Appl. Radiat. Isot. Vol. 37, No. 9, pp. 955-959, 1986 Int. J. Radiat. Appl. lnstrum. Part A Printed in Great Britain

0883-2889/86 $3.00 + 0.00 Pergamon Journals Ltd

Recoil Chemistry of 1281 Atoms in Cesium Iodate under (n, 7) Process

S H U D D H O D A N P. M I S H R A * and A R C H I T A P A T N A I K

Nuclear and Radio Chemistry Laboratory, Department of Chemistry, Banaras Hindu University, Varanasi-221005, India

(Received 18 November 1985; in revised form 29 January 1986)

The increase in initial retention of 128I in solid CslO~ upon pre-heat treatment prior to neutron irradiation is ascribed to the participation of inherent crystal defects, and an exciton mechanism is proposed to explain the thermal annealing data. Results of the sulphate additive studies are explicable on the basis of gO 3, gO~- ion-radical involvement, and possible reactions are suggested.

Introduction

Despite 51 years of hot-atom research, an under- standing of the recoil reactions has proved quite tricky due to various concomitant extraneous pro- cesses that could participate in the recoil stabilisation process. The general tendency of (n,),) reaction in solution phase is the interaction of the intermediate transient chemical species with the medium, oxidising or reducing. A general theory would be possible if we could study and identify the nature of all or some of these transient intermediates formed, using indirect means like activations in the presence of various inert, oxidising or reducing additives. Such studies also contribute to the understanding of solid state recoil reactions, since the lattice stable precursors obtained in solid phase irradiated target cannot exist in solution phase irradiation because of their reaction with water and/or the additives.

A review of the literature on recoil iodine in iodates reveals the lack of systematic work on the sta- bilisation of 128I in CsIO3. Although solid phase tl 3) studies give us some information, absolutely no work has been undertaken to study the behaviour of recoil 128I in CsIO3 in aqueous and frozen aqueous phases. The present work, involving pre-heat treatment in solid phase prior to neutron irradiation in aqueous and frozen states of neutron irradiated CsIO 3 with additive Cs2SO4, is an effort to meet some of the above requirements, providing data on the response of the recoil atom to the varied environment and enabling us to understand the nature of the metasta- ble species formed and the recombination reactions.

* Author to whom correspondance should be addressed.

ARI 3 7 / 9 ~

Experimental Procedure

The technique for the irradiation of solid C s I O 3 at ambient temperature and 77 K by thermal neutrons and the annealing procedures are given elsewhere: 4) Pre-heat treatment of the sample was performed in bulk with the help of an electronically controlled oven at 423 K for 1 h. The heated iodate was then cooled in a desiccator and aliquots were used for individual experiments.

For solution phase study, a 0.048 M solution of CsIO3 was prepared by addition of 20 mL of doubly distilled water to 300 mg of the sample. The mol fraction of Cs2SO 4 additive was varied from 0.00 to 12.4 x 10 -4. The solutions with varying concen- trations of Cs2SO4 were irradiated at ambient (304 K) temperature as well as at 77 K. For the latter case, the solution was added dropwise to the test tube dipped inside the Dewar flask containing liquid nitrogen.

Fractional precipitation ~5) method was followed for chemical analysis of the irradiated samples, with the pH of the dissolution media kept at ll.0. Radio- activities in the three stable forms 128IO3, 128I- and 128IO4- as their silver salts were counted through an end window GM counter having a constant geome- try. The retention values were computed after the usual necessary corrections.

The experimental results were refined by applying the least squares fit method with the help of an ICl 1904 Computer facility available at the B.H.U. Com- puter Centre.

Results and Discussion

The retention and yield values in various phases reported (cf. Table 1) are an average of at least three

955

956 ~HUI)DHOI)AN P. MISHRA and ARCHITA PATNAIK

Table I. Initial 2fields in (n, ?') irradiated CslO~

Percentage ),ieid State of the sample Temperature of Concentration during irradiation irradiatinn (K) (M) ~*IO, '~*I ~"~IOa

Crystalline solid 304 62.X t4.0 t 2 Crystalline solid 77 52.7 47.:~ Neutral aqueous solution 3(14 0.048 12.6 87 4 Frozen aqueous solution 77 0.048 40.6 >9.5

independent experiments with an accuracy of + 2°/,. A retention value of 62.8% in CslO~ at 304 K irra- diation in the solid phase is comparable with the previous values, 66 _+ 1%~2~ and 68.2% ~3~ obtained for room temperature activations. The 52.7% retention at 77 K is markedly less than the room temperature activation value, showing that annealing occurs even at room temperature, in spite of the fact that an apparent threshold of I l O C '< exists for thermal annealing in bromates.

Binary collision approximation of collision cas- cades in computer simulation of (n, 7) irradiated KIO 3 has given a value of 9% retention due to direct replacement reactions/> Hence, in the present value of initial retention, c a 9% contribution would come from direct replacement. Kortl ing e t a l . ~ observed that at least 21% of the (n, ;') events in iodine nuclei produce positively charged iodine species, while Wexler tg) and Rack ~m~ found it to be 45 and 36%, respectively. The 133 keV level of ~:Sl is associated with a high internal conversion coefficient. ~ - 0.5j s~ and occurs in 42% of neutron capture events out of which 21% lead to internal conversion/~l! For an atom of iodine (Z = 53), the probability of Auger electron emission against x-ray fluorescence is 14.5%3 TM Therefore, about 3% (21 x 14.5/100= 3.045%) of neutron capture events lead to pure Auger charging. It has been agreed that recoil ~-~I exists in the irradiated solid in either ~:Sl or ~2si~ state3 TM The charged state of iodine is to be preferred because in insulating ionic solids like CsIO~, the Auger charge neutralisation would be delayed and the recoil ~2Sl atom could stay in the ionic form. Hence, the most probable process loading to the formation of parent 12SlO~ may be the addition of one of the unshared electron pairs of molecular oxygen to 12~|~ to form ~:slO:) which further under- goes dissolution induced reaction to form ~2SlO~.

12Sl + -P O ~ - + I2~IO~ ( I )

I:qO+ + H20 --+ 128|O 3 + 2H + (2)

Such reactions have also been proposed in the EC decay of 125I+ in X e - H 2 0 system/TM

The effect of pre-heat treatment prior to neutron activation results in an increase of the initial retention from 62.8 to 66.7%, which finds support from the work of Arnikar e t a l . ) TM who found an increased retention of pre-neutron-irradiation heating of HIO~. A similar trend has also been reported by Campbell and Jones, ~61 who observed a 20.9% retention after

pre-heating NaBrO) at 245 ( ' for 2h as againsl a value of 17% for the untreated sample, and a similar trend has been confirmed through an independent studyJ ~7~ The present findings could be explained by considering the role played by inherent crystal de- fectsJ ls '~ In ionic crystals, electrons and holes are generally considered as inherent defects incorporated into the lattice during crystal preparation. On pre- heat treatment, the concentration ~;l thc reducing defects is decreased, thereby leading to an increased number of oxidized recoil atoms: this may be the reason why an increased retention is obtained for the pre-heated CslO~.

An inspection of the thermal annealing isotherms for both the samples irradiated at 3114 K (of. Fig. 1) reveals the usual trend, i.e. a last initial rise followed by a temperature dependent pseudo-plateau. Similar results have been reported by earlier workers on various inorganic halate targets/6~ At any annealing temperature, the plateau values (R , ) for the pre- heated CslO~ shifted to higher values ('Fable 2) than those for the untreated CslO~. A similar trend has also been reported by the present authors in the case of Li lO 3 and Cu( lO3)2 . (2m The annealing rate con- stants for both samples were computed from the slope of the plots of log (R> - R,) against time of h e a t i n g , R t being the retention at a particular time of the annealing isotherm. The plot revealed the pres- ence of a combinat ion of two first order processes,

9o~_ (a) - o - - - 3 9 3 K

L~

' / / - - - ~ c $ 25

o=60

g 5O

cr

9O

(b)

80 353 K

70

6 0 I [ ~ . _ I ~ i ___ i . . . . . . .

0 5 10 15 2 0 25 50

T i m e of h e o t i n g ( r a i n )

Fig. 1. Annea l ing i so therms for samples i r rad ia ted at room temperature (304K) by thermal neutrons, fa) Untreated

CslO~; (b) Pre-heated CsIO~

Recoil chemistry of 12sI atoms 957

Table 2. Isothermal annealing data* for CslO3 irradiated by thermal neutrons at 304 K [R 0 = 62.8% (66.7%)]

Slow component

Temperature R~ R~ - R 0 ll/2 k/ lO -2 (K) (%) (%) (min) (min ~)

393 87.5 24.7 3.84 18.03 4- 0.45 (89.5) (22.7) (3.77) (18.35 + 0.63)

373 83.0 20.2 4.12 16.79 4- 0.32 (87.0) (20.3) (3.48) (19.91 4- 0.43)

353 78.5 15.7 4.37 15.854-0.19 (82.0) (15.2) (3.97) (17.42 4- 0.21)

323 70.0 7.2 4.68 15.13 _ 0.16 (72.5) (5.7) (4.05) (17.09 4- 0.92)

Activation energy Arrhenius kinetics Fletcher-Brown model

kJ mol -I eV x 103 kJ mol -I eV

2.57+0.15 26.70_+1.65 41.67+1.63 0.43+0.02 (I.62 + 0.42) (16.86 4- 4.38) (41.38 + 2.55) (0.42 4- 0.03)

* Values inside the parenthesis are for the pre-heated sample.

one being much faster than the other. The rate constant at a particular temperature is found to be greater for the pre-heated sample. Classical Arrhenius plots provide the values of activation energies (Table 2) for both the samples studied. Least squares fit method was applied to refine the kinetic parameters and activation energies and to identify the associated probable errors. To confirm the trends of activation energies in the two samples, the annealing data were further fitted to the well-known Fletcher-Brown model3 nt The composite annealing curves are shown in Figs 2 and 3. The contributions of fast and slow first order processes to annealing were known from the theoretical equations (cf. Figs 2 and 3) obtained through curve fitting. Thus, in the untreated target, 85.5 and 12.7% are the con- tributions to annealing from slow first order and fast first order processes, respectively, whereas in the pre-heated CslO 3, these contributions are 82.3 and 19.1% respectively. The Fletcher-Brown activation energies were calculated from the slope of the linear plots of log z (ref)/z(T) (where ~ is the average jump time of the vacancy in the lattice) against I/T (cf. Table 2).

A 3O %

x

-0- 2O

== 8 ~1o

ta-

t~ • 0.85.5 ( 1-e-r~'541'71)÷ 0 . 1 2 7 ( 1 - e - t ' / 7 1 4 )

Stortdord dev ia t ion of the f i t • 0 . 9 4 9 x 10 -2 - - /

i ,¢

! x . , I 1~ o 3 2 3 K

-- _ ~ j ~ " x 353 K ~ o o t, 3 7 3 K

• 3 9 3 K E xperlmentol curve

- - ' - - T h e o r e t i c a l curve 0 I I I I I I l = l I = = = = = = i l I

10 100 200

E q u i v a l e n t a n n e a l i n g t i m e a t 3 5 3 K (m in ) t '

Fig. 2. The Fletcher-Brown composite annealing curve for (n, ; ) irradiated CslO~.

~'~ 3 0 - - o x

-0-

2 0 -

t~

__. 10

u_

I 1.5

dp • 0 . 8 2 3 ( 1 - e -z7148214) + 0.191 ( I - e - tF579)

Standard deviat ion o f the f i t • 0 . 6 4 7 X 10 - 2

_ / ~ - , ' / x 3 5 3 K / ~ ~ 3 7 3 K

o ~ : ¢ ~ % o o • 3 9 3 K

IFTI' l i t i lltll l I i i , [JJl

1.0 10 100

E q u i v a l e n t a n n e a l i n g t i m e a t 3 5 3 K ( m i n ) t '

Fig. 3. The Fletcher-Brown composite annealing curve for (n, 7) irradiated pre-heated CsIO 3.

Our data on thermal annealing of pre-heated CslO 3 appear to be explicable on the basis of the exciton mechanisms. Since the defects produced by internal conversion and radiolysis/self radiolysis are common to both the samples, we can assume that by virtue of annealing out inherent defects in pre-heating, the lattice of the pre-heated target is more ordered than that of the untreated one and hence there is a greater probability that the exciton will reach the recoil site at which it deposits its energy and enables the an- nealing reactions to occur. In the case of untreated CsIO3, because of the relatively large number of defects, excitons are stopped or captured and thus are not able to reach the recoil sites. As expected from the exciton mechanism, we have obtained higher plateau and rate constant values with lower activation ener- gies for the pre-heated CslO 3.

Experiments performed in the solution phase of CslO 3 irradiated at 304 K resulted in a retention value of c a 12.6%, whereas ay 77K the value amounted to c a 40.6% (cf. Table 1); these values are very much different from those observed in pure crystalline target. A retention value of 9.4% is re- ported by the present authors (4) in the case of neutron irradiated LilO 3 solution. Ambe and Saito a) found 22 _ 1 and 23 ___ 1%, respectively, for 0.05 and 0.1 M NalO 3 solutions irradiated at 0°C, while 36 + 1 and 38 _ 1% are the values obtained for the respective frozen solutions irradiated at -78°C. In the present investigation no significant change was observed on changing the concentration of target solution, as is further supported by the observation of Sankpal and Rao. (22) Cleary e t al . (231 reported that retention in aqueous iodate solution is independent of concen- tration and pH and obtained c a 20 _+ 1% for NaIO 3 solution, which is in partial support of the present results. Lower retention values have also been re- ported in neutron irradiated solutions of LiIO3, a4) HIO3 and KIO3 .t25) Radiation damage studies on aqueous solutions of iodates and inorganic materials irradiated at 77 K have shown a glassy character, and ESR studies (26) on such systems show that electrons in the neutral and alkaline glasses have a shorter life- time and range. Thus, these electrons are effectively trapped in frozen aqueous iodate target, resulting in

958 SHUDDHODAN P. MISHRA a n d ARCHITA PATNAIK

94 N I (a)

9©-

r ~ ~-

8

~, Io L IO~- "~ J . . . . L . . . . J__ _ _ ~ 2 L 1 I

9)0

"~ (b)

IOn-

i 2 0

• ~ L I I I I I I c,o 0 2 0 4 0 6 0 8 1 0 12 1 4 1 6

[Additive] / [Target]

Fig. 4. Effect of Cs~SO 4 additive on (n, 7) activation of CslO~ solution. (a) Irradiation temperature: 304 K;

(b) Irradiation temperature: 77 K.

the oxidation of recoil iodine fragments and thereby enhancing the retention. This effect can also be visualised by considering the efficient caging ability of the ice lattice, where ~2sl remains quite close to its ligands. Thus, enhanced retentions obtained for 77 K activation in frozen aqueous iodate targets are sub- stantiated.

Anbar and Neta ~27) observed the rates of reaction of iodate ions with hydrated electrons and found the specific rate constant for iodate ions of the order of 109 l O l O M I s I.

IO3 + e ~ I O ~ (3)

In pure iodate solution, retention can arise as the oxidation of 128I (major recoil species in the (n, 7) reaction of aqueous iodates) by OH radicals formed through the radiolysis of water:

OH + i28I --* 1281 q- OH (4)

5IO~ +2.5128I,--}51:8IO~ + 5 I (5)

The effect of additive (Cs2SO4) concentration is depicted in Fig. 4 (cf. Table 3). Solutions irradiated

Table 3. Activity distribution of 1"8I in (n ,? ) irradiated CsIO~ solutions containing Cs:SO 4 additive

Percentage yield

Cone. of Cs,SO 4 in mf 77 K 304 K Conc. of

CslO~ Conc. of CsIO 3 in mf IO 3 l IO 3 I

0.048 M

0.00 40.6 59.3 12.6 87.4 0.28 22.6 77.3 12.9 87.1 0.56 26.3 73.6 12.4 87.6 0.85 36.7 63.3 I I. 1 88.8 1.13 46.6 53.4 11.5 88.5 1.41 52.3 47.6 10.3 89.6

at 304 K (cf. Fig. 4a) did not show any measurable change in the activity distribution of ~:SlO~ solution, since the sulphate ionic lattice is known to have high radiolytic stability. (-~s~ In contrast, Bera, ~-'~ on irra- diating the solution of (KIO 3 + Na:SO4) at roonl temperature, reported a change of retention from 20.1 to 27.2%. Patil et al. (s"> have reported thal dissolution of ?-irradiated potassium sulphate in aqueous solution of NaNO.~ results in the rednction of nitrate to nitrite, and the nitrite yield was found to be linearly related to the increasing concentration of irradiated K2SO 4. They have explained their rcsuhs on the basis of colour centres produced in the salts during irradiation. A recent work c~) on (n. 7) recoil reactions of iodine in IO3/S_~O~ binary solution has shown that gO4 ion-radical is responsible fl)r the change in retention of iodate.

Gromov and Karaseva <3z~ suggested that at 77 K. gO4 and ~O~ ion radicals are formed upon radialion decomposit ion of sulphates according to the follow- ing steps:

SO~ ~ O ~ + e (trapped) 16)

gO4 + e -**SO~ -,SO~ + O + e (7) (excited)

The sharp decrease of % R l¥om 40.6 to 22.6% at an [additive]/[target] ratio of 0.28 (cf. Fig. 4b) noted at 77 K irradiation is attributed to ihe reducing action of ~O 3 ion-radical acting as an electron donor. Species such as gO4, gO, and O~ are also formed with lesser concentration/~') With an m- creasing concentration of Cs2SO4, more gO~ ion- radicals are formed and are annihilated, ~4) reacting with themselves according to the reaction

Hence, no further decrease in % R is observed. The relatively larger number of gO~ ion-radicals at higher concentrations of Cs:SO~ undergo dimerisation u) form $20~ ~31)in the glassy matrix, which participates in reactions (9) and (10) with ~-'~I to result in an increase in retention as observed in the present study.

SzO~ + IZSl -+ gO4 + IZ~l + SO~ (9)

~ : s I + 5 g O 4 + 3 H , O - * H ~ ' s I O ~ + 5 S O 4 + 5 H ' (10)

Acknowledgement One of us (Miss Archita I'atnaik) is thankful to the C.S.I.R., New Delhi, for the award of a Senior Research Fellowship.

References 1. Saito N., Ambe F. and Sano H. Radiochim. ,tcta 7, 131

( 1967). 2. Ambe F. and Saito N. Ibid. 13, I05 (1970L 3. Dupetit G. A. and Aten A. H. W. Jr. Ibid. 7, 165 (1967) 4. Mishra S. P., Patnaik A., Sharma R. B. and Wagley D.

P. Ibid. 34, 189 11983). 5. Boyd G. E. and Larson Q. V. J. Am. ("hem. Soc. 91,

4639 (1969).

Recoil chemistry of 12sI atoms 959

6. Owens C. W. Chemical Effects of Nuclear Trans- formations in Inorganic Systems (Eds Harbottle G. and Maddock A. G.) Chap. 7, p. 145 (North-Holland, Amsterdam, 1979).

7. Bera R. K. and Shukla B. M. Radiochim. Acta 30, 29 (1982).

8. Kortling R. C., Auria J. D., Jones C. H. W. and Isenhour T. Nucl. Phys. A138, 392 (1969).

9. Wexler S. and Davies T. H. J. Chem. Phys. 20, 1688 (1952).

10. Rack E. P. and Gordus A. A. Ibid. 34, 1855 (1961). 11. Jones C. H. W. J. Phys. Chem. 74, 3347 (1970). 12. Braundle C. R. and Baker A. D. Electron Spectroscopy

(Academic Press, New York, 1977). 13. Adloff J. P. and Friedt J. M. Panel on Applications of

Mossbauer Spectroscopy, p. 301 (IAEA, Vienna, 1972). 14. Halpern A. Chemical Effects of Nuclear Transformations

in Inorganic Systems, Chap. !6, p. 303 (1979). 15. Arnikar H. J., Dedgaonkar V. G. and Shrestha K. K.

Proc. Chem. Syrup. Chandigarh, India, Vol. 2, p. 222 (1969).

16. Campbell 1. G. and Jones C. H. W. Radiochim. Acta 9, 7 (1968).

17. Arnikar H. J., Dedgaonkar V. G. and Shrestha K. K. J. Univ. Poona Sci. Tech. 38, 177 (1970).

18. Jones C. H. W. and Warren J. L. J. Inorg. Nucl. Chem. 30, 2289 (1968).

19. Jones C. H. W. and Warren J. L. Ibid. 32, 2119 (1970). 20. Mishra S. P., Patnaik A. and Wagley D. P. J. Chem.

Soc. Faraday Trans. 1 80, 47 (1984).

21. Fletcher R. C. and Brown W. L. Phys. Rev. 92, 585 (1953).

22. Shankpal S. K. and Rao B. S. M. Radiochem. Radio- anal. Lett. 45, 289 (1980).

23. Cleary R. E., Hamil W. H. and Williams R. R. J. Am. Chem. Soc. 74, 4675 (1952).

24. Arnikar H. J., Patnaik S. K, Rao B. S. M. and Bedekar M. J. Radiochim Acta 23, 121 (1976).

25. Singh R. N. and Shukla B. M. Ibid. 17, 135 (1980). 26. Ginns I. S. and Symons M. C. R. J. Chem. Soc. Dalton

Trans. I 514 (1975). 27. Anbar M. and Neta P. J. Inorg. Nucl. Chem. 28, 1645

(1966). 28. Huang S. and Johnson E. R. ASTM Symposium on

Effects of High Energy Radiation on Inorganic Sub- stances, Seattle, Wash. Oct. 31-Nov. 5, 1965. ASTM Special Tech. Publ. No. 400.

29. Bera R. K. Ph.D. Thesis (Banaras Hindu University, Varanasi, 1975).

30. Patil S. F., Ravishanker D., Bhatia P. and Chowdhary I. B. Int. J. Appl. Radiat. Isot. 35, 459 (1984).

31. Sankpal S. K. and Rao B. S. M. Radiochem. Radioanal. Lett. 46, 391 (1980).

32. Gromov V. V. and Karaseva L. G. Khim. Fysokikh Energii 1, 51 (1967).

33. Kamali J. and Walton G. N. Radiat. Eft. 84, 171 (1985).

34. Kamali J. and Walton G. N. Radiochim. Acta 36, 109 (1984).