Recoil iodine-128 in anhydrous copper iodate

J.RADIOANAL.NUCL.CH~M.,LETTERS 94 /i/ 33-44 /1985/

RECOIL IODINE-128 IN ANHYDROUS COPPER IODATE

R.B. Sharma x A. Patnaik, S.P. Mishra

Nuclear and Radiation Chemistry Laboratory, Department of Chemistry, Banaras Hindu University,

Varanasi - 211 005, India

Received 31 July 1984 Accepted 29 August 1984

The thermal annealing of the damage caused by /n,y/ reaction has been investigated for anhydrous copper iodate in crystalline phase. Solvent extraction and fractional precipitation methods were employed to distinguish the recoil species containing recoil iodine-128 atoms. The role of reaction intermediates have been dis- cussed.

INTRODUCTION

Chemical effects of nuclear recoil have been studied

in the gaseous, liquid and solid state for a variety of

organic and inorganic compounds I. The increase in re-

tention on heating or exposure to electromagnetic radiation

of a solid which has undergone radiative neutron capture

is a widely observed phenomenon and detailed investi-

gations have been reported on permanganate 2 3 , chlorate ,

bromate 4 and iodate 5-7 targets. The results of these in-

Xpresent address: Department of Chemistry, The Johns Hop- kins University, Baltimore, Maryland-21218, USA.

3 33

SHARMA et al. : RECOIL 128I IN COPPER IODATE

vestigations differ slightly in number, identities and

proportions of the different fragments formed depending

on the conditions of irradiation and on the methods of

analysisemployed.

Cleary et a l . 8, Libby 9 and D 'Agos t i no 10 s u c c e s s f u l l y

made use of~different iodates to enrich iodine activity.

Since then a number of investigations have been reported

on the chemical state of recoil iodine atoms arising

from radiative neutron capture in iodatesil'17. However,

most of these investigations have dealt with the be-

haviour of recoil iodine at room temperature irradiation,

not much attention has so far been paid at lower tem-

perature of irradiation. Also, from this laboratory it

has been earlier reported that thermal annealing takes

place at room temperature and even at -80 ~ under /n,y/

process 18'19 Keeping the above idea in mind present stud-

y was aimed at finding initial retention,in crystalline

Cu/I03/2 free from inherent annealing which usually occurs at room temperature and for this the neutron acti-

vation of the target was carried out at liquid nitrogen

temperature /-196 ~ To learn further about the radio-

activity of the radioactive intermediate, subsequent

thermal annealing studies were performed, which might be

able to shed some light on the nature of fundamental

physical and chemical processes which are responsible

forthethermal annealingreaction in the crystals during

and after irradiation~

EXPERIMENTAL

300 mg of crystalline Cu/IO3/2 was taken for the

thermal neutron activation and kept in a sealed ampoule

under the neutron irradiation facilities. The thermal

34


neutrons needed for the irradiation of the target were

obtained from a 3OO mCi /Ra-Be/ neutron souce of the

integrated flux 3.2xi06 n cm-2s, I. The neutron source

was surrounded by a Cylindrical block of paraffin for

thermalizing the fast neutrons.

A brief description of experimentaljprocedure for

thermal annealing has been described previously 6 . Frac-

tional precipitation and solvent extraction methods 18

were used for determining the distribution of recoil

iodine activity between iodide~ iodate and periodate.

All the fractions were collected as their silver salt

by using silver nitrate solution. Activity was counted

with the aid of an end window G.M. counter possessing

constant geometry.

RESULTS AND DISCUSSION

A retention value of 59.0% is obtained when the target

was irradiated with thermal neutrons at room temperature.

The retention observed is as low as 39.O% when the ir-

radiation was performed at -196 ~ Comparison of re-

tention at room temperature and at low temperature reveals

that almost half of the retention observed at room tem-

perature is due to annealing processes taking place in

the target during irradiation. Data on thermal annealing

is illustrated in Fig. i. Typical isothermal annealing ~

curves show the retention rising rapidly initiaily and

then more slowly to a pseudo-plateau whose level depends

on the temperature of annealing. Annealing for identical

time but at lower temperature leads to lower plateau level.

This also reveals that solid state reactions leading to

parent reformat• are possibly due to two different

rates one being faster than the other. From the slope

3* 35

SHARMA et al. : RECOIL,128I IN COPPER IODATE

80 - ~ ~ , n , .,, ~ o 150oC

c 9 , , ' ~ . , , ~ , ' ~ " " ~ " - ' ~ o ~" .o 125

u o r~ 10

rr 60

5O

40V , . , i O 10 20 30

Time of h e a t i n g ~ min

Fig. i. Kinetics of isothermal annealing of iodate in thermal neutron irradiated Cu/IO 3/2

plots ig /R -Rt/ VS. time of heating, the reaction rate

constants were computed and the values are given in Table i.

In contrast with KCIO 3 and KBrO 3 a small but significant

yield of radioperiodate ion was produced in Cu/IO3/2 both

at liquid nitrogen temperature irradiation, Table 2,

which disappeared at higher temperature of annealing and

the activity was only observed in iodide and iodate forms.

The variation in retention was attributed to the dif-

ferences in

-the polarizing power of the cation,

- the relative ease of transferring electronic exci-

tation energy to the vibrational mode of the lattice

and

- in the closeness of packing of the crystal lattice.

Other factor such as trapping of electrons, the ef-

fect of radiation damage in the lattice, stability of the

primary products and immediate electronic and spatial en-

vironment of the iodate ion should be taken into consider-

ation for the interpretation of the observed results. Ma- 19

chado et al. proposed that on preannealing the cobalt

and chromium compounds trap sites were thermally generated

36


~o

0

O~

II

0

t'N

r ~

0 H

U

4~

>

.p

0

<

rd

0

,-I

@

0

D 4~ -a

~J

O E- ,~

0

0

I

8

I

~ o ~ (D.,~

0

o d 0

I I ~ 0

S S S

I I

N .N

>

O m 0 m O O ~ ~ ~ O

~ ~ ~ ~ O

> O m O ~ O

g ~ d d S ~ 0

O ~ O ~ O

O O O m O ~ O ~

I

Cq

O~

O H

U

O

0

4J

O

37


TABLE 2

Distribution of activity of recoil 128I in thermal activ- ated anhydrous Cu/I03/2 without any heat treatment

Chemical Temperature of Fraction of total fractions irradiation, activity

~ C

I- + 12 25 0.38

IO3 0.59

I04 0.03

I + I -196 0.57 2

I03 0.39

IO~ 0.04

in the crystal and these traps would then compete with the

recoil fragments for the electron or positive holes liber-

ated on heating the crystal following the neutron irra-

diation. The same mechanism for the thermal annealing ap-

pears to be plausible in the present case. At the begin-

ning of the annealing recoil iodine is present and the in-

herent trap sites in the crystal wiil be partially occupied

by electrons and positive holes generated in the radio-

active decay. On heating electrons and positive holes are

promoted from their trap sites and the annealing of re-

coil 128I fragments occurs leading to an increase in re-

tention. At the same time the trap sites may be thermally

removed from the crystal. Thus, at the time of heating in-

creases the number of trapped electrons and holes remain-

ing to participate in the annealing reactions decreases.

On the basis of the above idea following reactions have

been proposed on the line of Lin and Wiles 20.

§ § e - / 1 /

38

SHARMA et al.: RECOIL 128I IN COPPER IODATE

IO 3 + @ > I0~ + e- 121

xI + IO 3 > xI+ + IO 3 /3/

xI+ + IO 3 > XIO- + IO 2 /41

The overall reaction could proceed either by a series of

reactions 3 followed by three reactions 4 or more likely

in an alternative series such as 3~3,4 3,3,4 etc. In Fig.

i. we see the temperature dependence of the maxima /i.e.

plateau/ in the isothermal annealing curves which occurs

after lOmin of heating. One possible cause for this is

that on heating all the traps in the crystal may not be

removed and enough may survive to subsequently trap

electrons and holes generated during radioactive decay.

The formation of iodide, iodate and periodate can also

be explicable on the basis of radiolytic products formed

due to internal y-rays associated with neutron source. The

primary steps in the radiolytic process are assumed to be

the ionization and excitation of the iodate ion as:

IO 3 > XIo 3 ~ IO 2 + 0 /5/

- - > I O + 02 161 I +02+0

IO 3 or xI03

or

I - + 30 /7/

> IO 2 + O- /8/

> I o + 02 191

> IO 2 + 0 /I0/

IO + 02 /ii/

~ I + 0 2 + 0

or

I + 30 1121

39


TABLE 3

x Electron affinities of few iodine and oxygen species

Species Electron affinities, Type of measurement eV

I + 10.5 I.P.

I 3.29 E.A.

0 1.465 E.A.

O + 13.6 I .P.

02 O.87 + 0.13 Cal.

IO 2.6 - 2.9 E.A.

IO 2 2.8 E.A.

IO 3 3.96 E.A.

XData from V.I. Vedeneev et al. Bond Dissociation Ener- gies, Ionization Potentials and Electron Affinities, Translated fcom Russian by Scripta Technica Ltd., Ed- ward Arnold Publs. Ltd., N.Y., 1966.

Some of the above reactions probably do not occur and

several products are unstable and would not exist in the

iodate crystal lattice at room temperature. The electron

affinities of the species I02 and IO are substantially

larger than the affinities for O and 02 , respectively,

so that reactions /8/ and /9/ should not be favoured,

Table 3. From the table it can be also seen that the

acquisition of electrons by I + and O + is so favoured

energetically that neutralization may be expected during

the slowing down. Because of relatively large electron

affinity, iodine atom is much more likely to capture an

electron to form I than will an oxygen atom to form an

O- ion. Thus, the tendency of the recoil 128I to appear

in different oxidation states of iodine can be understood.

40

SHARMA et al. : PZCOIL 128I IN COPPER IODATE

Various other possible reactions of the fragments

can be envisaged which account for the formation of

iodate and other recoil fragments of iodine. The trap-

ping of electrons produced by ionization in the solids

is important. Thus, electron capture by free radicals

may lead to highly excited ions:

+ eaq ~ XIo; /13/ IO 3

In addition, electron trapping at anion vacancies in the

lattice of iodate can be also expected,

- ~- IO; + eaq --2 IO /14/

and the specific rate constant for the iodate ion for

the above reaction is 109-1010 M-is -I as has been re-

ported by Anbar and Neta 21. The reactive intermediate

IO formed may be responsible for the oxidation of re-

coil iodine and its protonated form may undergo dehyd-

ration to yield IO 2 /Ref. 22/.

Although it is not possible to determine the con-

centration of I02 and IO- in solution, their role during

annealing is important in the solid-state reaction. The

first order disappearance of the hypoiodite ion as has

been formed in reaction /6/ is possibly the reaction of

species with oxygen atom forming I as

u

I O + O = > I + 0 2 / 1 5 /

Also, oxygen atoms produced in reactions /5/, /7/, /i0/

and /12/ will react in the crystal lattice. Periodate

ions may be formed by addition

io] + o I04 /16/

or by oxygen atom transfer reaction

+ To; >. To; + /17 /

41


The first order reaction of iodite may be the recombi-

nation reaction involving adjacent oxygen atom

zo~ + o ~ zo~ 1181

In chemical radiation damage of a molecular ion, the

fragment with the charge in a halate crystal is situated

in the lattice position of the original ion and the

neutral fragments such as oxygen, some distance away.

In the annealing process the neutron fragment undergoes

diffusive movement interstitially and reacts with the

charged fragments reforming parent ion. Chemical anneal-

ing is thus also diffusive controlled whatever be the

nature of the reacting entities or the chemical reaction

involved.

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42


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Documents

Recoil iodine-128 in anhydrous copper iodate