . . . 4 4

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CONTENTS

4 .2 .2 Th e 4 70 k eV r es on a n ce . . . . . . . . . . . . . . . . . . . . . 5 1

5 AS TROPHYS ICAL IMPLICATIONS 5 5

5 .1 St ella r r ea ct ion r a t es . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5

5 .2 F a t e of O d u rin g s tella r He b u r n in g . . . . . . . . . . . . . . . . . 6 1

6 CONCLUDING REMARKS 6 3

Re fe re n c e s 6 5

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List of Tables

2 . 1 E ve n t s u p p r e s s ion p r o ce s s es in c lu d e d i n t h e LiMD A c od e . F or

e a c h k in d o f s u p p r e s s i on , a l l Clo ve r m o d es o f o p e r a t io n a r e c on -

s id er ed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1

2 .2 D iffe r en t e xp er im e n t a l r u n s in s e a r ch of t h e low e n er gy r e s -

on a n c es a s well a s t h os e of t h e r efer en c e r es on a n c es in t h e O( , - ) Ne r e a c t io n . R u n s o n n a t u r a l ox yge n t a r g et s w e r e u s e d

to e xa mine the ba c kground c ontribution. The lis te d va lue s for the

b e a m c u r r en t a n d t a r ge t t h ic k n es s a r e r e pr e s en t a t ive . . . . . . . . 2 8

3 . 1 P a r a m e t er s o f t h e r e fe r en c e r e s on a n c es i n t h e O( , - ) Ne re a c -

t ion ava ila b le fr om th e lit er a t u r e. . . . . . . . . . . . . . . . . . . . 3 0

3 . 2 B r a n c h in g r a t io s o b t a in e d in t h i s w or k for t h e t h r e e r e fe r en c e

r e s o n a n c es . Th e r e s u l t s fr o m t h e a n a l ys i s of c os m ic s u p p r e s s e d

C lo ve r s p e c t r a in d i r ec t a s w ell a s in a d d b a c k m o d e a r e lis t e d . . . 3 33 . 3 F r a c t io n a l c on t r ib u t io n o f d iffe r e n t d e c a y m o d e s t o t h e t o t a l d e -

ca ys for t h e h igh en e rgy r es on a n c es (in %). . . . . . . . . . . . . . 4 0

3 . 4 Ne t a r e a u n d e r t h e 1 2 7 4 k e V lin e i n c o in c i d en c e w it h a 3 - 1 1 M eV

B G O w i n d o w a n d t h e c a l c u l a t e d s t r e n g t h s ( c o r r e c t e d f o r b y p a s s

o ve r t h e fi r s t e xc it e d s t a t e ) for t h e 7 5 0 a n d 7 7 0 k e V r e s on a n c e s

r e la t i ve t o t h a t o f t h e 6 6 0 k e V r e s o n a n c e. Th e va l u e s i n b r a c k e t s

a r e t a k en fr om Ta b le 3 .1 for com p a r is on . . . . . . . . . . . . . . . 4 2

4 . 1 Lo w e n e r gy r e s on a n c e p a r a m e t er s fou n d in t h e lit e r a t u r e . . . . . . 4 4

4 .2 S t r en g th - o f t h e 5 6 6 k e V r e s o n a n c e d e t e r m i n e d r e l a t i v e t o t h e

r e s o n a n c es a t h igh e r e n e r gie s . Th e lo we r l im i t i s c a l cu la t e d b ya p p lyin g t h e b yp a s s c or r e ct io n fo r t h e r e fe r en c e r e s on a n c e a n d

a s s u m in g n o b yp a s s e s for t h e low e n er gy r es on a n c e. . . . . . . . . 4 8

4 .3 S t r en g th - o f t h e 4 7 0 k e V r e s o n a n c e d e t e r m i n e d r e l a t i v e t o t h e

h igh e n e rgy re s on a n c e s . Th e lowe r lim it is c a lc u la te d by a p plyin g

t h e b yp a s s c or r e ct io n fo r t h e r e fe r en c e r e s o n a n c e a n d a s s u m i n g

n o b yp a s s es for th e low en e rgy r es on a n c e. . . . . . . . . . . . . . . 5 1

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LIST OF TABLES

5 . 1 Th e t o t a l r e a c t io n r a t e (c m m ol s ) a n d t h e c on t r i b u t ion o f t h e

4 7 0 a n d 5 6 6 k eV r es on a n c es a s a fu n ct ion of t em p er a t u r e T

(=10

K). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 9

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List of Figures

2 .1 D own s t r e a m a n d la t er a l v ie ws of t h e e xp er im e n t a l s e t u p a r e

r e p r es e n t e d s c h e m a t ic a lly. Th e c old t r a p in s i d e t h e b e a m lin e

is o m it t e d fo r b e t t e r r e a d a b ilit y o f t h e fi gu r e . Wh i le t h e le a d

s h ie ld a n d a l l d e t e ct o r s a r e t o s c a l e, t h e b e a m t u b e a n d t h e t a r g et

a r e e m p h a s ize d . Aft e r s e ve r a l t es t ge om e t r ie s , t h is fi n a l s e t u p

h a s b e en u lt im a t e ly a d o p t e d t o s e a r c h for t h e w ea k low -e n e r gy

r e s on a n c es in t h e O( , - ) Ne r e a c t ion . E n e r gy w in d o ws h a v e

be e n s e t in the BGO s pe c tra to look for c a s c a de c oinc ide nc e s de -

te c te d in the Clove r Ge -a rra y. The pla s tic s c intilla tors ha ve be e n

u s e d t o e x c l u d e e v e n t s t h a t c o r r e s p o n d t o s e c o n d a r y c o s m i c - r a y

in t e r a c t io n s in t h e C lo ve r - BG O a s s e m b l y o r in t h e p a s s i ve s h i eld . 6

2 . 2 C lo s e p a c k in g o f t h e fo u r Ge c r ys t a ls in a Clove r d e t ec t or. . . . . . 7

2 .3 S ch e m a t ic r e pr e s en t a t ion of t h e d a t a a c qu i s it ion s ys t em . . . . . . 8

2.4 Elim ina tion of Ran dom an d r eservin g Tru e Clover -BGO coin ci-d e n ce s in a b a n d d efin e d by t h e t im e r es olu t i on of t h e s e tu p . . . . 9

2.5 Virtu al rea lity (VRML) visu alizat ion of th e Boolean cons tru ction of

C love r c rys t a ls for d et a ile d G ea n t 4 s im u la t ion s . . . . . . . . . . . 1 2

2 . 6 Th e 1 1 1 5 k e V lin e o f t h e Zn s ourc e a long with its Compton c on-

t in u u m , b o t h i n a d d b a c k a n d d ir e ct m o d e . Th e a d d b a c k ga in in

pe a k e ffic ie nc y is i llus tra te d in the e xpa nde d pa rt of the s pe c trum

c on t a in i n g t h e f u l l en e r g y p e a k (r igh t p a n e l). Th e c or r e s p o n d i n g

C o m p t o n s c a t t e r e d p h o t o n s r e s t o r e d t o t h e f u l l e n e r g y p e a k i n

a d d b a c k m o d e r e s u l t in a c l ea r r e d u c t i on o f t h e C o m p t o n c on t in -

u u m (left p an el). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 42.7 Th e contr ibu tion of diffe re nt m u ltiplic ities to the ph otope a k in t h e

t ot a l a d db a ck s p ect ru m of Fig. 2 .6 . . . . . . . . . . . . . . . . . . . 1 5

2 . 8 H igh e n e r gy p a r t o f t h e s p e c t r u m a c q u i r e d on t o p of t h e 7 5 0 k e V

r e s o n a n c e of t h e O( , - ) Ne re a c tion, both in a ddba c k a nd dire c t

m o d e . Th e ga i n i n t h e fu ll en e r g y p e a k i s c l ea r in a d d b a c k m o d e

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

for b id d en tr a n s it ion to t h e gr ou n d s ta t e. . . . . . . . . . . . . . . . 1 6

2 . 9 C lo ve r e ffi cie n c y c u r ve s fo r b o th a d d b a c k a n d d ir e ct m o d e s . . . . 1 8

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LIST OF FIGURES

2 . 1 0 S u m m in g e ffe ct s i n a d d b a c k m o d e . E ve n t s s u m m i n g in t o t h e R 0

e ne rgy re gion a r e dis tinguis h e d (in dire c t m ode ) by s e tt in g a win -

d o w o n t h e C lo ve r a d d b a c k s p e c t r u m . E v en t s s u m m in g o u t o f t h e177 9 ke V lin e a re th os e of mu ltiplicities 2 while setting windows

on in d ivid u a l cr ys ta l s pect ra . . . . . . . . . . . . . . . . . . . . . . 1 9

2.11 Tota l e ffic ie nc y in a d dba c k m ode for m onoe ne rge tic ph otons ob-

t a i n e d fr o m G ea n t 4 s i m u la t i on s , illu s t r a t in g t h e la r g e e ffe c t o f

s c a t t er in g a n d a n n ih i la t io n p h o t on s fr o m t h e s u r r o u n d i n g le a d

s h ield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 0

2.12 Left L and right R halves of the Clover as defined for Doppler

c or r e c t io n . E ve n t s in a n y h a lf a r e e it h e r o f s i n g le o r d o u b l e m u l-

t ip lic it ie s (e ve n t s 1 , 2 & 3 in t h e l e ft h a lf). E ve n t s in t h e m id d l e

M a r e of doub le m u ltiplic ity c orre s p onding t o a-

- r a y s c a t t e r i n g

b e t we en t h e t wo h a l ve s of t h e C lo ve r a r r a y (e ve n t 4 ). . . . . . . . 2 2

2 . 1 3 D op p l er s h i ft i llu s t r a t e d b y t h e t r a n s it io n fr o m t h e r e s o n a n t s t a t e

t o t h e 5 .3 26 MeV s t at e in t h e O( , - ) Ne r e a ct io n (7 5 0 k e V

r e s o n a n c e ) . a ) F o r a m e a s u r e m e n t w i t h t h e C l o v e r a x i s a t 4 5 o t o

the be a m dire c tion illus tra ting the diffe re nt e ne rgy s hifts on both

h a lve s o f t h e C lo ve r. b ) Fo r t h e s t a n d a r d s e t u p (F ig. 2 . 1 ) a t 0 o

w h e r e s ym m e t r y a r o u n d t h i s a n gle r e s u lt s in e qu a l s h ift s s e en

on both h a lve s . Th e L, M a nd R de te c tion pos itions a s we ll

a s t h e c o r r e s p o n d i n g m u l t i p l i c i t i e s a r e d e fi n e d i n F i g . 2 . 1 2 a n d

exp la in ed a ls o in t h e t ext . . . . . . . . . . . . . . . . . . . . . . . . 2 3

2 . 1 4 Ta r g et s t a b ilit y t e s t u s in g t h e yie ld ob t a in e d fo r t h e 1 1 6 9 k e Vr e s on a n c e o f th e O(p, - ) F r e a c t io n v s . t h e m a gn e t i c fie ld o f t h e

a n a l yzin g m a g n e t . Als o i n d i ca t e d is t h e s u r fa c e d a m a g e r e gio n

of 3.3 ke V proton e ne rgy width whic h is e quiva le nt to 21 ke V for

a lp h a b ea m a t 6 00 k eV. . . . . . . . . . . . . . . . . . . . . . . . . 2 5

2.15 Ve rific a tion of the ta r get s ta b ility u s ing th e 66 0 ke V re s on a n c e of

t h e O(

,-

) Ne r e a c t io n . Th e b e a m e n e r gy wa s k e p t w e ll a b o ve

t h e r es o n a n c e e n e r gy t o a vo id t h e s u r fa c e d a m a ge r e gio n . . . . . . 2 5

2 . 1 6 S u p p r e s s i on o f t h e s e c on d a r y c o s m i c- r a y c om p o n e n t in t h e b a c k -

ground s pe c trum us ing the pla s tic s c intilla tor pa ne ls on top of the

s et u p (Fig. 2 .1 ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 6

3 . 1 C os m ic s u p p r e s s e d s p e c t r u m a c q u i r e d fo r t h e 6 6 0 k e V r e s o n a n c e

u s i n g a t h ic k t a r ge t a n d a n in t e gr a t ed b ea m c u r r e n t of 1 4 C. . . . 3 0

3 . 2 C os m ic s u p p r e s s e d s p e c t r u m a c q u i r e d fo r t h e 7 5 0 k e V r e s o n a n c e

u s i n g a t h in t a r ge t a n d a n in t e gr a t ed be a m cu r r e n t of 4 .8 8 C. . . 3 1

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LIST OF FIGURES

3 . 3 C os m ic s u p p r e s s e d s p e ct r u m a c qu ir e d fo r t h e 7 5 0 a n d 7 7 0 k e V

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

3.3 C. Th e a s t e ris ks in dic a te tra n s itions th a t r e c e ive contr ibu tion sfr o m b o t h r e s on a n t s t a t e s . Th e u n m a r k e d t r a n s it ion s b e lo n g to

t h e 7 5 0 k eV r e s on a n c e o n ly a n d a r e id e n t ifi ed in F ig. 3 . 2 . . . . . . 3 2

3 . 4 E x cit a t i on fu n c t i on o f t h e 7 5 0 a n d 7 7 0 k e V r e s o n a n c es m e a s u r e d

u s ing th e Clove r de te c tor (127 4 ke V s e c onda ry tra n s ition a n d h igh

e n e r g y g a m m a s ) a s w e l l a s t h e h i g h e n e r g y t r a n s i t i o n s u s i n g t h e

BGO d et ect or s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4

3 . 5 Th e n e t c on t r i b u t io n o f t h e 7 7 0 k e V r e s on a n c e t o t h e t o t a l s p ec -

t r u m a c qu i r ed on t op of b ot h 7 5 0 a n d 7 7 0 k e V r e s on a n c es a s

o b t a in e d b y s u b t r a c t i on o f t h e c om p o n e n t d u e t o t h e r e s o n a n c e a t

7 5 0 k eV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5

3 . 6 D ec a y m o d e s id e n t ifi ed for t h e r e s on a n c es a t 6 6 0 , 7 5 0 a n d 7 7 0

k eV - en e r gy. Th e m u c h w ea k e r r e s on a n c e s a t low er e n e r gie s ,

w h i c h a r e t o b e s t u d i e d i n C h a p t e r 4 , a r e d e n o t e d b y q u e s t i o n

m a r k s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 6

3 . 7 Th e G e Clo ve r s p e ct r u m t a k e n o n t h e 7 5 0 k e V r e s o n a n c e w it h a n

in t e gr a t e d b e a m c u r r e n t o f 4 . 8 8 C . Th e t o p p a n e l s h o ws t h e s i t u a -

tion if only th e plast ic scint illators covering th e experim en ta l set u p

a r e u s e d for r e d u c t io n o f t h e s e co n d a r y c os m i c r a y b a c k gr o u n d .

Th e b o t t om p a n e l s h o ws t h e c oin c i d en c e s p e c t r u m o b t a in e d w it h

a B GO w in d o w of 3 - 1 1 Me V. Th e im p r o ve m e n t in t h e p e a k -t o-

b a c kgr ou n d r a t io for th e 12 7 4 k eV lin e is ob viou s . . . . . . . . . . 3 73 . 8 Le ft p a n e l: S a m e d a t a a s i n F ig. 3 . 7 , b u t zo om e d o n t h e 1 2 7 4 k e V

t r a n s it io n fr o m t h e fi r s t e xc it e d s t a t e t o t h e gr o u n d s t a t e . R igh t

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

u s i n g a n a t u r a l Al

O

t a r ge t . B ot h s p e ct r a c or r e s p on d t o a c ol-

l e c t e d c h a r g e o f 1 C . T h e s q u a r e s c o r r e s p o n d t o t h e s p e c t r a i n

w h ic h o n ly t h e s e con d a r y c os m i c r a y b a c k gr o u n d c om p o n e n t is

s u p p r e s s e d w h i l e t h e t r i a n g l e s r e p r e s e n t t h e C l o v e r c o i n c i d e n c e

s p e ct r a w it h a 3 - 1 1 M eV w in d o w in t h e B GO d a t a . C los e d a n d

op en s ym b o ls c or r es p on d t o t h e d ir e ct a n d a d d b a ck m o d e, r e -

s pect ively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 83 . 9 Ma t r i x s p e c t r a c a lc u l a t e d fo r t h e t h r e e h igh e n e r gy r e s o n a n c es . A

clear peak in the 3-6 MeV region (both Clover and BGO) for the

7 5 0 k e V r e s on a n c e is c on s i s t en t wit h t h e s t r o n g m o d e 3 t r a n s i-

t io n s f or t h is r e s o n a n c e . Th e r igh t - m o s t c o lu m n r e p r e s e n t s c oin -

c id en c es in a BG O win d o w a r ou n d th e 1 27 4 k eV lin e . . . . . . . . 4 1

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LIST OF FIGURES

4 .1 Th e u p p er p a n e l s h o ws t h e s p e ct r u m for t h e c om b in e d r u n s a t

E

= 6 0 0 , 6 2 0 a n d 6 4 0 k e V w it h 8 7 . 5 C t o t a l in t e gr a t e d c u r r e n t

a n d 2 1 3 h o u r s o f b e a m t im e o n t a r ge t . Th e s e r u n s w er e c a r r ie do u t in s e a r c h o f t h e w ea k r e s on a n c e a t 5 6 6 k e V. Th e s e co n d a r y

c os m i c r a y b a c k gr o u n d c om p o n e n t is s u p p r e s s e d in t h i s s p e c-

t r u m a n d t h e a p p e a r i n g - - lin e s c or r e s p on d t o t h e b a c k gr o u n d

c on t r i b u t io n s i d en t i fie d i n S e ct io n 2 . 5 . Th e s h a d e d a r e a r e p r e-

s e n t s t h e e n e r g y r e g i o n w e r e t h e fi r s t e x c i t e d s t a t e t r a n s i t i o n i s

e xp e c t ed . Th e c or r e s p o n d i n g c oin c i d en c e s p e ct r u m is s h o wn in

t h e l o w e r p a n e l , w h i c h i l l u s t r a t e s t h e s t r o n g i m p r o v e m e n t o f t h e

pe a k -to-ba c kgroun d ra tio a llowin g the c lea r de te c tion of th e 127 4

k e V t r a n s it ion in t h e ve ry we a k 5 6 6 k eV r e s on a n c e. . . . . . . . . 4 6

4.2 Coinc ide n c e option a p plie d to the low e ne rgy re s on a n c e (566 ke V)

a s w ell a s t o t h e b a c k gr o u n d wit h b e a m s p e ct r u m . S h o wn a ls o

a r e t h e c or r e s p on d i n g s p e c t r a w it h o n ly c o s m ic r a y b a c k gr o u n d

s u p p r e s s i on . Th e fi r s t e xc it e d s t a t e t o gr o u n d s t a t e t r a n s it io n fo r

th e 566 ke V re s on a n c e ca n be c le a rly ide n tifie d in th e c oin c ide n c e

s p ect r u m . All s p ect r a a r e in a d db a ck m o de . . . . . . . . . . . . . . 4 7

4 . 3 a ) Ne t c ou n t s u n d e r t h e 1 2 7 4 k e V lin e i n c o in c i d en c e w it h 1 M e V

BGO win dows . b) Th e n u m be r of c oin c ide n c e s in a 1 Me V win dow

a s a fr a c t i on t o t h e t o t a l n u m b e r o f c oin c i d en c e s i n t h e 2 - 1 2 M e V

en er gy r egion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 9

4 .4 Ma t r ix s p ect r u m for t h e 5 6 6 k eV r es on a n c e. . . . . . . . . . . . . . 5 0

4 . 5 Mo d e 3 d e c a ys c om p a r e d t o t r a n s it ion s t h r o u g h t h e fi r s t e xc it e ds t a t e (m o d e s 1 & 2 ) fo r t h e 5 6 6 k e V r e s o n a n c e a n d t h e r e fe r e n c e

r es on a n ces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 0

4 . 6 S e a r c h fo r t h e 4 7 0 k e V r e s o n a n c e . S h o w n is t h e co in c i d en c e s p e c -

tru m (win dow 3-11 MeV) with a n d withou t re binn in g (s e e Su bs e c -

tion 2.3.1). The c ounting s ta tis tic s a re limite d be c a us e of the low

b e a m c u r r e n t . Th e cu r ve is a fit of t h e 1 2 7 4 k eV t r a n s it io n . . . . . 5 2

5 . 1 C on t r i b u t io n o f s in g le r e s on a n c e s t o t h e r e a c tion r a t e . Th e r a t e

i s r e c a l c u l a t e d u s i n g r e s o n a n c e s t r e n g t h s t a b u l a t e d i n T a b l e 4 . 1

a nd ta ke n from [KWG94]. Als o s hown is the CF88-ra te . The ra tio

b e t we en b o th r a t e s i s d e p ic t ed i n t h e low er p a n e l. Th e s h a d e d

a r e a s re pr e s e nt th e un c e rta intie s due to the diffe re nt pos s ible s pin

a s s ign m e n ts a s d is cu s s ed in Sect ion 4.1 . . . . . . . . . . . . . . . 5 6

5 .2 Th e fa r ext r em e s of t h e r ea ct ion ra t e. . . . . . . . . . . . . . . . . . 5 7

x


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LIST OF FIGURES

5 . 3 S t e lla r r e a c t io n r a t e s c a lc u l a t e d in t h is w or k a n d c om p a r e d t o t h e

lit er a t u r e. Th e wid t h of t h e s h a d e d b a n d s in t h e u p p er p a n el

r e p r es e n t t h e u n c e r t a in t y in t h e c o n t r ib u t io n o f t h e 4 7 0 a n d 5 6 6k e V r e s on a n c es . Als o s h o wn in t h e u p p e r p a n e l is t h e r e com -

m e n d e d t o t a l r e a ct io n r a t e . T h e r a t e d e d u c e d in t h is w o r k is c o m -

p a r e d t o t h e lit e r a t u r e in t h e lo we r p a n e l a n d t h e c or r e s p on d i n g

u n cer t a in t y is r ep r es en t ed b y t h e d a s h ed a r ea s . . . . . . . . . . . 5 8

5 . 4 Th e r a t io of t h e s t e lla r r a t e s fo r t h e O( , - ) Ne re a c tion obta ine d

i n t h i s w o r k a n d O( ,n ) Ne r e a c t io n t a k e n fr o m [C F 8 8 ]. Th e

r a t i o o b t a i n e d f r o m [ C F 8 8 ] d a t a f o r b o t h r e a c t i o n s i s s h o w n f o r

com pa ris on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1

xi


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Ch a p t e r 1

INTRODUCTION

1 . 1 Pre am ble

O n e o f t h e fu n d a m e n t a l p r o b le m s o f a s t r o p h y s ic s i s t h e q u e s t f or t h e o r igin o f

t h e c h e m ic a l ele m e n t s . Th e a s h e s of t h e B ig B a n g c o n s is t p r e d om i n a n t ly of

h y d r o ge n a n d h e l iu m . Aft e r t h e fo r m a t io n o f ga l a xie s t h e s e a s h e s a r e s u b s e -

qu e ntly bur n e d in th e s te lla r int e riors . All ele me n ts u p to th e re gion of th e iron

g r o u p c a n b e s y n t h e s i z e d i n s u c h e n v i r o n m e n t s b y c h a r g e d p a r t i c l e i n d u c e d

n u c le a r r e a c t io n s (h y d r o ge n b u r n in g , h e liu m b u r n in g , e t c .. ). Th e s e r e a c t io n s

a r e in c re a s ingly im pe de d by the Coulomb ba rrie r, a fa c t th a t e xpla in s th e s h a rp

d r o p i n t h e e le m e n t a l a b u n d a n c e s fr o m A = 1 2 t o A = 50 [Rol88 ]. Ultima tely,

s i l i c o n b u r n i n g g i v e s r i s e t o t h e c l e a r a b u n d a n c e p e a k a t A = 5 6 w h e r e t h e

m o s t s t a b le n u c le i a r e p r o d u c e d . F u r t h e r b u ild - u p of m u c h h e a vie r n u c le i b yc ha rge d pa rtic le re a c tions is s trongly inhibite d due to the s trong Coulomb ba r-

rie r a s well a s th e de c re a s in g bin ding e ne rgy pe r n u c le on [K p9 9].

B e y o n d t h e i r o n p e a k , t h e o b s e r v e d a b u n d a n c e d i s t r i b u t i o n o f t h e h e a v y

e l e m e n t s e x h i b i t s a m u c h s l o w e r d e c r e a s e w i t h m a s s n u m b e r , t h e a b u n d a n c e s

b e in g m u c h h i gh e r t h a n e xp e c t ed fr o m c h a r g e d p a r t i c le in d u c ed r e a c t io n s . In

t h is r e gio n , t h e e x is t e n c e o f s eve r a l t w in p e a k s in t h e a b u n d a n c e d is t r i b u t i on

a s s o cia t e d w it h n e u t r o n s h e l l fi llin g a t t h e m a g ic n e u t r o n n u m b e r s (N = 5 0 , 8 2

a n d 1 2 6 ) le d t o t h e e x t en s i on o f n u c le os yn t h e s is in s t a r s b y n e u t r o n c a p t u r e ,

w h ic h is o b vio u s l y u n a f fe ct e d b y C ou lo m b r e p u ls i on . Th e s e t w in p e a k s a r e

t h e s i g n a t u r e o f t h e s l o w (s ) a n d r a p i d (r) n eu t r o n c a pt u r e p r oc es s e s . B ot h

p r o c es s e s p r o vid e a b o u t 5 0 % o f t h e o b s e r ve d h e a vy e le m e n t a b u n d a n c e s .

Th e r- p r o c e s s o c c u r s i n r e g i o n s o f e x t r e m e l y h i g h n e u t r o n d e n s i t i e s , p r e -

s u m a b l y d u r i n g s t e l l a r e x p l o s i o n s i n t y p e I I s u p e r n o v a e , w h e r e t h e n e u t r o n

c a p t u r e t i m e s a r e s o s h o r t , c om p a r e d t o - d e ca y , t h a t n e u t r o n - r ic h n u c le i fa r

o ff s t a b i lit y c a n b e r e a c h e d . At t e m p t s t o d e s c r ib e t h e r- p r oc es s a r e h a m p e r ed

b y t h e la r g e u n c e r t a in t i e s in t h e n u c le a r p h ys i cs d a t a fa r fr o m s t a b ilit y a n d b y

proble ms re la te d to de ta ile d m ode ling of s te lla r e xplos ions ([K p9 9] a n d re fe r -

e nc e s the re in).

O n t h e c o n t r a r y , t h e s -proc e s s ope ra te s a t re la tive ly low ne utron de ns itie s

1


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CHAPTER 1. INTRODUCTION

a n d t h e n e u t r o n c a p t u r e t im e s a r e m u c h l on g er t h a n t yp ic a l -decay half-lives.

Therefore, the s -process reaction path follows the stability valley. The develop-

in g s a b u n d a n c es a r e d e t er m in e d b y t h e r e s p e c tive (n ,-

) c r o s s s e c t i o n s , s u c ht h a t is o t op e s w it h s m a l le r c r o s s s e c t io n s b u ild u p l a r ge r a b u n d a n c e s . Th i s e x -

p la in s t h e a b o ve m e n t io n e d p e a k s in t h e a b u n d a n c e d is t r ib u t io n c o r r es p o n d -

in g t o n e u t r o n s h e ll fillin g a t t h e m a g ic n u m b e r s . S in c e (n , - ) cross sections for

t h o s e m a in l y s t a b le n u c le i ca n b e m e a s u r e d i n l a b o r a t o r y ex p er im e n t s , a la r g e

pile of da ta le d t o a fa irly qua nt ita t ive pic tur e of the s -proc e s s .

An im p o r t a n t r e m a in i n g p r o b le m f or o u r u n d e r s t a n d i n g of s - p r o ce s s n u c le -

o s y n t h e s i s i s t h e q u e s t f o r t h e e f fi c i e n c y o f t h e n e u t r o n p r o d u c i n g r e a c t i o n s

t h a t a r e fe ed in g t h e n e u t r o n c a p t u r e c h a i n s i n t h e s e s ce n a r io s .

1 . 2 Ne u t ro n s o u rc e s fo r t h e s p r o c e s s

It w a s a s e a r ly a s 1 9 5 7 w h e n t h e p r o d u c t of t h e s t e lla r n e u t r o n c a p t u r e c r os s

s e c t i o n t i m e s t h e s a b u n d a n c e < (A )>Ns (A ) w a s id e n t i fie d b y E . M. B u r b id g e,

G.R. Bu rb idge, W.A. Fowler an d F. Hoyle (B F H ) a s t h e c h a r a c t e r is t i c qu a n t it y

o f t h e s proc e s s [B F H 5 7 ]. Th e c on c e p t o f t h is c a n o n ic a l a p p r o a c h h a s b e e n

m e a n while im pre s s ive ly confirm e d, in pa r ticu la r by the qu a n tita tive de s c ription

o f t h e a b u n d a n c es o f t h e s o -c a lle d s -o n ly n u c le i, w h ic h c a n b e u n a m b igu o u s l y

a s s i gn e d t o a p u r e s proc e s s origin [K p99]. From a c ompa ris on be twe e n c a l-

c u l a t e d Ns va l u e s a n d t h e c or r e s p o n d i n g e m p i r ic a l p r o d u c t s fo r t h e s -only

nuc le i, a t le a s t two diffe re nt s p r o c e s s c o m p o n e n t s c a n b e i n f e r r e d ; t h e s e a r eth e m a in a n d th e wea k c ompon e nts [K p8 9, K p9 9]. Th e s e two c om pone n ts

in d i ca t e t h a t t w o s ep a r a t e s it e s c on t r i b u t e d t o t h e a b u n d a n c e o f s -nuc le i. The

w ea k c om p o n e n t is b e lie ve d t o b e r e la t e d t o t h e p r o d u c t i on o f n u c le i u p t o Z r

(A < 9 0 ) d u r in g H e b u r n in g i n t h e c or e s o f m a s s i ve s t a r s ( 1 5 M

), while th e

m a i n c o m p o n e n t (A = 90 to 209 ) is like ly to oc c ur in th e He bu rn in g s he ll in low

ma s s s ta rs [Me y94, Ga l98].

Th e Ne(

, n ) n e u t r o n s o u r c e p l a ys a n im p o r t a n t r o le in b o t h s c en a r io s . Th i s

r e a c t io n c om p le t e ly d om i n a t e s t h e n e u t r o n p r o d u c t io n in m a s s i ve s t a r s . O n t h e

o t h e r h a n d , t h e lo we r t e m p e r a t u r e s in low m a s s s t a r s fa vo r t h e C( , n ) r e a c -

t io n t h a t p r o vid e s a b o u t 9 5 % o f t h e t o t a l n e u t r o n e x p os u r e . H ow eve r, t h e fi n a la b u n d a n c e p a t t e r n s a r e m o d i fi e d i n a c h a r a c t e r i s t i c w a y b y t h e m a r g i n a l a c t i -

va tion of th e Ne( , n ) r e a c t i o n a t t h e e n d o f e a c h n e u t r o n p r o d u c t i o n e p i s o d e

in t h e r m a lly p u ls i n g AG B s t a r s [G a l9 8 ]. At t h i s e vo lu t io n a r y s t a g e, r e c u r r e n t

H e s h e ll fl a s h e s le a d t o s u ffi cie n t l y h igh t e m p e r a t u r e s o f T ~ 3 x1 0 K (T

~ 0 . 3 )

t h a t Ne(

, n ) r ea c t ion s a d d 5 % t o t h e t ot a l e xp os u r e , b u t a t a m u c h h igh e r

n e u t r o n d e n s i t y t h a n i n t h e p r e v i o u s C(

, n ) p h a s e .

A c c o r d i n g l y , t h e a b u n d a n c e p a t t e r n s t h a t w e o b s e r v e a r e s t r o n g l y i n fl u -

e n c e d b y t h e Ne( , n ) n e u t r o n s o u r c e. It is t h i s i n t e r p l a y b et w ee n t h e c om p le -

m e n t a r y n e u t r on s ou r c es , C( , n ) a n d Ne( ,n), whic h re pre s e nts a pe rs is ting

2


17/82

1 . 3 .

-CAPTURE ON O

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

to la c k of da ta in th e re leva n t te m pe ra tu re re gions of th e re s p e c tive s te lla r s ite s .

This work is de vote d to a n improve d s tudy of the re a c tion s e que nc e le a dingt o t h e b u ild - u p o f Ne d u r in g s t e l la r H e b u r n in g .

In t h e e a r lie r C NO c yc le d u r in g c or e h y d r oge n b u r n i n g i n m a s s i ve s t a r s ,

pra c tic a lly a ll of th e CNO n u c le i a re tr a n s form e d into N s i n c e p r ot o n c a p t u r e

on N is th e s lowe s t re a c tion in th e c yc le [Rol88]. Afte r h ydroge n e xha u s tion,

t h e c or e c o n t r a c t s u n t i l it s d e n s it y a n d t e m p e r a t u r e a r e s u ffi cie n t fo r h e liu m

burning to be ignite d. In this s ta ge of s te lla r e volution, the ma in e ne rgy s ourc e

is the triple r e a c t i o n . T h e N a s h e s o f h yd r o ge n b u r n in g a r e n o w co n ve r t e d

t o Ne b y t h e r e a c t i on s e q u e n c e N(

,-

) F(

) O(

,-

) Ne . Th e r e s u lt in g Ne

a b u n d a n c e a n d h e n c e t h e e f fi c i e n c y o f t h e s u b s e q u e n t Ne(

,n ) M g n e u t r o n

s o u r c e fo r t h e s p r o c e s s t h u s d e p e n d s o n t h e p r e c e d i n g

c a p t u r e r e a c t io n s o n N a n d O . Th e r e a c t io n r a t e s a t t h e t e m p e r a t u r e s o f c or e h e liu m b u r n in g a r e

e xpe c te d to be domina te d by c ontributions of low e ne rgy re s ona nc e s [K p98].

Wh ile the firs t re a c tion is we ll u n de rs t ood [Gr00, Gr01], - c a p t u r e o n O is

s till a ffe c te d by c on s ide ra ble u nc e rta in ties .

1 .3 -c a p t u re o n O

P r e v i o u s d i r e c t m e a s u r e m e n t s o f t h e O(

,-

) Ne re a c tion c ove re d the e ne rgy

ra n ge from 0.6 to 2.3 MeV [Tra 78] a n d from 2.15 to 3.7 0 Me V [Ada 6 9]. Th e re -

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

=0 . 3 - 4 . 3 . At t h e r e le va n t s - p r oc es s t e m p e r a t u r e s

(T

< 0.3), howe ve r, the ra te might be s ignific a ntly e nha nc e d if a dditiona l re s o-

n a n c e s c ou ld b e d e t e ct e d a t e n e r gie s b e lo w 6 0 0 k e V [R ol8 8 , T r a 7 8 ]. At t h e s e

e n e r g i e s , p e n e t r a t i o n t h r o u g h t h e C o u l o m b b a r r i e r r e d u c e s t h e - wid t h (

)

s h a r p l y t o v a l u e s m u c h l e s s t h a n t h e - -width (

-

). S in c e t h e r e s on a n c e

s t r e n gt h is d e fin e d b y t h e p r o d u c t o f t h e s t a t is t i ca l fa c t o r a n d t h e w i d t h r a t i o

-

-

-

, the s tre n gth of th e s e low e ne rgy re s on a n c e s de pe nd s only on

. It wa s indic a te d [Tra 78] th a t du e to th e -

d e p en d e n c e a n d d u e t o t h e

ba rrie r pe ne tra tion fa c tor c onta ine d in

, t h e l ow e n e r g y r e s on a n c e s w e r e i n -

a c c es s i b le t o t h e e xp e r im e n t s a im i n g a t a d ir e c t s t r e n g t h m e a s u r e m e n t . Th e s epos s ible re s ona nc e le ve ls , tha t would domina te the re a c tion ra te a t a s trophys i-

cal energies, have been so far studied only indirectly. The O( Li,d) Ne t r a n s -

fe r re a c tion ha s be e n us e d to inve s tiga te the le ve l s truc ture of Ne a r o u n d t h e

-thr e s h old [Gie 94]. Atte m pts to m e a s u re th e s e low e ne rgy re s on a n c e s dire c tly

c ou ld p r o vid e o n ly u p p e r lim i t s fo r t h e r e s o n a n c e s t r e n g t h s [Vo g9 0 , G ie 9 4 ].

T h e p r e d i c t e d r e s o n a n c e s t r e n g t h s a r e l o w (

1 .7

e V) a n d a r e a g a in r e p o r t ed

a s e xtre me ly diffic ult to be a c c e s s e d in dire c t m e a s u re m e nts [K p 98].

I n g e n e r a l , t h e r e i s a s t r o n g d e m a n d f o r r e l i a b l e c h a r g e d p a r t i c l e r e a c t i o n

r a t e s fo r s t e lla r n u c le os y n t h e s is c a l cu la t i on s . Th e m a i n p r o b le m i s t h a t t h e s e

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CHAPTER 1. INTRODUCTION

m e a s u r e m e n t s a r e h a n d i ca p p e d b y h i gh b a c k gr o u n d s , w h ic h i n c lu d e n a t u r a l,

c os m ic r a y in d u c ed , a s w e ll a s b e a m in d u c ed c o m p o n e n t s . B a c k g r ou n d r e d u c -

tion a n d e ve nt s u ppr e s s ion te ch n iqu e s a re th u s impe ra tive for the inve s tiga tionof extr em ely weak rea ction s [Wie01].

Th i s w or k a t t e m p t s t o m e e t t h e s e r e q u i r em e n t s . In o r d er t o im p r o ve t h e

ga mm a d e te c tion s e ns itivity, or in othe r words to im prove the c a p tu re yie ld-to-

b a c k g r ou n d r a t i o, d i ffe r e n t e ve n t s u p p r e s s io n m e t h o d s h a ve b e e n c on s id e r e d .

T h e m e t h o d c o n c e n t r a t e s o n p a r t i c u l a r f e a t u r e s o f t h e c a s c a d e t r a n s i t i o n s

t h a t d is t in g u is h t h e m fr om t h e b a c kgr ou n d . Wit h s t a n d a r d s p e ct r os c op y,

ba c kgroun d le ve ls e ve n with h e a vy pa s s ive s h ie lding a r e ove rwhe lmingly mu c h

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

u n d e r s t a n d i n g o f t h e d i f f e r e n t c o m p o n e n t s i n t h e b a c k g r o u n d s p e c t r u m a n d

o f t h e e xp e c t ed t r a n s i t io n s in t h e Ne c om p o u n d n u c le u s , t h e s e t u p h a s b e en

d e s ign e d t o s e le ct ive ly e x clu d e a l l e ve n t s t h a t d o n o t s a t i s fy c e r t a in p h ys i ca l

c r it e r ia c h a r a c t er is t ic o f t h e s o u gh t t r a n s i t io n s . S in c e t h e d e c a y o f t h e r e s -

o n a n t s t a t e , u n l ik e c om m o n b a c k gr o u n d s , in c lu d e s c a s c a d es o f we ll d e fin e d

- -line s , the c orre s ponding c oinc ide nc e s pe c tra s e le c tive ly c onta in the de s ire d

in fo r m a t i on w it h s u b s t a n t ia l b a c k gr o u n d e lim i n a t io n . Th e m e t h o d u t i lize s

h i gh e ffi cie n c y B G O s c in t illa t o r s a n d h i gh r e s o lu t io n gr a n u la r G e d e t e c t or s

from which all information is recorded event by event for later off-line analysis.

As a re s u lt, we we re a ble to s ign ific a nt ly e nh a n c e the de te c tion s e ns itivity to

a d e gre e th a t e n a ble d th e dire c t ide n tific a tion of a t le a s t th e firs t e xc ite d s t a te to

gr o u n d s t a t e t r a n s it io n i n t h e c om p o u n d n u c le u s Ne for th e e xtre m e ly we a k

r e s o n a n c e s i n t h e c a p t u r e r e a c t i o n .T h i s d i r e c t m e a s u r e m e n t p r o v i d e s t h e i m p o r t a n t r e s o n a n c e s t r e n g t h s f o r

-pa rtic le e ne rgie s be low 600 ke V, a nd c ons e que ntly a more a c c ura te de te rmi-

na tion of the s te lla r re a c tion ra te .

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Ch a p t e r 2

EXPERIMENTAL TECHNIQUES AND

DATA REDUCTION

2 . 1 Ge n e ra l fe a t u re s

Th e e xp e r im e n t h a s b e e n c on d u c t e d a t t h e 3 . 7 MV Va n d e G r a a f f a c c ele r a t o r

fa c ilit y o f t h e I n s t i t u t f r Ke r n p h y s ik , F o r s c h u n g s ze n t r u m Ka r l s r u h e . P r ot o n -

a n d a lp h a - b ea m s , g u i d ed t h r o u g h a Ta - co llim a t or a n d a LN

-c oole d Cu tra p,

we re s we pt horiz onta lly a nd ve rtic a lly a c ros s a n e ffe c tive ta rge t a re a of 1 c m

t o i llu m in a t e t h e t a r ge t h o m oge n e ou s ly. In t h e fi n a l m e a s u r e m e n t of t h e l ow

e n e r gy r e s on a n c e s , t a r g et s w e r e m o u n t e d a t 0 o t o t h e b e a m d i r ec t io n a n d w e r e

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

in g t h e in t e gr a t e d b e a m c u r r e n t o n t a r ge t . A n e ga t ive v olt a ge o f 3 0 0 V wa s

a p p l ie d t o t h e C u t r a p t u b e t o s u p p r e s s s e c on d a r y e le ct r o n e m is s i on . D e t a ile d

d e s c r i p t i o n o f t h e e x p e r i m e n t a l s e t u p a n d d a t a r e d u c t i o n p r o c e d u r e s , g a m m a

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

s i on w ill fo llo w in t h e s u b s e q u e n t s e c t io n s o f t h i s c h a p t e r. Th e c oin c i d en c e

c a l c u l a t i o n s a n d a l l r e l a t e d a s p e c t s w i l l b e d i s c u s s e d t h o r o u g h l y i n c o n j u n c -

t io n w it h t h e r e s u lt s o f t h e O( , - ) Ne m e a s u r e m e n t s (C h a p t e r s 3 & 4 ).

2 . 1 . 1 Ex pe ri m e n t a l s e t u pTh e e xp e r im e n t a l s e t u p wa s o p t im i ze d in s e ve r a l s t e p s a n d w a s e ve n t u a lly

a d o p t e d i n t h e ve r s io n s k e t c h e d i n F i g. 2 . 1 . Alp h a p a r t i c le b e a m s o f 5 0 - 1 2 0

A we r e u s e d t o ir r a d ia t e Al

O

ta r gets of high s ta bility (s e e Se c tion 2.4). - -

r a y s f r o m c a p t u r e r e a c t i o n o n O w e r e d e t e c t ed b y a h i gh p u r it y G e -d e t e ct o r

ar ra y in clover geometr y (Fig. 2.2 ) as well as by a large solid an gle ar ra y of BGO

s c intilla tors for t h e c oin c ide n t de te c tion of - - r a y c a s c a d e s . B a c k g r o u n d s w e r e

minimiz e d by a he a vy le a d c a s tle c ombine d with la rge a re a pla s tic s c intilla tor

p a n e ls fo r s e c o n d a r y c o s m i c- r a y s u p p r e s s io n .

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CHAPTER 2 . EXPERIMENTAL TECH NIQUE S AND DATA REDUCTION

F ig u re 2 . 1 : Do w n s t re a m a n d l at e r al v i e w s o f t h e e x p e ri m e n t a l s e t u p a re re p re s -

e n t e d s c h e m a t i c a ll y . Th e c o l d t ra p i n s id e t h e be a m l i n e i s o m i t t e d

f o r b e t t e r re a d a bi l i t y o f t h e fi g u re . Wh i l e t h e l e a d s h i e l d a n d a l l d e t e c -

t o r s a r e t o s c a l e , t h e b e a m t u b e a n d t h e t a r g e t a r e e m p h a s i z e d . A f t e rs e v e r a l t e s t g e o m e t r i e s , t h i s fi n a l s e t u p h a s b e e n u l t i m a t e l y a d o p t e d

t o s e a r c h f o r t h e w e a k l o w - e n e r g y r e s o n a n c e s i n t h e O( , - ) Ne re-

a c t io n . En e rg y w i n do w s h a v e b e e n s e t i n t h e BGO s p e c t ra t o l o ok

f o r c a s c a d e c o i n c i d e n c e s d e t e c t e d i n t h e Cl o v e r Ge -a rra y . Th e p la s -

t i c s c i n t i l l a t o r s h a v e b e e n u s e d t o e x c l u d e e v e n t s t h a t c o r r e s p o n d t o

s e c o n d a r y c o s m i c - r a y i n t e r a c t i o n s i n t h e C l o v e r - B G O a s s e m b l y o r i n

t h e p a s s i v e s h i e l d .

Th e m a i n c om p o n e n t i n t h e ga m m a d e t e c t io n s y s t e m i s t h e E u r is y s Me s u r e s

C lo ve r a r r a y (F ig. 2 . 2 ). It c on s is t s o f fo u r c oa x ia l n - t yp e H PG e d i od e s o f 5 0m m d i a m e t e r a n d 7 0 m m l e n g t h c u t i n a w a y ( S e c t i o n 2 . 2 ) t h a t e n a b l e s c l o s e

p a c k in g w it h a G e- Ge d i s t a n c e o f o n ly 0 . 2 m m . Th e c r ys t a ls a r e m o u n t e d in

a c om m o n c r y os t a t . Th e t o t a l a c t ive G e v olu m e is 4 7 0 c m [Du c99, J os97 ],

which results in an absolute efficiency in close geometry of 7% a t 1 MeV - - r a y

energy.

Th e s e c on d p a r t o f t h e - -detection s ystem cons ists of fou r S cionix BGO (Bis-

m u t h G er m a n a t e B i

Ge

O

) c r y s t a l s , 1 3 1 3 3 c m i n s i z e , s u r r o u n d i n g t h e

u p s t r e a m s id e o f th e t a r ge t . Th i s i s a h i gh Z, h igh d e n s it y m a t e r ia l c on s i d -

e re d a s a n e ffic ie nt-

-ra y a bs orbe r. Th e BGO de te c tors toge the r with th e pla s tic

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2.1 . GENERAL FEATURES

F i gu re 2 . 2 : C lo s e p a c k i n g o f t h e f o u r Ge c r y s t a l s i n a C lo v e r d e t e c t o r.

s c intilla tor p a n e ls on top of th e s e tu p (60 5 0 2 cm ) ser ve to pr ovide coinciden t

h its t h a t c a n be u s e d to s u ppr e s s th e Clove r s ign a ls a c c ording to diffe re nt c rite -

ria . Th e s e a c tive de te c tors we re u s e d, for e xa mp le , in c os mic -ra y a nd Compton

s u ppr e s s ion a s we ll a s for ide ntifyin g c oin c ide n c e s be twe e n t h e Clove r a n d s ig-

n a ls in c or r e s p o n d i n g e n e r gy w in d o ws o f t h e B G O s p e c t r u m . Th e p a s s i ve l ea d

s h i e l d c o n s i s t e d o f a 1 0 c m t h i c k l a y e r f o r r e d u c i n g t h e a m b i e n t - - r a y b a c k -

ground level.

The ge ome try of the fina l s e tup ha s be e n optimiz e d for Clove r-BGO c oinc i-

d e n c es , a s C om p t o n s u p p r e s s io n w a s fo u n d t o b e n o t a s im p o r t a n t a c co r d in g

t o a p r e lim i n a r y t e s t m e a s u r e m e n t (S u b s e c t io n 3 . 2 . 1 ). Th e C lo ve r G e d e t e ct o r

c ove rs forwa rd a n gles a n d th e BGO s c in tilla tors a r e orie nt e d s o th a t th e y cove r

a l a r g e s o l i d a n g l e u p s t r e a m t h e O t a r ge t . Th e fi n a l ge om e t r y a d o p t ed fo r

t h e e x p e r i m e n t a l s e t u p a s s k e t c h e d i n F i g . 2 . 1 h a s b e e n m o d e l e d i n d e t a i l f o r

e xte ns ive Ge a nt4 Monte Ca rlo s imula tions whic h a re de s c ribe d in Se c tion 2.2.

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F ig u re 2 . 3 : S c h e m a t i c re p re s e n t a t i o n o f t h e d a t a ac q u i s i t io n s y s t e m .

2 . 1 . 2 D at a a c q u i s i t i o n a n d d at a a n a ly s i s s o f t w a re (Li MDA)

Th e s i gn a ls fr o m a l l d e t e ct o r s , i .e . t h e t im i n g a n d e n e r gy ou t p u t s o f t h e fo u r

C l o v e r c r y s t a l s , t h e t i m i n g a n d e n e r g y o u t p u t s o f t h e f o u r B G O s c i n t i l l a t o r s

(pa irs of two c ou pled t oge the r) a s we ll a s th e timing s ign a ls from th e two pla s -

t ic s c i n t illa t o r s a r e p r o c es s e d a s s h o wn s c h e m a t i ca l ly in F ig. 2 . 3 . Th e s i gn a l

proc e s s ing is inte nde d to re c ord a ll re le va nt e ne rgy a nd timing informa tion for

la t e r o ff- lin e t r e a t m e n t . F or t h i s p a r t ic u la r r e a s on , n o h a r d w a r e ve t os h a v e

b e e n in c l u d e d , s o t h a t a l l n e ce s s a r y i n fo r m a t io n i s p r e s e r ve d fo r r e p e a t e d a n dflexible software analysis.

Th e da ta a c qu is ition s ys te m c ons is ts of e ight a m plitu de -to-digita l c onve rte rs

(ADC):

AD C 1 , 2 , 3 , a n d 4 a re u s e d f o r t h e f o u r Cl o v e r e n e r gy o u t p u t s .

AD C 5 a n d 6 a re u s e d f o r t h e t w o p a i rs o f c o u p l e d B GO e n e r gy o u t p u t s .

A D C 7 is u sed fo r th e Ge-B GO timin g sig n a ls.

AD C 8 i s u s e d f o r t h e Ge - p la s t i c s c i n t i l l at o r t i m i n g s i g n a l s

All even ts ar e record ed in list m ode: If a h it is r egist ered in a n y ADC, all oth er

ADCs a re r e a d within a s p e c ifie d c oin c ide n c e tim e a n d t h e whole s e qu e nc e is

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2.1 . GENERAL FEATURES

F i gu re 2 . 4 : E li m i n a t i o n o f R an d o m a n d re s e r v in g Tru e Cl o v e r-B GO c o i n c i -

d e n c e s i n a b a n d d e fi n e d b y t h e t i m e r e s o l u t i o n o f t h e s e t u p .

stored. The final list file thus consists of blocks (called events) of eight registers

pe r e ve nt re pre s e nting the e ight ADC c onve rte d digita l outputs . Obvious ly, a n

e ve n t c ou ld c on t a i n o n e o r m o r e fi r in g AD Cs b u t n o n - fi r in g AD C s a r e a l s o

re gis te r e d. In th is wa y, one e ve nt c ons is ts of 8 words (16 byte ) a n d e a c h 2 -byte

word is re la te d to one s ingle ADC whe re the lowe r 13 bits re gis te r the c ha nne l

n u m b e r ( c o r r e s p o n d i n g t o 8 k a n a l o g - t o - d i g i t a l c o n v e r s i o n ) a n d t h e h i g h e r 3

b i t s g ive t h e AD C n u m b e r. Th e d e s ir e d c o m p o s it e s p e c t r a a r e t h e n ge n e r a t e d

offline us ing a s pe c ia l s oftwa re (Lis t Mod e Da t a Analysis LiMDA) written for

t h i s p u r p o s e.

F r o m e a c h s i n gle l is t fi le , a n d u s i n g t h e p r o p er e n e r gy c a lib r a t i on fo r e a c h

in d ivid u a l c r ys t a l , d i ffe r e n t (s u p p r e s s e d a n d u n s u p p r e s s e d ) a d d b a c k o r d i r e ctsp ectra (Section 2.3.1 ) can be genera ted a n d an alyzed in a flexible way. To bu ild

th e tota l a ddb a c k or dire c t s pe c tru m , the firs t four words in a n e ve nt whic h c or-

r e s p o n d t o t h e fo u r C lo ve r e n e r gy ou t p u t s a r e u s e d . Th e s e gm e n t a t i on o f t h e

Ge Clove r de te c tor a llows the re c ons truc tion of a n origina l-

- r a y i f t h e C om p -

ton s c a tte re d photon or the a nnihila tion photons following pa ir produc tion a re

de te c te d in one or m ore of th e ne igh boring Ge c rys ta ls . This s o-c a lle d a d dba c k

m ode re s u lts in a n in c re a s e of th e ove ra ll de te c tion e ffic ie nc y. Th e othe r four

words c a n be u s e d to s u ppr e s s th e e ve nts a c c ording to diffe re nt c rite ria de fine d

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b y t h e u s e r. Th e u n s u p p r e s s e d C lo ve r s p e c t r u m c a n b e g en e r a t e d s i m p ly b y ig-

noring the BGO a nd the pla s tic s c intilla tion byte s in the bina ry lis t file e ve nts .

T h e a n t i c o i n c i d e n c e a n d / o r t h e c o i n c i d e n c e s p e c t r a w i t h a n y c o m b i n a t i o n o f s c intilla tion byte s c a n a ls o be ge ne ra te d.

I n e a c h k i n d o f c a l c u l a t i o n , t h e d e fi n e d c r i t e r i a g i v e n a s a n i n p u t t o t h e

c od e a r e fi r s t u s e d t o r e d u c e t h e o r igin a l lis t fi le . Th i s r e d u c e d l is t fi le i s t h e n

a n a l yze d . In a d d i t io n t o t h e s i n gle c r ys t a l s p e c t r a , t h e a n a l ys i s in c l u d e s t h e

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

a s fo r p a r t i a l a d d b a c k u s e d fo r D o p p le r c o r r e ct io n (S e ct io n 2 . 3 . 3 ). Th e B G O

e n e r gy s p e c t r a t h a t s a t i s fy t h e s a m e r e d u c t io n c r i t er ia a r e a ls o g en e r a t e d .

The a bove me ntione d s uppre s s ion c rite ria c ould inc lude e ne rgy windows in

t h e B G O s p e c t r a . Th i s is e s p e c ia l ly u s e fu l w h e n t h e c oin c i d en c e c a l cu la t i on

i s i n t e n d e d t o s e a r c h f o r a-

- lin e w h i ch is p a r t of a c a s c a d e . F or e xa m p le ,

t h e B G O e n e r g y w i n d o w c a n b e s e t t o i n c l u d e t h e e x p e c t e d p r i m a r i e s i n t h a t

cascade. This clearly helps in minimizing background in the coincidence Clover

s p e ct r u m . F u r t h e r b a c k gr o u n d r e d u c t io n c a n b e a c c om p l is h e d b y a d d it io n a l

s u p p r e s s io n u s i n g t h e p la s t i c s c in t i lla t o r s ign a ls . Th u s , C lo ve r - B G O e n e r gy-

win dow c oin c ide n c e s wh ic h a r e in a n ticoin c ide n c e with th e pla s t ic s c intilla tors

a r e u s e d t o d e fin e t h e c a s c a d e e ve n t s fr o m t h e O( - ) re a c tion .

T h e n o n - c o s m i c b a c k g r o u n d c o m p o n e n t i n t h e c o i n c i d e n c e s p e c t r a c a n b e

e v e n f u r t h e r r e d u c e d . T h i s i s t h e r a n d o m c o i n c i d e n c e c o m p o n e n t . I n o r d e r t o

m in i m i ze G e -B GO r a n d o m c o in c i d en c e s , 3 D p lo t s h a ve b e e n p r e p a r e d a s p a r t

of th e lis t file a n a lys is s oftwa re LiMDA whic h s h ow the nu m be r of c oin c ide n c e s

a s a fu n c tion of bot h e n er gy a n d t im e . F ig. 2 . 4 is a n e xa m p le . In t h e s u p -pre s s e d s pe c tru m , only eve nt s ins ide a ba n d, la be le d True in th e figu re , a re

u s e d i n t h e e ve n t r e d u c t i on . Th i s b a n d is d e fi n e d i n t h e h i gh e n e r gy p a r t t o b e

s lightly wide r tha n the time re s olution of the s ys te m whic h is a bout 12 ns . The

c l e a r w a l k p r o d u c e d b y t h e l o w p u l s e - h e i g h t G e s i g n a l s w i d e n s t h e b a n d a t

low e ne rgie s . All Ge -BGO c oin c ide n c e e ve nt s out s ide th is ba nd a r e c ons ide re d

a s r a n d o m a n d t h e c o r r es p o n d in g G e r a w d a t a a r e t r e a t e d a s h a v in g n o B G O

c oin c i d en c e s . In t h e c a s e o f co s m i c s u p p r e s s io n , a n d s i n c e t h e r e a r e o n l y t im e

s i gn a ls r e gis t e r e d i n t h e l is t fi le , a s i m p le r e c t a n gu la r s t r i p i n t h e TAC o u t p u t

s p e ct r u m is u s e d t o d is t in gu i s h b et we en r a n d o m a n d t r u e e ve n t s . D ep en d -

in g o n t h e s u p p r e s s i on c r it e r ia o f c oin c i d en c e (a n t ic oin c i d en c e ) a n a lys e s , t h es oftwa re ke e ps (re jec ts ) e ve nts with tru e c oin c ide n c e s in th e n e wly gen e ra te d

s u p p r e s s e d t e m p o r a r y l is t fi le .

An othe r pos s ible c a lc u la tion int e gra te d in LiMDA is worth m e ntioning h e re .

If w e c o n s i d er t h e w h o le a s s e m b l y c on s is t i n g o f t h e C lo ve r a n d t h e B G Os a s

o n e c a l or im e t e r, t h e n a Q -va lu e e n e r gy w in d o w c a n b e s e t w h e r e e ve n t s a r e

r e gis t e r e d in t h e s u p p r e s s e d lis t fi le o n ly if t h e t o t a l e n e r gy d e p o s it io n in t h e

c a lorime te r lie s within th is wind ow. This m e thod will be c om pa re d t o th e BGO

e ne rgy-window c oinc ide nc e c a lc ula tion a nd to Compton s uppre s s ion whe n a p-

p l ie d t o r e s o n a n c es i n t h e O( , - ) Ne reaction (Subsection 3.2.1).

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2. 2. GE ANT4 SIMULATION

On th e c on tra ry to th e BGO win dow s u ppr e s s ion, a n e n e rgy win dow c a n a ls o

b e s e t fo r t h e C love r a d d b a c k s p e c tr u m a s a s u p p r e s s io n c r it e r io n . Th i s n o t

o n ly p r o vid e s t h e c oin c i d en t B GO s p e c t r a , b u t c a n b e u s e d t o t e s t s u m m a t i one ffe c t s i n t h e C lo ve r i t s e lf. If t h e w in d o w is s e t a r o u n d a - -line in the Clover

a d d b a c k s p e c t r u m , t h e d ir e c t m o d e C lo ve r s p e c t r u m o b t a in e d f r om t h e r e d u c e d

l i s t fi l e c o n t a i n s i n f o r m a t i o n a b o u t l o w e r e n e r g y g a m m a s t h a t s u m i n t o t h e

s p e c ifi ed w in d o w. Na t u r a l ly, t h is i n c lu d e s C o m p t o n c on t in u a in a d d i t io n t o

a n n i h ila t io n p h o to n s a n d e s ca p e p e a k s . An y o t h e r l in e s d is t in g u is h e d in t h e

s p e c t r u m a r e c le a r ly m e m b e r s o f a c a s c a d e a n d t h e c a lc u l a t io n o f t h e y ie ld o f

t h e - -line in the window s hould ta ke this s umma tion c ontribution into a c c ount.

Fina lly, the a bove me ntione d a na lys is provide s informa tion, for a ny s ingle

lin e , a bou t th e c ontr ibu tion of s u m m in g-in e ffe c ts . As a n a lte rn a tive a n a lys is

t h a t t a k e s in t o a c c ou n t t h e s u m m in g - ou t e ffe ct s , t h e d e s ir e d e n e r gy w in d o w

c a n be s e t in LiMDA on individu a l Clove r c rys ta ls in s te a d of th e tota l a d dba c k

s p e c t r u m . Th e r e s u lt a n t a d d b a c k c a lc u l a t io n o f m u lt ip l ic it ie s 2 obvious ly

p r o vid e s t h e s p e c t r u m o f e ve n t s s u m m in g o u t o f t h e w in d o w s et o n t h e s i n gle

c rys ta l e ne rgy s pe c tra .

Ta b l e 2 . 1 s u m m a r ize s t h e d iffe r e n t p o s s ib ilit ie s o f eve n t s u p p r e s s io n in -

c lude d in LiMDA. For e a c h s ingle te mpora ry lis t file ge ne ra te d in a ny kind of

s u p p r e s s io n , a ll s p e c t r a c or r e s p o n d i n g t o t h e d iffe r e n t C lo ve r m o d e s o f op e r -

a t i on a r e ge n e r a t e d . E x a m p l e s i llu s t r a t in g t h e s e c a l cu la t i on s w ill b e p r o vid e d

t h r o u g h o u t t h e t e x t.

Ta bl e 2 . 1 : Ev e n t s u p p re s s i o n p ro c e s s e s i n c l u d e d i n t h e Li MD A c o d e . F o r e ac h

k i n d o f s u p p r e s s i o n , a l l C l o v e r m o d e s o f o p e r a t i o n a r e c o n s i d e r e d .

T y p e o f S u p p r e s s i o n Co m m e n t s

N o s u p p r e s s i o n Scintillation data ignored

Anticoincidences:

C os m i c s u p p r e s s ion Us e p la s t ic s c in t illa t or d a t a

C om p t o n s u p p r e s s ion Us e B GO d a t a

Coincidences:*

Q-va lu e Win d ow on Clover + BGO

BGO win dow Ca s ca d e coin cid en ces

Clove r a ddba c k win dow Dire c t mode give s s u m m ing-in Clover crysta l win dow Add ba ck (M 2) gives summing-out

* With or w ithout cos m ic s upp ress ion.

2 . 2 Ge a n t 4 S im u l at i on

The Ge a n t4 obje c t orien te d s imu la tion toolkit [GEANT4] wh ic h u s e s th e Mon te

C a r l o t e c h n i q u e t o s i m u l a t e t h e e x p e r i m e n t a l s e t u p a n d r e s p o n s e o f d i f f e r -

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e n t d e t ec t or s h a s b e en t e s t ed fi r s t b y c o m p a r in g t h e s im u la t e d r e s p o n s e o f a

s i n g le G e c r ys t a l t o t h e e xp e r im e n t a l r e s u lt s . Th i s c o m p a r is o n , w h ic h s h o w s

r e m a r k a b le a g r e em e n t , p r o vid e s c o n fi d e n c e in t h e s u b s e q u e n t s im u la t i on s fo rth e diffe re nt m ode s of ope ra tion of th e c ompos ite s e tu p.

In o r d e r t o s im u la t e t h e p e r fo r m a n c e o f t h e m a i n d e t e c t io n u n i t , t h e r a t h e r

c om p le x C lo ve r ge om e t r y w a s c a r e fu lly m o d ele d . F or e xa m p le , t h e b e ve ls a t

th e front fa c e of e a c h Ge c rys ta l ge ne ra te , on t wo s ide s , a s e m i-e lliptic s u rfa c e .

The othe r two s ide s of e a c h c rys ta l a r e m os tly fla t t o a llow for th e c los e pa c kin g

wit h t h e a d ja c en t c rys t a ls . Th e fr on t fa c e of t h e c r ys t a l h a s t h u s a q u a s i-

s q u a r e s h a p e w h i l e i t s r e a r f a c e i s q u a s i - c i r c u l a r s i n c e t w o s i d e s i n t h e b a c k

h a lf c on s e rve the 5 0 m m dia m e te r of th e origin a l Ge c ylin de r. The s e d e ta ils h a d

t o b e c o n s i d er e d i n t h e s i m u la t i on .

F i gu re 2 . 5 : Vi rt u a l r e a li t y (VR ML) v i s u a li z a t i o n o f t h e B o o l e a n c o n s t r u c t i o n o f

Cl o v e r c ry s t a l s f o r d e t a i l e d Ge a n t 4 s i m u l a t i o n s .

A s i g n i fi c a n t f e a t u r e i n G e a n t 4 i s t h e a b i l i t y t o d o B o o l e a n o p e r a t i o n s o n

s ta nd a r d s olids c a lle d CSG (Con s tr u c te d Solid Ge ome try) to produ c e more c om -

ple x s ha pe s . Th e s e Boole a n ope ra tions like Union a n d In te rs e c tion on s ta n -

da rd ge ome tric a l volume s (Box, Cylinde r, Cone ) ha ve be e n us e d a s de s c ribe d

in F ig. 2 . 5 t o r e p r o d u c e t h e fi r s t C lo ve r c r ys t a l . Th e c on c e p t o f p o s it io n e d

volu m e s , u s e fu l for s a ving me m ory whe n de s c ribin g comple x or r e pe a te d s tru c -

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2.3 . GAMMA DETECTION SYSTEM

t u r e s , h a s b e e n u s e d t o b u ild t h e ot h e r t h r e e cr y s t a ls e n d in g u p w it h t h e c o m -

ple te ge ome try of the Clove r. This proc e dure e ns ure s pre s e rving the s ha pe a nd

d i m e n s i on s o f t h e c r y s t a ls . In t h i s s i m u la t e d vo lu m e , t h e c oa x ia l h o le o f ea c hd i od e a s w ell a s t h e d e a d m a t e r ia l we r e in c l u d e d . Th e w h o le s t r u c t u r e is t h e n

m o u n t e d in a 1 . 5 m m t h i c k a lu m i n i u m c a n . Th e s p a c e b e t we e n t h e Al b o x a n d

th e c rys ta ls is filled with va c u u m .

T h e B G O s c i n t i l l a t o r s a n d t h e i r a l u m i n i u m h o u s i n g a s w e l l a s t h e o t h e r

c om p o n e n t s in t h e s e t u p a r e r a t h e r m o r e s t r a igh t fo r wa r d t o r e p r o d u c e. Th e

0 . 4 m m T a t a r g e t b a c k i n g , t h e t a r g e t h o l d e r , t h e C u c o l d t r a p a n d t h e b e a m

lin e w h i ch a c t a s a b s o r b er s a n d s c a t t er e r s b e t we en t h e t a r ge t a n d b ot h t yp e s

o f d e t e ct o r s w er e a ls o c a r e fu lly s t r u c t u r e d . Th e p a s s i ve l ea d s h ie ld is fi n a l ly

a dde d a s we ll.

T h e s i m u l a t i o n i s o f c o u r s e i n t e n d e d t o t r a c k t h e-

- r a y s p r o d u c e d i n t h e

t a r g e t . T h e r e fo r e r oo m a n d c os m ic b a c k g r ou n d s w e r e n o t i n c lu d e d i n t h e s i m -

u la tion. Ac c ordingly, the pla s tic s c intilla tors a b ove th e p a s s ive s h ie ld we re n ot

d e fi n e d in t h e s im u la t i on .

T h e b e a m s p o t o n t a r g e t , w h i c h i s a b o u t 1 c m i n d i a m e t e r , h a s b e e n s i m u -

la t e d b y d e fin i n g t h e ga m m a s o u r c e t o b e d i s t r ib u t e d r a n d o m l y in a p e r p e n -

dic ula r two-dime ns iona l pla ne a t the ta rge t pos ition.

Th e e n e r gy d e p os it e d i n e a ch G e a n d B GO vo lu m e is t h e n c olle ct e d a n d

h i s t o gr a m m e d t o o b t a in t h e r e le va n t s p e c t r a . In a d d i t io n , t h e o r igin a l d a t a c a n

b e m a n ip u la t e d t o o b t a i n a l l d e s ir e d c o m b i n a t i on s o f h i t s . Th e a d d b a c k a n d

d ir e c t s p e c t r a o f t h e C lo ve r r e p r e s e n t a s i m p l e e xa m p le . All k i n d s o f eve n t

s u p p r e s s io n d e s c r ib e d i n t h e p r e vio u s s e c t io n a r e a ls o i m p l em e n t e d .T h e d e t e c t i o n e f fi c i e n c i e s o b t a i n e d f r o m t h e s i m u l a t i o n a r e p r e s e n t e d a n d

c ompa re d to e xpe rim e nt a l re s u lts in th e followin g s e c tion.

2 . 3 Gam m a de t e c t io n s y s t e m

Th e d e t a i le d d e s c r ip t i on o f t h e d e t e ct io n s y s t e m give n in t h i s s e c t io n s u m -

m a r i z e s t h e e x p e r i m e n t a l o b s e r v a t i o n s t o g e t h e r w i t h t h o s e o b t a i n e d f r o m t h e

G e a n t 4 s i m u l a t i o n s . T o s t a r t w i t h , t h e m a i n f e a t u r e s o f t h e C l o v e r s p e c t r a a s

we ll a s its diffe re nt mode s of ope ra tion a re dis c us s e d, illus tra ting the ir a dva n-t a ge s a n d d r a w b a ck s .

2 . 3 . 1 Th e Clo v e r d e t e c t o r: Fe a t u re s a n d m o d e s o f o p e ra t io n

Even ts in th e Clove r de te c tor re gis te r e d in lis t m ode a s de s c ribe d in Se c tion 2 .1

m a y e xhibit diffe re nt m u ltiplic ities . Mu ltiplic ity 1 m e a n s a h it in one c rys ta l

w it h n o c o in c i d en t h i t s i n a n y ot h e r . Mu lt ip l ic it ie s 2 , 3 a n d 4 c o r r e s p o n d t o

s i m u lt a n e ou s h it s d e t e ct e d i n 2 , 3 o r 4 c r ys t a l s , r e s p e c t ive ly, d u e t o C o m p t o n

s c a t t e r in g o r p a i r p r o d u c t io n .

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F ig u re 2 . 6 : Th e 1 1 1 5 k e V l in e o f t h e Z n s o u r c e a l o n g w i t h i t s C o m p t o n c o n -

t i n u u m , b o t h i n a d d ba c k a n d d i re c t m o d e . Th e a d db ac k g ai n i n p e a k

e f fi c i e n c y i s i ll u s t ra t e d i n t h e e x p a n de d p a rt o f t h e s p e c t r u m c o n t a i n -

i n g t h e fu l l e n e r gy p e a k (ri g h t p a n e l ). Th e c o r re s p o n d i n g C o m p t o ns c a t t e r e d p h o t o n s r e s t o r e d t o t h e f u l l e n e r g y p e a k i n a d d b a c k m o d e

r e s u l t i n a c l e a r r e d u c t i o n o f t h e C o m p t o n c o n t i n u u m ( l e f t p a n e l ) .

If e a c h c rys ta l in th e Clove r de te c tor is c ons ide re d ind e pe nd e ntly a s a s ingle

de te c tor, e ve n if the multiplic ity wa s highe r tha n 1, the n the c ompos ite Clove r

d e t ec t or is s a i d t o b e o p e r a t ed in t h e d ir e ct m o d e . Th u s t h e p h o t op e a k d e -

te c tion e ffic ie nc y is s imply the s um of the individua l photope a k e ffic ie nc ie s of

e a c h of the four c rys ta ls .

O n t h e o t h e r h a n d , t h e c l o s e p a c k i n g o f t h e C l o v e r d e t e c t o r f a c i l i t a t e s i t s

o p e r a t io n i n t h e s o c a l le d a d d b a c k m o d e . In t h is m o d e , c o in c i d en t e ve n t sin d iffe r e n t c r ys t a l s a r e s u m m e d (e n e r gy s u m ) r e s u lt in g i n t h e r e c on s t r u c t io n

o f fu ll- en e r g y s i gn a ls if C om p t o n - s c a t t e r e d a n d 5 1 1 k e V a n n ih i la t i on p h o t on s

e s c a p in g fr o m o n e c r ys t a l a r e d e t e ct e d i n a n o t h e r c r ys t a l . T h e s u m m e d s i gn a ls

a r e s tore d in t he a d dba c k s pe c tru m , wh ic h re s u lts in im prove d fu ll e ne rgy pe a k

e ffic ie nc y on t h e e xpe n s e of th e Compt on c ontinu u m a nd e s c a pe pe a k s . Fig. 2.6

i l l u s t r a t e s t h e p h o t o p e a k e f fi c i e n c y g a i n o f a f a c t o r 1 . 6 i n a d d b a c k m o d e

a t 1 1 1 5 k e V ( Z n s o u r c e ) a n d t h e c o r r e s p o n d i n g r e d u c t i o n o f t h e C o m p t o n

c on t in u u m . F ig. 2 . 7 s h o ws t h e c on t r ib u t io n o f d iffe r e n t m u lt ip lic it ie s t o t h e

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F ig u re 2 . 7 : Th e c o n t r i bu t i o n o f d i ffe re n t m u l t i p li c i t i e s t o t h e p h o t o p e a k i n t h e

t o t a l a d d b a c k s p e c t r u m o f F i g . 2 . 6 .

fu ll- e n e r gy p ea k . Th e m e a s u r e m e n t w a s c o n d u c t ed w it h t h e Zn s our c e pla c e d

in th e pos ition of th e O ta rge t.

At high ga mma e ne rgie s , pa ir produc tion pre va ils a nd the a nnihila tion pho-

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

p e a k . Th i s e ffe ct is d e m o n s t r a t e d in F ig. 2 . 8 w h ic h s h o ws t h e h i gh e n e r gy

p a r t o f t h e ga m m a s p e ct r u m ob t a in e d on t op o f t h e 7 5 0 k e V r e s on a n c e o f t h e O( , - ) Ne r e a c t io n . Th e t r a n s fe r o f c ou n t s fr o m t h e e s c a p e p e a k s t o t h e fu ll-

e n e r g y p e a k b y d e t e c t i o n o f a n n i h i l a t i o n p h o t o n s i n a n o t h e r c r y s t a l i s q u i t e

obvious. The detection efficiency for the full-energy, single-escape and double-

e s c a p e p e a k s i n a d d b a c k a n d d i r e c t m o d e w i l l b e d i s c u s s e d i n d e t a i l l a t e r i n

t h i s c h a p t e r .Th e d i s a d va n t a g e o f t h e a d d b a c k m o d e b e co m e s c le a r in c a s e o f c oin c i d en t

c a s c a d e p h o t on s r e s u lt in g fr o m s u b s e q u e n t t r a n s it io n s e s p e cia l ly in t h e d e c a y

of a h ighly e xcite d n u c le a r s ta t e . Su m m in g effe c ts c omplic a te th e in te ns ity c a l-

c ula tion whe re the obs e rve d yie ld de pe nds not only on the de te c tion e ffic ie nc y

fo r m o n o en e r g et ic g a m m a s b u t o n t h e p o s s ib ilit y o f d e t ec t in g c oin c i d en t c a s -

c a de photons a s we ll. In s uc h a c a s e , howe ve r, the de te c tor c a n be ope ra te d in

t h e d i r e ct m o d e. Th i s s u m m in g e ffe ct c a n b e e a s i ly s e e n i n F ig. 2 . 8 w h e r e t h e

fo r b id d e n t r a n s i t io n fr o m t h e r e s o n a n t 0 s t a t e t o t h e 0 g r o u n d s t a t e i s s e e n

1 5


30/82


8 9 10

20

40

60

80

20

40

60

80

Direct

Energy (MeV)

Counts

summing-in effect

R=>1274 Addback

F ig u re 2 . 8 : Hi gh e n e rg y p art o f t h e s p e c t ru m a c qu i re d o n t o p o f t h e 7 5 0 k e V

r e s o n a n c e o f t h e O( , - ) Ne re a c t i o n , bo t h i n a d db ac k a n d d i re c t

m o d e . Th e g a i n i n t h e f u l l e n e rg y p e a k i s c l e a r in a d d ba c k m o d e w h i l e

t h e s u m m i n g e f fe c t i s i l l u s t ra t e d b y t h e a p pe a ra n c e o f t h e fo rb id d e nt ra n s i t i o n t o t h e g ro u n d s t a t e .

c le a r ly on l y in a d d b a c k m o d e. Th i s d r a w b a c k in a d d b a c k c a l cu la t i on s m a k e s

it ne c e s s a ry to c orre c t for los s e s in ph otope a k e ffic ie nc ies . Th e obs e rved e ffi-

cien cy at a given en ergy an d for th e sam e geomet ry is a ffected by th e proba bility

for de te c ting a c oinc ide nt c a s c a de photon a s we ll a s by its a ngula r c orre la tion.

This is s ue will be de a lt with qua ntita tive ly in the following s ubs e c tion. Pa rtia l

a d dba c k a n d d ire ct c a lc u la tions c ould h e lp, h owe ve r, in c orre c tin g for Doppler

b r o a d e n i n g a s e xp la i n e d i n S u b s e c t io n 2 . 3 . 3 .

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

w or d s w h ic h c or r e s p o n d t o t h e fo u r C lo ve r c r ys t a l s . Th e r e s u lt s a r e t h e n h i s -

togra m m e d with a b in width th a t c orre s pon ds to the a ve ra ge e ne rgy pe r c ha nn e l

n u m b e r fo r t h e fou r c r ys t a ls . Th e a d d b a c k s p e c t r u m o b t a in e d i n t h is m a n n e r

might ne e d re binning in orde r to s ta tis tic a lly re dis tribute the e ne rgy s um. This

is e s pe c ia lly us e fu l for fe a tu re s r e la te d to low yie ld p e a ks in th e s p e c tru m . Th e

re binning c a lc ula tion is de s igne d a s follows : Firs t, norma l (Ga us s ia n) ra ndom

n u m b e r s a r e g en e r a t e d wit h ze r o a s t h e ir a v er a g e va l u e a n d u n i t y a s t h e i r s t a n -

1 6


31/82


d a r d d e via t i on (fo r e x a m p l e 0 . 1 2 , 1 . 2 3 , - 0 . 3 3 , 0 . 3 4 , - 1 . 7 6 , . .. ). Th e s e r a n d o m

n u m be rs a re th e n d igitize d (0, 1, 0, 0 , -1. . .) to produ c e the c ha n n e l s h ift. Th e

r e b i n n i n g h a s b e e n a p p l i e d t o t h e c o i n c i d e n c e s p e c t r u m i n s e a r c h f o r t h e 4 7 0k e V r e s o n a n c e o f t h e O( , - ) Ne r e a c t io n w h e r e t h e 1 2 7 4 k e V p e a k fe a t u r e i s

extremely weak (Fig. 4.6).

2 . 3 . 2 Clo v e r d e t e c t i o n e ffi c i e n c y i n d if fe re n t m o d e s

As a s ta rting point, the de te c tion e ffic ie nc y of the Clove r de te c tor in the s ta n-

da rd ge ome try with out c a s c a de e ve nt s u ppr e s s ion (only c os m ic s u ppr e s s ion) is

firs t inves tiga te d. For this pu rpos e a (p, - ) r e a c t io n w it h a s t r o n g r e s o n a n c e t h a t

yields several - -ra ys ove r a la rge e ne rgy ra nge [Sc o75] is c ons ide re d. The 992

k e V r e s o n a n c e in t h e Al(p, - ) Si re a c tion ha s be e n us e d for the e ffic ie nc y c a l-

ibra tion of the Clove r de te c tor, both in a ddba c k a nd dire c t mode s . To a c hie ve

t h i s , a l i s t fi l e a c q u i r e d o n t o p o f t h e 9 9 2 k e V r e s o n a n c e f o r a t o t a l c h a r g e o f

2.5 mC on a 22 ke V thic k Al ta rge t ha s be e n a na lyz e d. This ta rge t thic kne s s is

m o r e t h a n s u ffi cie n t t o s t o p a l l r e c oils , a n d y et e n s u r e s t h a t n e a r b y r e s on a n c e s

a r e n o t e xc it e d , t h e n e a r e s t r e s o n a n c e b e i n g a t 9 3 7 k e V [E n d 9 0 ]. Th e r e la t i ve

i n t e n s i t i e s o f t h e - - lin e s a r e t a k e n fr o m [An t 7 7 ]. Th e d e t e ct io n e ffi cie n c y i s

c a lc u l a t e d u s i n g t h e r e l a t io n :

-

-

(2.1)

w h e r e

is t h e s t o p p in g p o w er o f t h e t a r g et ,

-

t h e r e la t i ve in t e n s i t y o f t h e

respective-

- l i n e a n d

is t h e m a x im u m o b s e r ve d y ie ld (p e r in c i d en t p r o t o n )

a t t h e p la t e a u of t h e t h i ck t a r ge t e x cit a t ion fu n c t io n . B y a d op t in g t h e r e s o -

n a n c e s t r e n gt h o f - eV [Pai79], th e a bs olu te d etection efficien cy is

c a lc ula te d us ing the re la tion

-

(2.2)

Th e re s u lting e xpe rime n ta l Clove r e ffic ie nc y da t a toge th e r with th a t obta ine d

f r o m t h e G e a n t 4 s i m u l a t i o n f o r f u l l e n e r g y , s i n g l e e s c a p e a n d d o u b l e e s c a p ep e a k s i n b o t h d i r ec t a n d a d d b a c k m o d es a r e s h o wn in F ig. 2 . 9 . Th e r e m a r k -

a b l e a g r e e m e n t b e t w e e n e x p e r i m e n t a n d s i m u l a t i o n i n d i r e c t m o d e p r o v i d e s

c on fi d e n c e i n t h e e f fic ie n c y c a lib r a t i on in t h is m o d e. All t h e o b s e r ve d e x p e r -

i m e n t a l e f fi c i e n c y v a l u e s f o r a d d b a c k m o d e l i e , o n t h e o t h e r h a n d , b e l o w t h e

m onoe ne rge tic e ffic ie nc y c ur ve obta in e d from th e s imu la tion.

It is e xp e c t ed t h a t t h e c a lc u l a t e d a d d b a c k e ffi cie n c y in t h e c lo s e g eo m e t r y

o f t h e p r e s e n t s e t u p a n d i n t h e p r e s e n c e o f c o m p l i c a t e d c a s c a d e s w i l l s h o w a

s t r o n g d e p e n d e n c e on s u m m in g e ffe c t s . Th e fl u c t u a t io n i n t h e va l u e s o f a d d

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