The chemical consequences of thermal neutron capture in alkali selenates—III

J. inorg, nucL Chem. VoL 40, pp. 1839-1844

0022-1902178/1101-1839/502.00

© Pergamon Press Ltd.. 1978. Printed in Great Britain

THE CHEMICAL CONSEQUENCES OF THERMAL NEUTRON CAPTURE IN ALKALI SELENATES--III~" RADIATION ANNEALING

I N K2SeO4

G. DUPLATRE~ D6partement de Recherche Fondamentale, Laboratoire de Chimie Physique Nuclgaire, C.E.N.G. 85X, 38041 Grenoble Cedex, France

(Received 15 January 1978; receivedfor publication 31 March 1978) Abstract--The effects of radiation on the distribution of the recoil species in K2SeO4 have been studied by

irradiating at different pile doses (77 K). The radiation induces annealing and also the transformation of some species into more thermostable ones. The analysis of thermal annealing curves of irradiated K2SeO4indicates that the effect of the radiation is to lower the activation energies of the annealing steps according to a linear law. INTRODUCTION

In Part II[2] of this work on selenates, the initial retention of the SEO42- ion in various matrices has been reported. It was shown that the retained fraction would include selenium species with an oxidation state equal to, or higher than, VI (such as SeO42- and SeO4-). A mechanical model was adopted, accounting for the primary processes following thermal neutron capture. The action of the ionizing radiation on the distribution and properties of the recoil species, determined by these primary processes, may also help in the understanding of the fate of the pile-irradiated SeO42- ion. Since the unretained species always appear as Se(IV) on chemical analysis, independent of their oxidation state in the solid matrix, thermal annealing experiments on samples irradiated at different pile doses have been made in order to obtain information about possible variations in the distribution and properties of the recoil species in the matrix. To distinguish the different steps in the thermal annealing spectrum [3], which should reflect the distribution of types of recoil species, the technique of isochronal heatings has been widely used. However, isothermal heatings have been made in some cases: although they do not provide detailed information on the tRef. [l] is to be considered as Part I. ~tPresent address: C.R.N. Laboratoire de Chimie Nucliaire, BP 20, 67037 Strasbourg Cedex, France.

kinetic parameters, the isothermal annealing curves prove to be very sensitive to a given choice of these parameters and may be used to confirm parameters from the analysis of the isochronal curves. In most cases, a large number of experimental points have been determined for better accuracy.

EXPERIMENTAL

Material, analytical procedure and heating. The potassium selenate has been prepared by the fusion of elementary selenium with KNO3 in a platinum crucible. The purification consisted of three successive precipitations by ethyl alcohol. The pure salt was dissolved and recrystallized. The dry ground crystals were sieved (0.1-0.2 ram) before storage in a dessicator. The chemical analysis of the irradiated samples was made by complexing Se(IV) with sodium diethyldithiocarbamate. The Se(IV) complex was extracted with carbon tetrachloride. The counting was made on the 401 keV photopeak of 7~Se, using a 4"--4" well-type NaI (TI) scintillator. The hearings were conducted in thermostatically controlled devices. The isochronal beatings lasted for 20 min. Details are to found in [1]. Irradiations. All irradiations were conducted at the Melusine Reactor of the C. E. N. Grenoble[l, 2], either at dry ice temperature (200 K) or at liquid nitrogen temperature (77 K). At 200 K, the fluxes were 9.05×10~2cm-Zs-~, and 8.7xl0~°cm-2s ~ respectively for thermal and fast neutrons, and the y-dose rate was 2.91 × 103 rad s-t. Table 1 collects the different conditions of irradiation at 77 K.

Table 1. Conditions of irradiation at 77 K Sample number 1 II II1 IV(V)t X VI VII VIII~: IX~:

Initial retention (%) 32.5 32.5 33.3 35.2 42.1 43.5 51.0 66.3 76.1

Integral y-dose (Mrad) 5.8 14.6 24.3 97.2 281 288 576 6998 > 6998

Thermal neutrons cm-: s-1

Fast neutrons cm-Z s-~

5.8 x 10*~ 6.1 x 1013 1.55 X 1013 6.1 × 1013 6.6x 10~2 3,25 x 10~3 5.8x 10j~ 6.1 x 10I~ --

7.0 x 10I" 9.5 X 1012 9.07 x 1012 9.5× 10.2 5.0x 10H 1.04 x I0 j2 7.0x 1012 9.5x 1012 --

Duration 1.5 rain 3 rain 5 min 20 min 3 hr I hr 2 hr 30 min 24 hr

fSamples IV and V come from two widely-spaced syntheses. ~;Samples VIII and IX correspond to two widely-spaced irradiations [1]. 1839

1840

G. DUPLATRE

1.22 x 10-3 Mrad-'. Unfortunately, these parameters predict n = 8% at 7000 Mrad, which is much lower than Figure I shows the changes in initial retention with the experimental value, 33.7%. dose at 200 K. A comparison with the retention values In a previous paper[l], we had tentatively proposed a given in Table 1, after irradiating at 77 K, indicates that the saturation law describing the radiation annealing in radiation annealing proceeds more readily at 200 K. The KESeO4. On examining Fig. 1, this hypothesis would difference cannot be ascribed to some thermal annealing indicate that, at 200K, saturation would be rapidly occuring at 200 K, since no increase of the retention was achieved, with a maximum retention of some 45-50%, observed in samples irradiated for a short time (Ss) and which is certainly too low a value. It thus appears that this simple model, as has already been observed in other stored in dry ice for one hour before analysis [4, 5]. This result may either indicate that the y-rays create systems [7-9], breaks down at higher doses. Although there are not many published data on the more of the mobile charges which are responsible for the annealing at higher temperatures, or merely reflect an subject, it seems that a great similarity is usually obserincrease in the mobility of these charges with tempera- ved between the radiation annealing curves and those obtained by heating the irradiated samples [10]. Maddock ture. Supposing the y-rays have a constant probability of et al.[ll, 12] have proposed a model which should acannealing, directly or indirectly, a recoil species, this count for the present results. This model suggests that probability being proportional to the dose rate, one is led the effect of the radiation is to reduce the activation to the relation between n, the unretained fraction [(100- energies for the populations of species liable to thermal annealing, such as are revealed by an isochronal annealR)], and the dose d: ing spectrum. This reduction would be a rather fast process 1 - e -rd n = no K d (I) and the rate-determining step for annealing would remain that of the thermally activated reactions. The model also This relation replaces the usual n = no e -re expression predicts the dependence of this effect on temperature, [v.e.g. 6] which is used for radiation annealing after and it is expected that at some low enough temperature, neutron-irradiation (the radioactive decays are the radiation annealing should become ineffective [9]. In the present system, which is somehow peculiar in this neglected). Relation I is displayed in Fig. 2 (solid line) for the last respect, the lowering of the activation energies by samples irradiated at 77 K (Table 1). A least squares the radiation should be very efficient, and temperature adjustment gave Ro = 32%, which is the value found for lower than 77 K would be needed for radiation annealing the lowest initial retention [1, 2], and K = to cease. Although the model may qualitatively account for the present results, the question remains whether all the populations are liable to the radiation processes. It was suggested in a previous paper[13] that in the case of the selenates, and very likely in other oxycompounds[14], several mechanisms are responsible for the thermal 40 annealing, according to which species are available in the solid to the recoil centers, at a given temperature. These mainly include the mobile species (electrons, holes excitons) at the lower temperatures, and the oxygen 35 ligands at the higher. In this framework, it is expected that the radiation processes would affect mainly, if not exclusively, those populations liable to annealing by the capture of the mobile entities. This image of the annealing mechanisms is further supported by the data on the Dose y, mrad transfer chemistry in ~lCr(III) doped manganese Fig. 1. Variation of retention in reactor-irradiated K2Se04 sulfates [15] which indicate that only that fraction of the transfer reactions which involves the defects of the (200 K). lattice can be affected by the radiation. On this basis, a model such as was early proposed by Nath et a1.[16-18] can be invoked: the role of the radiation would be to facilitate, directly or not, the detrapping of those mobile species responsible for the annealing as it is observed on -O1 heating. Then, the rate-determining step would be that of detrapping, and the temperature dependence of the processes would arise from the variation of the trap depths 8 8 -o2 with temperature. Quantitatively, the simplest expression for the variation of the unretained fraction with dose, can then be derived from relation(I), in which each of the ' T ' populations are liable to radiation annealing on a different rate, according to a parameter Ki: RESULTS AND DISCUSSION Distribution of the recoil species

-°31 -°%

16o

26o

36o

46o

r~o

6oo

y dose, mrad

Fig. 2. Variation of In [ I O 0 - R ] / [ I O O - R o ] K2SeO4 (77 K).

in reactor-irradiated

n = (I00- R) = ~'~ ni

I - e-K~d

Ki--'---~

(II)

where n~ represents the percentage of population 'T', when extrapolating to very low doses.

Chemical consequences of thermal neutron capture in alkali selenates--Ill

1841

50 i--¢-~

8 IO ok_~, ,

,

-200-70-'30

0

,

,5o

I00

i50 :,00 ~'~0 Temperature, C*

300

3~0

400

4~

Fig. 3. ]sochronal annealing of reactor-irradiated K.~SeO4 (77 K): ©, sample I (5.8 Mrad); ~7, sample I| 04.6 Mrad);

Q, sample III (24.3Mrad); A, sample V (97.2Mrad); , , sample VI (288Mrad); l , sample VII (576Mrad); &, sample VIII (6998 Mrad); [], sample IX (>6998 Mrad).

d

6O 50

6, ~.

40

30 I

20

fO

0

-2oo-7'o -.~o

o

50 IOO Temperature, *C

150

Fig. 4. Isochronal annealing of reactor-irradiated KzSeO4 (77 K): ~ , VII (576 Mrad).

Figure 3 collects the experimental isochronal curves. It must be noted that, for the sake of clarity, the figure only displays the best-fit annealing curves, due to the great number of experimental points that each curve should include. It can be seen that the unretained fraction which is not annealed below 60C increases at high doses. Figure 4 shows that this increase becomes detectable at about 600 Mrad. This indicates that as the dose becomes larger, some thermostable species are created. Since the total unretained fraction keeps decreasing with the dose, it is very likely that these species would be generated from the more annealable species. This picture is further supported by the E.P.R. spectra shown on Fig. 5, which indicates that the rather intricate spectrum of low-dose irradiated K2SeO4 (Fig. 5a)t is greatly simplified at high dose, giving an unique axially-symetric species (Fig. 5b). The change in the unretained fraction with dose would tThe numbers given on this figure correspond to the main g-factors detected in irradiated K2SeO4[I, 2].

200

2~50

300

sample VI, course (288 Mrad): ©, sample

then be described by: l - e-~ia

1 - e -Kja

-- C--Kkd + ~ ( n j + n k ) l Kkd + ~ nj (g~ 7~

1

Kj)d

(e - ~ - e-KJ~).

(III)

The indices i relate to the species which are annealed by the radiations in one step, the indices j to those species which are converted into others, while the k's correspond to these latter created species, also annealed by the radiation, according to the general scheme: gi.

n~ nj

....) retention

K~ ~
1842

G. DUPLATRE 15 p

experiments[19], shows that the kinetics may be first order. Annealing may then be represented by: n = Z

/'/i e ± K i t i

with K,

= Kol

e -Ei/kT

and

~

n, = 100-Ro Fig. 5. Room temperature E.P.R. spectra of reactor-irradiated KzSeO4 (77K): (a) sample VI (288Mrad); po) sample IX k is Boltzmann's constant, n, is the initial proportion of (>6998 Mrad). For the numbers, see Ref. [1,2]. each step, E, and Ko~ are respectively the energy of activation and frequency factor. T and t correspond to The complexity of the annealing curves unfortunately the temperature and time of heating. Whereas each does not allow the detailed investigation of the evolution annealing step has its own activation energy, it is not of each species with dose and hence, no estimate of the clear whether they have different frequency factors. different parameters (n, and K+) can be made. Since the shape of an annealing curve is not much modified by adopting any given value for Ko, at least Variation o[ the kinetic parameters with dose over a restricted range of values (such as 10'2-I0~+), and It is clear from the isochronal curves (Figs. 3, 4, 6, 7) taking into account the results of the linear tempering that thermal annealing proceeds stepwise. The abrupt- experiments [6], a unique Ko-value has been used for a ness of each step, also confirmed by linear tempering given annealing curve.

8o +

,

!

O

o

oL_~ -200

i

i

i

q

-'?l) - 5 0

I

0

50

IO0

i" -

~

"

150

200

t

,t

250

300

Temperoture, °C

Fig. 6. Isochronalannealing of reactor irradiated K2SeO4 (77 K, low doses): A, sample I (5.8Mrad); &, sample II (14.6 Mrad); ©, sample III (24.3Mrad); O, sample VI (288Mrad).

3O

20

o

I0

I

I

-2oo-7o-so

I

L

o

5o

Ioo

J

[

~

I

Ir,o

2oo

L:'5o

3oo

aso

400

•

'

450

~o

Temperoture, °C

Fig. 7. Isochronal annealing of reactor-irradiated K:SeO4 (77 K, high doses): O, sample VllI (6998Mrad); O, sample IX (>6998 Mrad).

1843

Chemical consequences of thermal neutron capture in alkali selenates--IIl In order to analyze the annealing spectra as a function of dose, two particular pairs of isochronal curves have been considered: --Samples III (24.3 Mrad) and VI (288 Mrad) for low doses (Fig. 6). --Samples VIII (6998 Mrad) and IX (>6998 Mrad) for high doses (Fig. 7). The isochronal curve of sample III has first been fitted with the help of the linear tempering results[6], particularly in the region of rapid annealing, to yield the number and weights of the steps. The mean value of the logarithm of the frequency factors in these latter experiments has been adopted, giving a frequency factor of 3.89 x 10~3, which is quite reasonable for ionic solids. Table 2 collects the parameters used for the best fitting (solid line on Fig. 6). They also account for the isothermal annealing[5].

Table 3, Annealing of K2SeO4: calculated kinetic parameters. Sample VI (288 Mrad) Ko = 3.89 × 10~ Reaction number

Activation energy Ei (eV)

Reaction weight n, (%)

0.735 O.78 0.815 0.855 0.875 0.923 0.96 0.975 1.031 1.171 1.286 1.45

3.28 3.92 7.96 5.32 5.91 5.60 2.19 2.82 3.53 2.18 4.36 2.58 3.0

1 2 +4 5 6 7 8 9 10 11 12 13 14

Table 2. Annealing of K2SeO4: kinetic parameters. Sample III (24.3 Mrad) Ko = 3.89 x 10z3 Reaction number 1 2 3 4 5 6 7 8 9 10 1t 12 13 14

Activation energy Ei (eV)

Reaction weight nl (%)

0.75 0.83 0.89 0.90 0.95 1.00 1.065 1.17 1.24 t.31 1.45 1.62 1.73

4.76 5.67 7.33 3.78 7.05 7.72 5.93 2.26 2.92 3.56 2.20 4.40 2.60 3.00

The direct reduction of the isochronal curve of sample III (24.3 Mrad) by the quotient of the initial unretained fractions does not fit the curve for sample VI (288 Mrad). Nor does any variation of the n~'s (linearly decreasing or increasing, distributed according to one or several Gaussian l a w s . . . ) . This was expected from Fig. 6 which shows that the increase of dose in samples I, II and III, although not much modifying the initial retentions, greatly facilitates annealing at high temperatures. This indicates that the action of the radiation is not only on the distribution of the species. However, since the general shape of the annealing spectra seems preserved in this interval of doses, it is very likely that the weights of each step would be conservative and decrease with dose as does the total unretained fraction. No change in the frequency factor was found and the unique way of fitting the isochronal curve of sample VI from that of sample III requires the modification of the activation energies according to a linear relation:

Table 4. Annealing of K2SeO4: kinetic parameters. Sample IX (>6998 Mrad) Ko = 3.89 × 10~3 Reaction number

Activation energy E, (eV)

Reaction weight n~ (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.832 0.912 0.976 1.027 1.095 1.15 1.274 1.375 1.474 1.535 1.594 1.679 1.752 1.792 1.837 1.904

0.87 2.52 0.60 0.55 0.52 0.53 0.86 0.88 1.09 0.60 0.49 2.14 0.66 0.99 1.98 4.97

17

2.01

2.42

18 19

2.126

0.65 0.57

22 J

m o.

o 1.4

v t ~~

>

.

ro

o

Ei (VI) = 0.6756Ei (lid + 0.2. This relation is shown in Fig. 8 and the parameters deduced for the isochronal curve fo sample VI are collected in Table 3 (solid line on Fig. 6). The treatment has been conducted in the same manner for sample VIII (6998 Mrad) and IX (>6998 Mrad). For sample IX, for which we had a wealth of experimental points, the kinetic parameters are given in Table 4 (solid

060.6

'

;o

'

;4 E, ( e V ) ;

'

p.'5

2'2

'~

sompleslTr,ErlT

Fig. 8. Variation of the activation energies with dose in reactorirradiated K2SeO4 (77K): O, samples III (24.3Mrad)/VI (288 Mrad); 0, samples VIII (6998 Mrad)/IX (>6998 Mrad).

1844

G. DUPLATRE

line on Fig. 7). The results also accounted for a set of isothermal curves [5, 13]. As for the preceding curves, the fitting of the isochronal curve of sample VIII from these parameters has been made by modifying the weights (nO of the reactions by the quotient of the initial unretained fractions and changing the energies of activation (E~) according to a linear law (Fig. 8): Ei (IX) = 0.882El(VIII) + 0.125. Table 5 displays the calculated parameters for sample VIII (solid line in Fig. 7). These results show that in a given interval of dose values the effect of radiation is to lower the activation energies of the annealing steps, without necessarily changing one kind of centre into another (samples I, II and III). Table 5. Annealing of K2SeO4: calculated kinetic parameters. Sample VIII (6998 Mrad) Ko = 3.89 x l0 Is Reaction number 1 2 3 4 5 6 7 8

9 10 11 12 13 14 15 16 17 18 19

Activation energy Ei (eV)

Reaction weight n~ (%)

0.832 0.916 0.984 1.038 1.065 1.080 1.303 1.412 1.521 1.588 1.768 1.797 1.830 1.875 1.926 2.003 2.126 2.263

1.23 3.55 0.85 0.78 0.73 0.75 1.21 1.24 1.54 0.85 0.69 3.02 0.93 1.40 2.79 7.01 3.41 0.92 0.80

This is in keeping with previous results on K2SeO4[ll] and might also be compared to the results obtained in hafnium tropolonate, wherein the activation energy of each step was found to follow the law[20]: Ed = Eo+ AEa. The complexity of the annealing curves of K2Se04 unfortunately does not allow a detailed investigation of the evolution of the reaction steps and hence, to an explicit dependence on dose d in the linear relations. It is to be noted that in the case of hafnium tropolonate, the activation energies increase with dose. This reflects the

oppesite role of the radiolytic defects in this compound to that in potassium selenate. As the dose increases, the recoil species begin to suffer annealing. Finally, the radiation induces the transformation of the recoil species into more stable ones (samples VI, VII, VIII, IX). It is not excluded that the frequency factors as well as the activation energies would be changed. However, this alteration must be small since no variation is detected in our experiments. If the measured activation energies are directly related to the trapping energies of impurity levels, it must be supposed that sgme interactions occur between the different defects as the dose increases. If these activation energies were the apparent energies of a series of kinetic steps, the observed effect cannot be explained in detail due to our lack of knowledge of these steps. Acknowledgements--The author wishes to thank Professors A. Moussa for laboratory facilities and encouragement, J. I. Vargas for his early participation and A. G. Maddock for his interest in this work.

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(1964). 17. A. Nath, K. A. Rao and V. G. Thomas, Radiochim. Acta 3, 134 (1964). 18. A. Nath, S. Khorana, P. K. Mathur and S. Sarup, Ind. J. Chem. 4, 51 (1966). 19. G. Duplfitre, Th~se de 3~me cycle, Universit6 de Grenoble, France (1969). 20. A. Tissier, Thi~se de 36me cycle, Universit6 de Grenoble, France (1970).