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Chemical effects of the (n, γ) reaction in alkali selenates
J. inore, nut1.Chem., 1972.Vol.34, pp. 3007-3014. PergamonPress. Printedin Great Britain
C H E M I C A L E F F E C T S OF T H E (n, 3') R E A C T I O N IN A L K A L I S E L E N A T E S F. R. A L - S I D D I Q U E * and A. G. M A D D O C K The Chemical Laboratories, Lensfield Road, Cambridge CB2 1EW (Received 19 Now, tuber 1971) Abstract-Thermal annealing isochronals have been measured for sodium and potassium selenates. They are quite dissimilar. The isochronal for a dehydrated sample of irradiated sodium selenate hydrate suggests that dehydration removes the more readily annealable fragment centres. The temperature dependence of the radiation annealing of the same two salts has also been explored. An explanation of this temperature dependence, which also accounts for various features of the isothermal radiation annealing process, is proposed.
INTRODUCTION
THE EFFECTS of radiative thermal neutron capture have been studied extensively in the salts of the oxyanions, partly because of their thermal and radiolytic stability and partly because of an anticipated relative simplicity of the effects in these simple compounds. The last few years' work has shattered our illusions on the last score, especially in relation to the chromates[l, 2]. However, the largest body of data refers to the chromates and phosphates, and it seems useful to explore how far the results for these salts are representative of the whole set of these compounds. The selenates and selenites share most of the favourable features of the oxyanions. The cross-section for 75Se activation (0.26 barns)[3] is adequate, and its half-life convenient (120 days)[3], but comparatively little has been published about effects in selenium compounds. Langsdorf and Segr6 measured [4] the isotopic separation following isomeric transition in slmSe in a solution of selenic acid in hydrochloric acid. The ground state species was separated after adding selenite cartier and reducing selenite to elemental selenium by sulphurous acid. Daudel found that radiative neutron capture in selenite produced radioactive elemental selenium [5] and also mentioned a recoil effect in selenate without giving details [6]. Selenium dioxide has been investigated by Apers et al.[7, 8] and also by Pinkerton and Green[9], both authors finding very high retentions. *Present address: Atomic Energy Centre, Dacca 2, East Pakistan. I. P. Giitlich and G. Harbottle, Radiochim. Acta 5, 70 (1966). 2. T. Andersen and G. SCrensen, Trans. Faraday Soc. 62, 3432 (1966). 3. C. M. Lederer, J. M. Hollander and 1. Perlman. Table oflsotopes, pp. 29-30. John Wiley, N e w York (1967). 4. A. Langsdorf and E. Segre, Phys. Rev. 57, 105 (1940). 5. R. Daudel, Compt. rend. 213, 479 (1941). 6. R. Daudel, Compt. rend. 214, 547 (1942). 7. D.J. Apers, P. C. Capron and L. J. Gilly,J. inorg, nucl. Chem. 5, 23 (1957). 8. D.J. Apers, C. Theyskens and P. C. Capron, J. inorg, nucl. Chem. 29, 858 (1967). 9. D.M. Pinkerton andJ. H. Green,J. inorg, nucl. Chem. 10, 334 (1959). 3007
3008
F . R . A L - S I D D I Q U E and A. G. M A D D O C K
More recently, Constantinescu et al. have studied [ 10] neutron capture effects in hydrated sodium seleA~ates and in ammonium selenate. They find the retention in the latter salt decreases with the time of irradiation. Finally, while this study was in progress, Duplfitre and Vargas have completed a much more detailed study of the alkali selenates [11]. EXPERIMENTAL
Materials Na2SeO4-10H~O, AnalaR grade sodium selenate, was recrystallised from water at room temperature Na2SeO4 was obtained by crystallisation above 48°C. K2SeO4, AnalaR salt was recrystallised, The two anhydrous salts were dried in a desiccator over phosphorus pentoxide.
Irradiations Samples sealed in silica ampoules were irradiated, cooled in liquid nitrogen, in B.E.P.O. at A.E.R.E. in a thermal neutron flux of approximately 1.4 × 1012n cm -2 see -1. Irradiation I was for 2 hr and gave the sample a dose of ionising radiation of about 4 Mrad. Irradiation II was for 1 hr under otherwise similar conditions. Samples were stored in liquid nitrogen until used. In all cases 2 weeks or more elapsed between irradiation and use. Many of the experiments were made on material that had been stored in liquid nitrogen for some months, but control experiments failed to reveal any difference between this material and fresher samples. Two further irradiations at the same temperature were made in Herald at A.W.R.E. at a flux of about 3.6 × 1013n cm -2 sec -~. In the first of them, III, the dose of ionising radiation was about 20 Mrads and in the other, IV, about half this value. However, the concurrent fast neutron irradiation was much greater (perhaps 100 X) in Herald compared with the B.E.P.O. irradiations.
Analysis Selenite-selenate separation. About 1 mg of the irradiated salt was dissolved in 0-5 ml of 2-5% potassium selenite solution. The total activity was measured. One ml of 3% potassium selenate was added and selenate precipitated by adding 5 ml. of 10% barium chloride in 0.1 M hydrochloric acid. The barium selenate precipitate was centrifuged down, the supernatant removed, and the precipitate washed with 3.5 ml of warm 10% barium chloride in 0.05 M hydrochloric acid. Finally, the barium selenate was separated after centrifugation and its activity measured. Control experiments showed that >/99 per cent of the selenate in a solution was recovered in this separation. In addition, in the presence of a large selenite activity on separation of inactive selenate the barium selenate carried between 0.5 and 0'7 per cent of the selenite activity. Tests on these irradiated selenates using (i) adsorption on freshly-precipitated red selenium, and (ii) separation of carrier selenocyanide after solution in potassium cyanide solution, showed that they contained less than 1 per cent of the activity as Se °. This is in agreement with the results of Duplatre and Vargas.
Activity measurements Measurements were made of the 75Se activity using a well-type NaI/TI scintillation counter and a single-channel analyser with the window set to span both the 265 and 280 keV ph0topeaks. The reproducibility of measurements of the retention was _+2%.
Annealing (a) Thermal. Between 20 ° and 100 ° samples were heated in a thermostatically-controlled oven. Between 100 ° and 240°C a similarly-controlled Dowtherm A bath was used. Both devices gave constant temperatures within _+0.5°C. Above 240°C a tube furnace was used and the thermostatic control was only good to within _+2°C. (b) lonising radiation. Samples were irradiated with e°Co radiation in a 2000 C Vickrad unit. The samples were sealed in thin-walled silica tubes and thermostatted while irradiating. Liquid nitrogen, 10. M. Constantinescu, O. Constantinescu, I. Pascaru and E. Gird, Rev. Roum. Phys. 11,249 (1966). 1 l. M. Cogneau, G. Dupl~tre andJ. I. Vargas J. inorg, nucl. Chem. 34, 3021 (1972).
Alkali selenates
3009
solid CO2-acetone mixture and ice were used for temperatures below room temperature. Above room temperature a small oil thermostat was built in the irradiation chamber. This gave control to -+I°C. The dose rates were determined using an oxalic acid solution dosimeter (G_ox = 4.9)[ 12]. (c) Ultraviolet light. Samples were irradiated in transparent silica tubes placed 20 cm from a Hanovia UVS 500 mercury lamp. They were maintained close to room temperature by a current of air. RESULTS AND DISCUSSION
Measurements were made of the initial retentions by dissolving samples as quickly as possible after removal from the liquid nitrogen storage vessel in cold carrier solution. Unlike potassium chromate, these measurements show that the retention changes with the time and conditions of neutron irradiation even at the temperature of liquid nitrogen. Thus, sodium selenate and hydrated sodium selenate gave Ro = 45"0 and 19-8 per cent, respectively, after irradiation I, but the values increased to 75.0 and 46.4 per cent respectively for irradiation IV. Potassium selenate did not show such big changes. This result confirms Duplatre and Vargas' observations [11 ], and implies that some radiation-annealing process is still possible at the temperature of liquid nitrogen. The hydrated salt invariably gave lower retention than the anhydrous salt for similar conditions of irradiation.
1. Thermal annealing results One hour isochronal annealing results for samples of irradiated potassium selenate from irradiations I and II are shown in Fig. 1. These curves clearly reveal the presence of two and very probably three, groups of recoil centres, distinguishable by their different energies of activation for thermal annealing. The proportions of these centres do not seem to change very substantially on doubling the dose of ionising radiation during neutron irradiation, but increasing the dose shifts the curves to lower temperatures. This suggests that the ionising radiation can change the activation energy for thermal annealing without necessarily converting one kind of centre into another.
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o
i
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,
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i
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Fig. I. T h e r m a l a n n e a l i n g i s o c h r o n a ] s f o r K~SeO4. © I r r a d i a t i o n I. + I r r a d i a t i o n I I . 12. |. D r a g a n i c , Nucleonics 21, 33 ( ] 9 6 3 ) .
3010
F.R.
AL-SIDDIQUE
and A. G. M A D D O C K
A similar isochronal was run for sodium selenate (Fig. 2). In this case only two kinds of centres can be distinguished, and these cannot be correlated simply with those found for the potassium salt. However, Andersen et al.[13] and Collins et al.[ 14] have found that the degree to which such isochronals reveal their structure is very dependent on the defect density in the crystals used. It is very possible that the samples of the two selenates used differed substantially in the perfection of the crystals. Since selenites begin to undergo aerial oxidation in the region not much above the temperature at which the second thermal annealing processes is noticed in sodium selenate, a comparison was made by annealing in one case in air and in the other in an evacuated tube at about 10 -3 mm Hg pressure of air. Tests were made at 350 ° and 426 °, but in neither case was there a significant difference in the amount of annealing. The behaviour of sodium and potassium selenates appears to be quite different in the low-temperature region. The latter does not seem to anneal much between room temperature and about 60 ° (plateau in the isochronal), although the retention increases from R0 --- 50.5 per cent to a plateau value of 63.3 per cent at room temperature. The sodium salt, however, anneals from room temperature upward, although changing only from 45.0 to 50.3 per cent between liquid nitrogen and 20°C. (Both salts from irradiation I.) Hydrated sodium selenate is too readily dehydrated for measurement of a
70
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t
I
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200
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I
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°C Fig. 2. T h e r m a l annealing isochronals for Na2SeO4. × A n h y d r o u s salt. + D e h y d r a t e d NazSeO410H20 Irrad. I. O D e h y d r a t e d Na2SeO410H20 Irrad. II. 13. P. Giitlich and G. Harbottle, Radiochim. Acta 8, 30 (1967). 14. R. E, Ackerhalt, C. H. Collins and K. E. Collins, Personal communication.
Alkali selenates
3011
similar isochronal. A sample was allowed to reach its plateau retention at room temperature and then dehydrated over phosphorus pentoxide. Very little change in retention accompanied dehydration (xR R'T" Hydrate 37-9 per cent R R~ Dehydrated 40.0 per cent). Weight changes showed that dehydration was more than 98 per cent complete. The dehydrated material was then used for isochronal annealing. The result is shown in Fig. 2. The curve suggests that during dehydration the more labile and readily thermally annealable centres are converted to more inert species, so that substantial thermal annealing only takes place at high temperatures (> 350°C). Furthermore, the high-temperature process is now enhanced by the presence of oxygen: R 418°in air 57.1 per cent and R 41s°in 10-3 mm Hg of air 40.8 per cent. Thus the high-temperature (> 350°C) process may be due in part, or indeed entirely, to oxidation of selenite. Certainly the dehydration process removes the major part of the more annealable centres.
2. Radiation annealing results Published data and the differing values for R0 obtained for different times of neutron irradiation suggested that the selenates were probably fairly sensitive to radiation annealing. Since there are few quantitative data on the effects of temperature on radiation annealing[15, 16], measurements were made using 6°Co radiation at liquid nitrogen temperature, 0 °, 31 °, 51 ° and 77°C, on both sodium and potassium selenates. For measurements above room temperature the material was stored for three days at the temperature chosen for the ionising irradiation so that thermal annealing had almost ceased before radiation annealing began. In separate experiments at room temperature it was verified that for radiation annealing at dose rates of 0.17 and 0-48 Mrad/hr the extent of annealing depended only on the total dose absorbed. The results, except for those at liquid nitrogen temperature, are recorded in Figs. 3 and 4. The logarithm of the fraction of the radioactive selenium remaining unannealed is plotted as a function of the dose at each temperature. The material used in the low-temperature experiments was stored for 3 days at room temperature before exposure to ionising radiation. The liquid nitrogen irradiation of 9 hr (up to 4 Mrads) failed to produce any change in retention. Since the retention increased with the length of neutron irradiation at liquid nitrogen temperature, one must suppose that the most sensitive fragments must all be recombined in the room temperature thermal annealing preceding the gamma irradiation. It is not valid to try to extract any kind of energy of activation from these data, since the populations of radioactive fragments are not the same at the beginning of each experiment, because of the previous thermal annealing. Thus for sodium selenate the initial retentions were 59-7 per cent at 31 °, 64.0 per cent at 57 ° and 70.7 per cent at 77 °. However, there are some interesting conclusions that can be drawn. The isochronal annealing data imply some distribution of energies of activation for thermal annealing, for instance, like that shown in Fig. 5. Isothermal annealing has been shown to drive a step function across this figure, reaching a particular position at a given temperature, e.g. CY at Tt °. All fragment centres 15. G. Harbottle,J. chem. Phys. 21, 33 (1954). 16. F. Baumg~irtner and A. G. Maddock, Trans. Faraday Soc. 64, 714 (1968).
3012
F. R. A L - S I D D I Q U E and A. G. M A D D O C K
0 0.8
9
~
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Fig. 3. y Annealing of K2SeO4. [] 273°K. + 304°K. × 324°K. O 350°K.
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1 14
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Fig. 4. Annealing of Na2SeO4. + 304°K. × 324°K. O 350°K.
lying to the left of this line will be annealed at T1°. The effect of radiation annealing is to displace centres from the right-hand to the left-hand sides of this line. Thus, if radiation annealing is conducted at T1°, those fragments transferred by the radiation from energies of activation in the region C - F to values less than C will anneal. If the radiation annealing is conducted at a lower temperature T2° a new population of thermally-annealable fragments will build up in the interval B - C (indicated by the dotted line B-E). These are revealed on subsequent thermal annealing at T1°, when another stage of rapid thermal annealing takes place [17]. It has also been shown that some of the fragments crossing CE from right to left, because of the ionising irradiation taking place at T2°, can return to the region to 17. A. G. Maddock and H. Miiller, Trans. Faraday Soc. 56, 509 (1960).
Alkali selenates
3013
r~
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I
B
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e~ Fig. 5. Distribution of energies of activation.
the right of C E , because the number of annealable centres in the area B E C when the ionising irradiation is conducted at T2°, and the prior and subsequent thermal annealing at T1°, is less than the number annealed if the irradiation is carried out at T1 °.
The two isochronals shown in Fig. 1 suggest that the additional dose received during the longer irradiation seems to displace the distribution bodily to lower energies without greatly changing its shape. The radiation annealing data at different temperatures lead to another interesting conclusion. As the temperature rises the step function moves to the right of Fig. 5. It seems reasonable to suppose that at a constant dose rate the number of fragments that will be affected per unit time in a way that can change their energies of activation for subsequent thermal annealing will be proportional to the number of fragments remaining unannealed. This assumption is supported by the observation that the fraction of radioactive fragments annealed by a given dose of ionising radiation does not change as the radioactive fragments decay, if all the material used is from a given neutron irradiation. If, therefore, the diminution in energy of activation always transfers the fragments from the right-hand to the left-hand side of the step function, then the fraction of unannealed fragments annealed by the ionising radiation per unit dose should be independent of temperature. The experimental data show that this is not the case, and suggest that the average excitation event must be likely to displace the energy of activation by some smaller amount. Let us explore the case where an ionising event reduces the energy of activation by some fixed amount, BE, which is very much less than the difference C F = AE. We can now divide the area C E F into ~ / S E equally spaced zones (to the nearest integral value). Individual radiation-induced events will only be able to anneal the population of the first of these zones. Suppose that the distribution is such that the initial number of fragments in the 1st, 2nd etc. zones, counted from the one immediately to the right of the step function, are N1, N2, Na etc. Then, if the probability of the ionising radiation exciting a fragment and displacing it to the zone next nearest to the left is the same for all groups and equal to k, dnl
dt = - k n ~ + kn2,
3014
F. R. A L - S I D D I Q U E and A. G. M A D D O C K
also at/2
dt = -kn2 + kna etc, etc;
where nl, n2 etc. are the populations of the different regions at time t. It can easily be shown that for AE/SE = n zones
dnl _ Ink"× ( n - 1)kn-lt "-2 dt - [ n ! Nnt"-I + (n - 1) ! -ke-kt{-~.N,t'+
} "
"
e-kt
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The quantity measured in Figs. 4 and 5 is actually ~ ndE~ N~ and the slopes of these plots is - k n i / ~ N~. This model clearly permits the initial slope of these plots to increase with temperature, at least until AE/~E - 1. Furthermore, it suggests that, according to the initial distribution of E,, the slope of a plot of log (100--R) vs. time may show minima and/or maxima. This is easily seen in extreme cases. Suppose N~ = 0; then the initial rate of radiation annealing should be zero and the rate will clearly pass through some maximum. Alternatively, if the first region is well populated but there are some nearly empty zones before another wellpopulated region, the slope of the radiation annealing plot may show a minimum, that is the rate of radiation annealing may at first slow down with time of annealing only to undergo acceleration at still longer times. Some experimental evidence for such behaviour has been obtained with the selenates. After about 25 Mrads at 57 ° radiation annealing of anhydrous sodium selenate had almost ceased, the increase from 12 to 25 Mrads being only about 2 ± 1% while 17 per cent of unannealed radioactive selenium remained. But between 25 and 30 Mrads the rate of radiation annealing increased appreciably and the percentage of unannealed radioactive selenium fell from 17 to about 6 per cent. The above model also accounts for the general lack of linearity of the plots of log ( 100 - R) vs. time during radiation annealing. Acknowledgement- One of the authors (F.R.A-S.) is indebted, for a Fellowship, to the Colombo Plan Authority.
C H E M I C A L E F F E C T S OF T H E (n, 3') R E A C T I O N IN A L K A L I S E L E N A T E S F. R. A L - S I D D I Q U E * and A. G. M A D D O C K The Chemical Laboratories, Lensfield Road, Cambridge CB2 1EW (Received 19 Now, tuber 1971) Abstract-Thermal annealing isochronals have been measured for sodium and potassium selenates. They are quite dissimilar. The isochronal for a dehydrated sample of irradiated sodium selenate hydrate suggests that dehydration removes the more readily annealable fragment centres. The temperature dependence of the radiation annealing of the same two salts has also been explored. An explanation of this temperature dependence, which also accounts for various features of the isothermal radiation annealing process, is proposed.
INTRODUCTION
THE EFFECTS of radiative thermal neutron capture have been studied extensively in the salts of the oxyanions, partly because of their thermal and radiolytic stability and partly because of an anticipated relative simplicity of the effects in these simple compounds. The last few years' work has shattered our illusions on the last score, especially in relation to the chromates[l, 2]. However, the largest body of data refers to the chromates and phosphates, and it seems useful to explore how far the results for these salts are representative of the whole set of these compounds. The selenates and selenites share most of the favourable features of the oxyanions. The cross-section for 75Se activation (0.26 barns)[3] is adequate, and its half-life convenient (120 days)[3], but comparatively little has been published about effects in selenium compounds. Langsdorf and Segr6 measured [4] the isotopic separation following isomeric transition in slmSe in a solution of selenic acid in hydrochloric acid. The ground state species was separated after adding selenite cartier and reducing selenite to elemental selenium by sulphurous acid. Daudel found that radiative neutron capture in selenite produced radioactive elemental selenium [5] and also mentioned a recoil effect in selenate without giving details [6]. Selenium dioxide has been investigated by Apers et al.[7, 8] and also by Pinkerton and Green[9], both authors finding very high retentions. *Present address: Atomic Energy Centre, Dacca 2, East Pakistan. I. P. Giitlich and G. Harbottle, Radiochim. Acta 5, 70 (1966). 2. T. Andersen and G. SCrensen, Trans. Faraday Soc. 62, 3432 (1966). 3. C. M. Lederer, J. M. Hollander and 1. Perlman. Table oflsotopes, pp. 29-30. John Wiley, N e w York (1967). 4. A. Langsdorf and E. Segre, Phys. Rev. 57, 105 (1940). 5. R. Daudel, Compt. rend. 213, 479 (1941). 6. R. Daudel, Compt. rend. 214, 547 (1942). 7. D.J. Apers, P. C. Capron and L. J. Gilly,J. inorg, nucl. Chem. 5, 23 (1957). 8. D.J. Apers, C. Theyskens and P. C. Capron, J. inorg, nucl. Chem. 29, 858 (1967). 9. D.M. Pinkerton andJ. H. Green,J. inorg, nucl. Chem. 10, 334 (1959). 3007
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F . R . A L - S I D D I Q U E and A. G. M A D D O C K
More recently, Constantinescu et al. have studied [ 10] neutron capture effects in hydrated sodium seleA~ates and in ammonium selenate. They find the retention in the latter salt decreases with the time of irradiation. Finally, while this study was in progress, Duplfitre and Vargas have completed a much more detailed study of the alkali selenates [11]. EXPERIMENTAL
Materials Na2SeO4-10H~O, AnalaR grade sodium selenate, was recrystallised from water at room temperature Na2SeO4 was obtained by crystallisation above 48°C. K2SeO4, AnalaR salt was recrystallised, The two anhydrous salts were dried in a desiccator over phosphorus pentoxide.
Irradiations Samples sealed in silica ampoules were irradiated, cooled in liquid nitrogen, in B.E.P.O. at A.E.R.E. in a thermal neutron flux of approximately 1.4 × 1012n cm -2 see -1. Irradiation I was for 2 hr and gave the sample a dose of ionising radiation of about 4 Mrad. Irradiation II was for 1 hr under otherwise similar conditions. Samples were stored in liquid nitrogen until used. In all cases 2 weeks or more elapsed between irradiation and use. Many of the experiments were made on material that had been stored in liquid nitrogen for some months, but control experiments failed to reveal any difference between this material and fresher samples. Two further irradiations at the same temperature were made in Herald at A.W.R.E. at a flux of about 3.6 × 1013n cm -2 sec -~. In the first of them, III, the dose of ionising radiation was about 20 Mrads and in the other, IV, about half this value. However, the concurrent fast neutron irradiation was much greater (perhaps 100 X) in Herald compared with the B.E.P.O. irradiations.
Analysis Selenite-selenate separation. About 1 mg of the irradiated salt was dissolved in 0-5 ml of 2-5% potassium selenite solution. The total activity was measured. One ml of 3% potassium selenate was added and selenate precipitated by adding 5 ml. of 10% barium chloride in 0.1 M hydrochloric acid. The barium selenate precipitate was centrifuged down, the supernatant removed, and the precipitate washed with 3.5 ml of warm 10% barium chloride in 0.05 M hydrochloric acid. Finally, the barium selenate was separated after centrifugation and its activity measured. Control experiments showed that >/99 per cent of the selenate in a solution was recovered in this separation. In addition, in the presence of a large selenite activity on separation of inactive selenate the barium selenate carried between 0.5 and 0'7 per cent of the selenite activity. Tests on these irradiated selenates using (i) adsorption on freshly-precipitated red selenium, and (ii) separation of carrier selenocyanide after solution in potassium cyanide solution, showed that they contained less than 1 per cent of the activity as Se °. This is in agreement with the results of Duplatre and Vargas.
Activity measurements Measurements were made of the 75Se activity using a well-type NaI/TI scintillation counter and a single-channel analyser with the window set to span both the 265 and 280 keV ph0topeaks. The reproducibility of measurements of the retention was _+2%.
Annealing (a) Thermal. Between 20 ° and 100 ° samples were heated in a thermostatically-controlled oven. Between 100 ° and 240°C a similarly-controlled Dowtherm A bath was used. Both devices gave constant temperatures within _+0.5°C. Above 240°C a tube furnace was used and the thermostatic control was only good to within _+2°C. (b) lonising radiation. Samples were irradiated with e°Co radiation in a 2000 C Vickrad unit. The samples were sealed in thin-walled silica tubes and thermostatted while irradiating. Liquid nitrogen, 10. M. Constantinescu, O. Constantinescu, I. Pascaru and E. Gird, Rev. Roum. Phys. 11,249 (1966). 1 l. M. Cogneau, G. Dupl~tre andJ. I. Vargas J. inorg, nucl. Chem. 34, 3021 (1972).
Alkali selenates
3009
solid CO2-acetone mixture and ice were used for temperatures below room temperature. Above room temperature a small oil thermostat was built in the irradiation chamber. This gave control to -+I°C. The dose rates were determined using an oxalic acid solution dosimeter (G_ox = 4.9)[ 12]. (c) Ultraviolet light. Samples were irradiated in transparent silica tubes placed 20 cm from a Hanovia UVS 500 mercury lamp. They were maintained close to room temperature by a current of air. RESULTS AND DISCUSSION
Measurements were made of the initial retentions by dissolving samples as quickly as possible after removal from the liquid nitrogen storage vessel in cold carrier solution. Unlike potassium chromate, these measurements show that the retention changes with the time and conditions of neutron irradiation even at the temperature of liquid nitrogen. Thus, sodium selenate and hydrated sodium selenate gave Ro = 45"0 and 19-8 per cent, respectively, after irradiation I, but the values increased to 75.0 and 46.4 per cent respectively for irradiation IV. Potassium selenate did not show such big changes. This result confirms Duplatre and Vargas' observations [11 ], and implies that some radiation-annealing process is still possible at the temperature of liquid nitrogen. The hydrated salt invariably gave lower retention than the anhydrous salt for similar conditions of irradiation.
1. Thermal annealing results One hour isochronal annealing results for samples of irradiated potassium selenate from irradiations I and II are shown in Fig. 1. These curves clearly reveal the presence of two and very probably three, groups of recoil centres, distinguishable by their different energies of activation for thermal annealing. The proportions of these centres do not seem to change very substantially on doubling the dose of ionising radiation during neutron irradiation, but increasing the dose shifts the curves to lower temperatures. This suggests that the ionising radiation can change the activation energy for thermal annealing without necessarily converting one kind of centre into another.
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i
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,
i 200 °C
i
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Fig. I. T h e r m a l a n n e a l i n g i s o c h r o n a ] s f o r K~SeO4. © I r r a d i a t i o n I. + I r r a d i a t i o n I I . 12. |. D r a g a n i c , Nucleonics 21, 33 ( ] 9 6 3 ) .
3010
F.R.
AL-SIDDIQUE
and A. G. M A D D O C K
A similar isochronal was run for sodium selenate (Fig. 2). In this case only two kinds of centres can be distinguished, and these cannot be correlated simply with those found for the potassium salt. However, Andersen et al.[13] and Collins et al.[ 14] have found that the degree to which such isochronals reveal their structure is very dependent on the defect density in the crystals used. It is very possible that the samples of the two selenates used differed substantially in the perfection of the crystals. Since selenites begin to undergo aerial oxidation in the region not much above the temperature at which the second thermal annealing processes is noticed in sodium selenate, a comparison was made by annealing in one case in air and in the other in an evacuated tube at about 10 -3 mm Hg pressure of air. Tests were made at 350 ° and 426 °, but in neither case was there a significant difference in the amount of annealing. The behaviour of sodium and potassium selenates appears to be quite different in the low-temperature region. The latter does not seem to anneal much between room temperature and about 60 ° (plateau in the isochronal), although the retention increases from R0 --- 50.5 per cent to a plateau value of 63.3 per cent at room temperature. The sodium salt, however, anneals from room temperature upward, although changing only from 45.0 to 50.3 per cent between liquid nitrogen and 20°C. (Both salts from irradiation I.) Hydrated sodium selenate is too readily dehydrated for measurement of a
70
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I
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I
I 400
I
I 500
°C Fig. 2. T h e r m a l annealing isochronals for Na2SeO4. × A n h y d r o u s salt. + D e h y d r a t e d NazSeO410H20 Irrad. I. O D e h y d r a t e d Na2SeO410H20 Irrad. II. 13. P. Giitlich and G. Harbottle, Radiochim. Acta 8, 30 (1967). 14. R. E, Ackerhalt, C. H. Collins and K. E. Collins, Personal communication.
Alkali selenates
3011
similar isochronal. A sample was allowed to reach its plateau retention at room temperature and then dehydrated over phosphorus pentoxide. Very little change in retention accompanied dehydration (xR R'T" Hydrate 37-9 per cent R R~ Dehydrated 40.0 per cent). Weight changes showed that dehydration was more than 98 per cent complete. The dehydrated material was then used for isochronal annealing. The result is shown in Fig. 2. The curve suggests that during dehydration the more labile and readily thermally annealable centres are converted to more inert species, so that substantial thermal annealing only takes place at high temperatures (> 350°C). Furthermore, the high-temperature process is now enhanced by the presence of oxygen: R 418°in air 57.1 per cent and R 41s°in 10-3 mm Hg of air 40.8 per cent. Thus the high-temperature (> 350°C) process may be due in part, or indeed entirely, to oxidation of selenite. Certainly the dehydration process removes the major part of the more annealable centres.
2. Radiation annealing results Published data and the differing values for R0 obtained for different times of neutron irradiation suggested that the selenates were probably fairly sensitive to radiation annealing. Since there are few quantitative data on the effects of temperature on radiation annealing[15, 16], measurements were made using 6°Co radiation at liquid nitrogen temperature, 0 °, 31 °, 51 ° and 77°C, on both sodium and potassium selenates. For measurements above room temperature the material was stored for three days at the temperature chosen for the ionising irradiation so that thermal annealing had almost ceased before radiation annealing began. In separate experiments at room temperature it was verified that for radiation annealing at dose rates of 0.17 and 0-48 Mrad/hr the extent of annealing depended only on the total dose absorbed. The results, except for those at liquid nitrogen temperature, are recorded in Figs. 3 and 4. The logarithm of the fraction of the radioactive selenium remaining unannealed is plotted as a function of the dose at each temperature. The material used in the low-temperature experiments was stored for 3 days at room temperature before exposure to ionising radiation. The liquid nitrogen irradiation of 9 hr (up to 4 Mrads) failed to produce any change in retention. Since the retention increased with the length of neutron irradiation at liquid nitrogen temperature, one must suppose that the most sensitive fragments must all be recombined in the room temperature thermal annealing preceding the gamma irradiation. It is not valid to try to extract any kind of energy of activation from these data, since the populations of radioactive fragments are not the same at the beginning of each experiment, because of the previous thermal annealing. Thus for sodium selenate the initial retentions were 59-7 per cent at 31 °, 64.0 per cent at 57 ° and 70.7 per cent at 77 °. However, there are some interesting conclusions that can be drawn. The isochronal annealing data imply some distribution of energies of activation for thermal annealing, for instance, like that shown in Fig. 5. Isothermal annealing has been shown to drive a step function across this figure, reaching a particular position at a given temperature, e.g. CY at Tt °. All fragment centres 15. G. Harbottle,J. chem. Phys. 21, 33 (1954). 16. F. Baumg~irtner and A. G. Maddock, Trans. Faraday Soc. 64, 714 (1968).
3012
F. R. A L - S I D D I Q U E and A. G. M A D D O C K
0 0.8
9
~
n.. fl¢o 0.7
0O0.6 0.5
0.4 I
I 2
I 5
I 4
, I 5
] 6
I 7
h
Fig. 3. y Annealing of K2SeO4. [] 273°K. + 304°K. × 324°K. O 350°K.
o6 F
x,
"18_'
0.4~ 0.~
0-2
I 2
I 4
I 6
I 8
I I0
I 12
1 14
h
Fig. 4. Annealing of Na2SeO4. + 304°K. × 324°K. O 350°K.
lying to the left of this line will be annealed at T1°. The effect of radiation annealing is to displace centres from the right-hand to the left-hand sides of this line. Thus, if radiation annealing is conducted at T1°, those fragments transferred by the radiation from energies of activation in the region C - F to values less than C will anneal. If the radiation annealing is conducted at a lower temperature T2° a new population of thermally-annealable fragments will build up in the interval B - C (indicated by the dotted line B-E). These are revealed on subsequent thermal annealing at T1°, when another stage of rapid thermal annealing takes place [17]. It has also been shown that some of the fragments crossing CE from right to left, because of the ionising irradiation taking place at T2°, can return to the region to 17. A. G. Maddock and H. Miiller, Trans. Faraday Soc. 56, 509 (1960).
Alkali selenates
3013
r~
r,
J
x~
Yl
I
B
¢
e~ Fig. 5. Distribution of energies of activation.
the right of C E , because the number of annealable centres in the area B E C when the ionising irradiation is conducted at T2°, and the prior and subsequent thermal annealing at T1°, is less than the number annealed if the irradiation is carried out at T1 °.
The two isochronals shown in Fig. 1 suggest that the additional dose received during the longer irradiation seems to displace the distribution bodily to lower energies without greatly changing its shape. The radiation annealing data at different temperatures lead to another interesting conclusion. As the temperature rises the step function moves to the right of Fig. 5. It seems reasonable to suppose that at a constant dose rate the number of fragments that will be affected per unit time in a way that can change their energies of activation for subsequent thermal annealing will be proportional to the number of fragments remaining unannealed. This assumption is supported by the observation that the fraction of radioactive fragments annealed by a given dose of ionising radiation does not change as the radioactive fragments decay, if all the material used is from a given neutron irradiation. If, therefore, the diminution in energy of activation always transfers the fragments from the right-hand to the left-hand side of the step function, then the fraction of unannealed fragments annealed by the ionising radiation per unit dose should be independent of temperature. The experimental data show that this is not the case, and suggest that the average excitation event must be likely to displace the energy of activation by some smaller amount. Let us explore the case where an ionising event reduces the energy of activation by some fixed amount, BE, which is very much less than the difference C F = AE. We can now divide the area C E F into ~ / S E equally spaced zones (to the nearest integral value). Individual radiation-induced events will only be able to anneal the population of the first of these zones. Suppose that the distribution is such that the initial number of fragments in the 1st, 2nd etc. zones, counted from the one immediately to the right of the step function, are N1, N2, Na etc. Then, if the probability of the ionising radiation exciting a fragment and displacing it to the zone next nearest to the left is the same for all groups and equal to k, dnl
dt = - k n ~ + kn2,
3014
F. R. A L - S I D D I Q U E and A. G. M A D D O C K
also at/2
dt = -kn2 + kna etc, etc;
where nl, n2 etc. are the populations of the different regions at time t. It can easily be shown that for AE/SE = n zones
dnl _ Ink"× ( n - 1)kn-lt "-2 dt - [ n ! Nnt"-I + (n - 1) ! -ke-kt{-~.N,t'+
} "
"
e-kt
'' "}
The quantity measured in Figs. 4 and 5 is actually ~ ndE~ N~ and the slopes of these plots is - k n i / ~ N~. This model clearly permits the initial slope of these plots to increase with temperature, at least until AE/~E - 1. Furthermore, it suggests that, according to the initial distribution of E,, the slope of a plot of log (100--R) vs. time may show minima and/or maxima. This is easily seen in extreme cases. Suppose N~ = 0; then the initial rate of radiation annealing should be zero and the rate will clearly pass through some maximum. Alternatively, if the first region is well populated but there are some nearly empty zones before another wellpopulated region, the slope of the radiation annealing plot may show a minimum, that is the rate of radiation annealing may at first slow down with time of annealing only to undergo acceleration at still longer times. Some experimental evidence for such behaviour has been obtained with the selenates. After about 25 Mrads at 57 ° radiation annealing of anhydrous sodium selenate had almost ceased, the increase from 12 to 25 Mrads being only about 2 ± 1% while 17 per cent of unannealed radioactive selenium remained. But between 25 and 30 Mrads the rate of radiation annealing increased appreciably and the percentage of unannealed radioactive selenium fell from 17 to about 6 per cent. The above model also accounts for the general lack of linearity of the plots of log ( 100 - R) vs. time during radiation annealing. Acknowledgement- One of the authors (F.R.A-S.) is indebted, for a Fellowship, to the Colombo Plan Authority.