STORED ENERGY IN SODIUM CHLORIDE SINGLE CRYSTALS AFTER PROTON IRRADIATION

A. A. VOROB'EV, E. K. ZAVADOVSKAYA, A. V. KUZ'MINA
E. M. GOLOVCHANSKII

(Presented by the seminar of the Laboratory of Energetics and Electronics of Ionic Structures,
Research Institute of Physics and Technology)

When radiations interact with solids, defects of the crystal structure are formed in the latter. This entails a change in certain physical properties of solids and an increase in their free energy. The increase in free energy determines the magnitude of the stored energy. Stored energy encompasses all defects of the crystal lattice and gives a measure of the overall disturbance of the solid.

Solids are complex combinations of substances with various types of chemical bonding. To establish the regularity of the influence of irradiation on complex structures, study must begin with the simplest model crystalline solids, which are single crystals of alkali-halide salts.

In the present work, single crystals of sodium chloride were investigated. In works [1, 2] it was shown that the stored energy in NaCl under identical conditions of proton and gamma irradiation is significantly greater than in such alkali-halide crystals as KCl, KBr, and KJ. This makes it possible to study the stored energy in NaCl crystals in greater detail.

Stored energy in NaCl single crystals was previously investigated after irradiation with protons of energy 350 MeV [2]. The kinetics of accumulation of stored energy was studied after gamma irradiation [3] and X-ray irradiation [4]. In NaCl polycrystals, stored energy was measured after reactor irradiation [5]. From the results of these works one may judge the influence of various types of irradiation on the magnitude of stored energy in NaCl.

The purpose of the present work was to study the regularities of the influence of exposure dose (absorbed energy), intensity of proton irradiation, purity of the crystal, and its defectiveness on the magnitude and nature of the accumulation of stored energy in NaCl.

Irradiation of NaCl single crystals was carried out on the cyclotron of the Tomsk Polytechnic Institute with protons of energy 4.5 MeV through a thin (7 μm) layer of foil, which protected the samples from bleaching during irradiation. The irradiation-dose range studied was from \(4.5 \cdot 10^{13}\) protons/cm\(^2\) to \(1.35 \cdot 10^{16}\) protons/cm\(^2\). The irradiation intensity was varied by a factor of 7, from \(1.5 \cdot 10^{11}\) protons/cm\(^2\)·sec to \(9.5 \cdot 10^{11}\) protons/cm\(^2\)·sec.

To measure the stored energy, the dissolution method was used. Two samples (irradiated and reference) were simultaneously dissolved in two wells of a microcalorimeter [6]. From the difference between the heats of dissolution, the magnitude of the stored energy was estimated. The experiments showed that in irradiated crystals the heat of dissolution is smaller than in unirradiated ones, i.e., the energy expended on destroying the crystals by dissolving them decreases after irradiation.

Fig. 1 shows the change in stored energy as a function of the absorbed energy in NaCl crystals after irradiation with protons of different intensities. As can be seen from this graph, the magnitude

Fig. 1. Change in stored energy as a function of absorbed dose in NaCl crystals irradiated by a proton beam of different intensity:
—○—○—○— intensity \(1.5\cdot 10^{11}\) protons/cm\(^2\),
—△—△—△— intensity \(3.0\cdot 10^{11}\) protons/cm\(^2\),
—×—×—×— intensity \(4.5\cdot 10^{11}\) protons/cm\(^2\).

of the stored energy depends in a complicated way on the absorbed energy and does not depend on the irradiation intensity. At all investigated values of the irradiation intensity, characteristic features are observed in the accumulation of energy: a) the rate at the initial, linear stage of accumulation does not depend on the irradiation intensity; b) at all irradiation intensities, a maximum appears at the same absorbed energy; c) the magnitude of the maximum does not depend on the irradiation intensity; d) the minimum value of the stored energy is observed at the same absorbed energy; e) after the decrease in stored energy, a tendency toward a renewed increase in its value is again observed, which

is most clearly manifested at an intensity equal to \(4.5 \cdot 10^{11}\) protons/\(\text{cm}^2 \cdot \text{s}\).

We previously confirmed experimentally the assumption that, under certain irradiation conditions of NaCl, the main contribution to the stored energy is made by \(F\)-centers [4]. It is known that the rate of accumulation of \(F\)-centers in alkali-halide crystals increases when divalent cation-substituting impurities are introduced into them. In the present work we investigated the effect of a calcium impurity on the stored energy in NaCl.

Figure 2 gives the dependence of the magnitude of the stored energy on the absorbed energy in NaCl with different contents of calcium impurity (in the charge). In this dependence the following regularities may be noted:

Fig. 2. Dependence of the stored energy on the absorbed dose in NaCl with different contents of calcium impurity: curve 1—calcium content (in the charge) \(1\ \text{mol}\%\), curve 2—calcium content (in the charge) \(0.1\ \text{mol}\%\), curve 3—a relatively “pure” crystal (possible calcium content \(0.001\ \text{mol}\% \sim 0.005\ \text{mol}\%\)).

a) the stored energy in NaCl crystals containing calcium impurity is considerably smaller than in “pure” crystals (by “pure” crystals we mean those in which the calcium content is not more than \(0.01\ \text{mol}\%\)); b) the rate of accumulation of stored energy at small absorbed energies increases with increasing impurity content; c) in impurity-containing crystals the maximum in the kinetics of stored-energy accumulation is considerably smaller and is shifted into the region of lower absorbed energies; d) at an absorbed energy approximately equal to \(16 \times 10^3\ \dfrac{\text{J}}{\text{g}}\), saturation of the stored energy is observed.

In the work, the kinetics of accumulation of stored energy was investigated in NaCl single crystals of different origin, namely: in single crystals grown from the melt by the Kyropoulos method and in rock salt. The results obtained are given in Fig. 3. As can be seen, in NaCl single crystals of different origin, under identical

under irradiation conditions (irradiation intensity \(1.5 \cdot 10^{11}\) proton/\(\text{cm}^2 \cdot \text{s}\)) both the magnitude of the stored energy and the character of its accumulation differ strongly.

From consideration of the dependences obtained (Figs. 2, 3) it may be concluded that the magnitude of the stored energy is strongly affected by the impurity and by its distribution in the crystal. It is known that in rock salt the impurity does not enter the lattice, but is located at defective sites of the crystal lattice, mosaic blocks, and dislocations. With such a distribution, it possibly does not take part in the generation of defects. Defects are generated in the crystal lattice itself, for example, by the Varley mechanism, and the accumulation of defects proceeds more slowly than in an impurity crystal and does not reach the critical concentration at which the coagulation process begins. Therefore, in the kinetics of stored-energy accumulation in rock salt there is no maximum. In impurity crystals the impurity affects both the magnitude and the position of the maximum.

Fig. 3. Accumulation of stored energy in NaCl single crystals grown from the melt and in rock salt as a function of absorbed energy: curve 1—NaCl grown from the melt, curve 2—rock salt.

Consequently, it may be assumed that the maximum in the kinetics of stored-energy accumulation is associated with the impurity located in the lattice. More precisely, the impurity provides an additional source for the generation of defects. Therefore their critical concentration is reached more rapidly.

After the critical concentration is reached, the process of coagulation of defects begins and, possibly, the process of their mutual annihilation under the influence of the irradiation itself. The critical concentration is reached at one and the same absorbed dose. This process also explains the presence of a maximum in the kinetics of stored-energy accumulation. The question then arises why, in impurity crystals, although the rate of accumulation is higher at small absorbed doses, the stored energy does not reach a value as large as in “pure” crystals.

In work [7] it was shown that the process of formation of complex defects under proton irradiation is determined by the multiplicity of localization of defects. The impurity in the crystal is distributed nonuniformly and with a high local concentration. Therefore, in impurity crystals the local concentration of defects will be higher, and the process of coagulation and mutual annihilation of defects should begin earlier; because of this the maximum in the accumulation kinetics appears earlier, and then saturation sets in, since the process of the transformation \(F \rightleftarrows M\) in impurity crystals is suppressed [8].

In the present work the stability of the stored ...

energy in NaCl. Figure 4 shows the dependence of the stored energy on the storage time of these crystals in the dark at room temperature. During the first week of storage the stored energy changes only slightly. It then gradually decreases to a stable value of $\sim 4$ J/g. The change in stored energy with storage time also indicates the presence in the crystals of a certain nonequilibrium concentration of defects. In the case of the dependences shown in Figs. 1 and 2, irradiation itself causes the stored energy to decrease to a certain stable state, corresponding to a stored energy equal to $\sim 4$ J/g. During storage of irradiated samples, possibly the same processes occur in them that lead to the formation of a more stable state of defects, which also corresponds to a stored-energy value of 4 J/g.

Fig. 4. Change in stored energy in NaCl with storage time in the dark

Conclusions

Thus, as a result of studying the accumulation of stored energy after proton irradiation of NaCl single crystals, the following conclusions can be drawn:

1. The accumulation of stored energy has a complex character, determined by the presence and distribution of calcium impurity in the crystal.

2. The presence of calcium impurity increases the rate of energy accumulation at the initial stage of irradiation, but decreases its magnitude at large irradiation doses.

3. The stored energy is determined by the absorbed dose and does not depend on the intensity of proton irradiation.

4. The change in stored energy during storage of irradiated crystals occurs in two stages.

References

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6. E. K. Zavadovskaya, A. V. Kuzmina, E. M. Golovchanskii. Instruments and Experimental Techniques, 5, 217, 1966.
7. D. I. Vaysburd. ZhTEF, Chem. 10, 1965.
8. V. M. Lisitsyn, I. Ya. Melik-Gaikazyan. Izv. TPI, vol. 140, 1966.