EFFECT OF IMPURITIES ON THE THERMAL ANNEALING OF RADIATION-CHEMICAL DEFECTS IN POTASSIUM NITRATE CRYSTALS

V. M. Lykhin, A. N. Oblivantsev, V. V. Boldyrev

(Presented by the scientific seminar of the Laboratory of Dielectrics and Semiconductors)

A number of works have shown that defects in crystals affect the behavior of recoil nuclei generated in solid ionic salts upon the capture of thermal neutrons by the Szilard—Chalmers reaction, and also affect the subsequent process of thermal annealing [1–3]. However, there are almost no data on the influence of defects on the annealing of chemical disturbances occurring in solid ionic salts under the action of ionizing radiation [4], and the influence of impurities on the annealing of radiation-chemical changes has not been studied at all, although it is known [5] that, in obtaining radioactive isotopes in a nuclear reactor by the Szilard—Chalmers reaction, radiation-chemical decomposition of the irradiated compounds also occurs under the action of the reactor gamma background, which complicates the separation of the isotopes obtained.

In the present work, the influence of heterovalent impurities \( \mathrm{Sr}^{++} \), \( \mathrm{Pb}^{++} \), and \( \mathrm{SO_4}^{--} \), introduced into potassium nitrate crystals, on the kinetics of isothermal annealing of its radiation-chemical defects after gamma irradiation was studied.

Experimental Part

To introduce impurities, potassium nitrate of “analytical grade” purity was brought to melting, after which, at a temperature of \(340^\circ\mathrm{C}\), the calculated amount of an additive of strontium nitrate, lead nitrate, or potassium sulfate was added to it with stirring. The resulting melt was cooled to room temperature. Preparations were made containing \(2.5\) mol % \( \mathrm{Sr}^{++} \), \(2.5\) mol % \( \mathrm{Pb}^{++} \), and \(1.5\) mol % \( \mathrm{SO_4}^{--} \). The additives [6], selected from the melting diagrams of salt systems, can, when fused with potassium nitrate, form solid substitution solutions with the generation, in the case of strontium and lead additives, of cation vacancies, and in the case of sulfate ion, of anion vacancies, similarly to what occurs when thallium, calcium, and lanthanum additives are introduced by fusion into potassium chromate [1].

The crushed samples of the fraction smaller than \(0.02\) mm were sealed in glass ampoules measuring \(3 \times 10\) cm and irradiated in a cobalt isotope gamma unit with a dose of 208 Mrad. The absorbed dose rate was 400 rad/sec, and the irradiation temperature was \(35^\circ\mathrm{C}\). The irradiated

samples were stored in a desiccator over \(P_2O_5\). In potassium nitrate, which upon radiation-chemical decomposition gives nitrite and oxygen, after dissolution in water the nitrite content was determined photocolorimetrically [7]; the amount of nitrite decreased during thermal annealing as a result of recombination of the nitrite ion with oxygen formed during radiolysis and trapped by the lattice. Isothermal annealing was carried out in a U-8 thermostat in an air atmosphere. The accuracy of temperature control was \(\pm 0.1^\circ\mathrm{C}\). The fraction annealed was calculated as the ratio [8]

\[ a=\frac{(\mathrm{NO}_2^-)_{0}-(\mathrm{NO}_2^-)_{t}}{(\mathrm{NO}_2^-)_{0}}, \]

where \((\mathrm{NO}_2^-)_{0}\) is the initial content of nitrite ion in the irradiated crystals,

\((\mathrm{NO}_2^-)_{t}\) is the content of nitrite ion after the annealing time has elapsed.

The results of annealing at \(130^\circ\mathrm{C}\), in coordinates of fraction annealed—annealing time, are shown in the figure. It is seen from the figure that practically no annealing is observed in pure potassium nitrate; the introduction of an anionic additive likewise does not affect the annealing process. The introduction of cationic additives, however, has a very strong effect on the annealing process, increasing the annealing rate at the initial moment, after which a slowing of the process is observed, leading to saturation and the fraction annealed. The fraction annealed after 3 hours of heating at \(130^\circ\mathrm{C}\) was 0.55 in the case of the strontium nitrate additive and 0.75 in the case of the lead nitrate additive.

Fig. 1. Isothermal annealing at \(130^\circ\mathrm{C}\) of potassium nitrate irradiated with a dose of 208 Mrad:

1 — pure \(\mathrm{KNO_3}\);
2 — \(\mathrm{KNO_3} + 1.5\) mol% \(\mathrm{K_2SO_4}\);
3 — \(\mathrm{KNO_3} + 2.5\) mol% \(\mathrm{Sr(NO_3)_2}\);
4 — \(\mathrm{KNO_3} + 2.5\) mol% \(\mathrm{Pb(NO_3)_2}\).

Conclusions

It has been shown that the introduction of heterovalent cationic additives into potassium nitrate increases its sensitivity to post-radiation thermal annealing.

Literature

1. T. Andersen, A. G. Maddock, Trans. Faraday Soc., 59, 2362, 1963.
2. A. G. Maddock, J. I. Vargas, Nature, 184, 1932, 1959.
3. T. Andersen, A. G. Maddock, Nature, 194, 371, 1962.
4. S. R. Mohanty, S. R. Upaghyay, Nature, 199, 4889, 1963.
5. A. N. Murin, R. V. Bogdanov, S. M. Tomilov. Uspekhi khimii, vol. 33, issue 5, 1964.
6. Handbook on the Melting Behavior of Systems of Anhydrous Inorganic Salts, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1961.
7. B. V. Mikhal’chuk and R. E. Osherovich. Zavodskaya laboratoriya, 9, 8, 1940.
8. S. R. Mohanty, S. R. Upaghyay, J. Inorg. Nucl. Chem., 25, 3, 1963.