EFFECT OF IMPURITIES ON THE MAGNITUDE AND KINETICS OF STORED-ENERGY ACCUMULATION IN NaCl CRYSTALS AFTER PROTON IRRADIATION

E. K. ZAVADOVSKAYA, A. P. ARBUNOV, A. V. KUZMINA, V. V. BORISOVSKII

The article presents the results of a study of stored energy in NaCl crystals doped with anion-substituting impurities \((\mathrm{S}^{--}, \mathrm{OH}^{-})\). It was found that in “pure” NaCl crystals the stored energy is greater than in doped crystals. The introduction of anion-substituting impurities reduces the rate of defect generation. Different anion-substituting impurities affect the process of defect generation in NaCl in the same way.

An this paper the results of investigation of storage energy in NaCl crystals doped by anion—substituting impurities \((\mathrm{S}^{--}, \mathrm{OH}^{-})\) are described. At is determined that the storage energy is greater in «pure» crystals NaCl than in the doped ones. The introduction of anion—substituting admixtures decreases the rate of defects generating. Different anion—substituting impurities influence on the process of defects generation in NaCl in the same manner.

For radiation materials science, it is of interest to study the properties of materials subjected to the action of large irradiation doses. The use of optical methods for this purpose is not always possible because of the high concentrations of defects formed in them by irradiation. One of the most convenient methods for studying the total concentration of radiation defects is the measurement of stored energy. The magnitude of the stored energy is determined by the change in the free energy of a solid under the action of various external factors: plastic deformation, heat treatment, ionizing radiation, and can serve as a measure of the overall disturbances in a substance.

Theoretically, the most radiation-resistant structures, i.e., those that do not change their properties under the action of radiation, are ideal crystalline structures. In such lattices of alkali-halide crystals, an electron cannot become localized, while a self-localized hole exists only at a temperature of \(120 \div 130^\circ\mathrm{K}\) [1]. At room temperature, color centers cannot form in perfect crystals.

It is known from the literature [2, 3] that the formation of radiation defects in a crystal can be regulated by introducing an impurity. Thus, the introduction of a divalent cation-substituting impurity increases the rate of accumulation of \(F\)-centers in comparison with “pure” crystals owing to an increase in the concentration of cation vacancies. Cation vacancies promote the localization of holes, i.e., in a crystal with such an impurity the process of radiation destruction of defects is slowed.

A divalent anion-substituting impurity lowers the rate of accumulation of \(F\)-centers, and their concentration is smaller than in a “pure” crystal [4]. When such an impurity is introduced into a crystal, the concentration of anion vacancies increases; by the law of mass action these reduce the concentration of cation vacancies. This is confirmed by experimental results on measurements of the electrical conductivity of “pure” KCl and \(\mathrm{KCl}\cdot\mathrm{S}^{--}\). In crystals with a sulfur impurity, the electrical conductivity is lower than in the “pure” crystals.

The aim of the present work is to determine the influence of anion-substituting impurities on the magnitude and kinetics of accumulation of stored energy in NaCl crystals after proton irradiation.

As the irradiation source we used the cyclotron of the Research Institute of Nuclear Physics at the Tomsk Polytechnic Institute, with a proton energy of 4.5 MeV. The specimens were irradiated in air. To prevent luminescence, the crystals were wrapped in aluminum foil 7 microns thick. The crystals used for the investigation were “pure” NaCl, \(\mathrm{NaCl}\cdot\mathrm{S}^{--}\), and \(\mathrm{NaCl}\cdot\mathrm{OH}^{-}\), grown by the Kyropoulos method from chemically pure-grade salts.

The stored energy was measured by the dissolution method in a Calvet-type differential calorimeter manufactured at the Tomsk Polytechnic Institute [5].

Figure 1 presents kinetic curves for the accumulation of stored energy in “pure” NaCl, \(\mathrm{NaCl}\cdot\mathrm{S}^{--}\) (0.05 mole %), and \(\mathrm{NaCl}\cdot\mathrm{OH}^{-}\) (0.5 mole %)—curves 1, 2, and 3, respectively—at a radiation intensity of \(4.5\cdot10^{14}\) protons/cm\(^2\) per hour.

From Fig. 1 it is seen that the stored energy in “pure” NaCl is greater than in doped crystals. In investigating series of alkali-halide crystals, a regular relationship was found between the energy stored by the crystals and their lattice energies [6], namely: the greater the lattice energy, the higher the stored energy. When an impurity is introduced, the crystal lattice is loosened [7] and the lattice energy decreases. It follows from this that the stored energy in doped crystals will be less than in “pure” ones.

Fig. 1. Stored energy in “pure” NaCl crystals (curve 1), \(\mathrm{NaCl}\cdot\mathrm{S}^{--}\) (00.5 mole %) curve 2, and \(\mathrm{NaCl}\cdot\mathrm{OH}^{-}\) (0.5 mole %) curve 3.

When a crystal is irradiated, two processes occur simultaneously in it: the process of generation and the process of destruction of defects. In the initial period of irradiation, the generation process predominates over the destruction process. This is evidenced by the section of the kinetic curve for accumulation of stored energy in “pure” NaCl up to an absorbed dose of \(4\cdot10^{3}\) J/g. With an increase in absorbed energy, the process of defect destruction becomes more significant and appreciably affects the accumulation of defects; and at an absorbed energy of \(24\cdot10^{3}\) J/g a dynamic equilibrium is established between generation and annealing of defects. Saturation is characteristic of all the crystals investigated.

The accumulation of energy in $\mathrm{NaCl}\cdot\mathrm{S}^{--}$ (curve 2), up to an absorbed dose of $20\cdot 10^3$ J/g, is linear in character. With the introduction of a divalent anion-substituting impurity ($\mathrm{S}^{--}$), the concentration of cation vacancies decreases, i.e., the number of hole-localization sites decreases. In connection with this, the process of destruction of defects will be facilitated due to recombination of holes and electrons of $F$-centers and, as a consequence of this, the sensitivity to radiation coloration of $\mathrm{KCl}\cdot\mathrm{S}^{--}$ crystals will be lower in comparison with “pure” KCl [8]. An analogous result was obtained by us for NaCl and $\mathrm{NaCl}\cdot\mathrm{S}^{--}$ crystals. At one and the same absorbed energy ($8\cdot 10^3$ J/g), the defect concentration in $\mathrm{NaCl}\cdot\mathrm{S}^{--}$ is lower than in “pure” NaCl; this is indicated by the magnitude of the stored energy: 2 J/g and 5 J/g, respectively.

The rectilinear portion of the energy accumulation indicates that the process of defect generation may be associated with the action of a single mechanism—the Varley mechanism [9], according to which an ionized halogen is displaced from regular lattice sites into an interstice. Since, in a crystal doped with anion-substituting impurities, the process of destruction of radiation defects is facilitated, they will be formed only in the case when the holes created by irradiation are removed from the sites of their formation.

The additional defectiveness of the anion sublattice may promote the diffusion of chlorine into defective sites of the crystal.

Etzel [10] observed, in crystals with an impurity of hydroxyl ions, yellow fluorescence, which is attributed to oxygen ions, as well as growth of the $U$ band under the action of x-ray irradiation. This indicates that, in crystals with an impurity of hydroxyl ions, there are dissociated oxygen and hydrogen ions. Hydrogen ions, possessing electron-acceptor properties, localize electrons that appear in the process of interaction of radiation with the substance, with the formation of atomic hydrogen, which, apparently, subsequently does not exert a substantial influence on defect generation. Consequently, in the process of defect generation in $\mathrm{NaCl}\cdot\mathrm{OH}^{-}$, the predominant influence is exerted by oxygen ions, whose mechanism of action is analogous to the action of sulfur ions. This is indicated by the kinetic curves of stored-energy accumulation in $\mathrm{NaCl}\cdot\mathrm{S}^{--}$ and $\mathrm{NaCl}\cdot\mathrm{OH}^{-}$ (curves 2, 3, respectively).

Conclusions

1. An anion-substituting impurity decreases the stored energy. In “pure” NaCl the stored energy reaches 6.6 J/g, and in doped crystals 5.8 J/g, in the investigated dose interval.

2. The introduction of an impurity changes the kinetics of stored-energy accumulation.

3. Different anion-substituting impurities ($\mathrm{S}^{--}$, $\mathrm{OH}^{-}$) affect the process of defect generation in NaCl in the same way.

4. The rectilinear portion on the kinetic curve of energy accumulation indicates that there is a linear dependence between the stored and absorbed energies; i.e., from the measured value of the stored energy one may judge the absorbed energy in the dose interval investigated.

Literature

1. W. Kanzig, J. Phys. Chem. Solids, 17, 88 (1962).
2. W. Hayes, Q. M. Nichols, Phys. Rev., 174, 4, 993 (1960).
3. I. Ya. Melik-Gaikazyan, V. M. Lisitsyn, Solid-State Physics, 7, No. 7 (1965).

1. I. Ya. Melik-Taikazyan, L. I. Roshchina, M. I. Ignat’eva. FTT, vol. 7, 3465, 1965.
2. A. V. Kuz’mina, E. M. Golovchanskii. PTE, No. 5, 1966.
3. A. V. Kuz’mina. Dissertation, Tomsk, 1964.
4. G. A. Andreev. FTT, vol. 8, issue 2, 1966.
5. L. I. Roshchina, V. P. Klyukvina. FTT, 7, 2262, 1965.
6. Color Centers, Izd. IL, Moscow, 1958.
7. H. W. Etzel, D. A. Patterson. Phys. Rev., 112, 4, 1112 (1958).