Recoil chemistry of 128I atoms in cesium iodate under (n, γ) process

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Text of Recoil chemistry of 128I atoms in cesium iodate under (n, γ) process

  • 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

    SHUDDHODAN P. M ISHRA* and ARCHITA PATNAIK

    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 37/9~

    Experimental Procedure

    The technique for the irradiation of solid Cs IO 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 lOC '< 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, ca 9% contribution would come from direct replacement. Kortling et al . ~ 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 Xe-H20 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 et 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 heat ing , R t being the retention at a particular time of the annealing isotherm. The plot revealed the pres- ence of a combination of two first order processes,

    9o~_ (a) -o - - - 393 K

    L~

    ' / / - - - ~ c $ 25

    o=60

    g 5O

    cr

    9O

    (b)

    80 353 K

    70

    60 I [~ ._ I ~ i ___ i . . . . . . . 0 5 10 15 20 25 50

    T ime of heot ing ( ra in )

    Fig. 1. Anneal ing isotherms for samples irradiated 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 fo