THERMAL STABILITY OF RADIATION DEFECTS IN SODIUM CHLORIDE

E. K. ZAVADOVSKAYA, E. M. GOLOVCHANSKII, A. V. KUZ’MINA, A. P. AR’YANOV

The annealing of defects in heavily irradiated alkali-halide crystals was studied in works [1, 2]. In work [1] it was shown that the annealing process in NaCl crystals irradiated with high-energy protons occurs in three stages.

The release of energy is associated with the destruction of Frenkel defects, \(F\)-centers, and vacancy aggregates in the first, second, and third stages, respectively. During annealing of NaCl heavily irradiated with gamma rays it was found that the stored energy is released in two stages [2, 3]. In these works the authors consider that both Frenkel defects and \(F\)-centers, which are formed in the second stage of coloration, are responsible for the low-temperature peak of energy release (maximum at \(150^\circ\mathrm{C}\)). The \(F\)-centers formed at pre-radiation vacancies are responsible for the high-temperature maximum of energy release.

In the present work, isothermal and nonisothermal annealing of defects was investigated in NaCl crystals heavily irradiated with protons (proton energy 4.5 MeV, exposure dose \(6.8 \cdot 10^{14}\) protons/cm\(^2\)).

1. Isothermal annealing

Isothermal annealing of defects was carried out at temperatures of 100, 150, and \(200^\circ\mathrm{C}\) in the following manner. A batch of samples for investigation of the stored energy and absorption spectra was placed in a furnace heated to the above temperatures, held in it for a definite time, and then, by means of a special device, the samples were thrown onto a cotton-wool pad. After the samples had cooled, the release of stored energy was determined and the change in optical absorption was studied.

In Fig. 1 (\(a, b\)) are shown the dependences of the change in stored energy and microhardness on annealing time at different temperatures. In Fig. 2 is presented the isothermal annealing (\(200^\circ\mathrm{C}\)) of color centers. Let us compare the dependences obtained at an annealing temperature of \(200^\circ\mathrm{C}\).

During 2 minutes of annealing at this temperature, the microhardness of the irradiated crystal is practically equal to the microhardness of unirradiated NaCl, and in the optical absorption spectrum a number of complex bands appear, which overlap with the \(F\)- and \(M\)-bands in such a way that it is impossible to follow the annealing of \(F\)- and \(M\)-centers. During this same time the stored

the energy decreases to 50% of the initial value. Further annealing leads to complete destruction of the \(E\)- and \(M\)-bands and to the appearance of a band at \(578\ \mathrm{m}\mu\); in this case a large part of the stored energy is released (about 75%). After 47 minutes of annealing at a temperature of \(200^\circ\mathrm{C}\), the crystals were completely bleached. Complete destruction of the color centers

Fig. 1. a) Dependence of the change in stored energy on annealing time at different temperatures for NaCl crystals.
Fig. 1, b) Change in the microhardness of irradiated crystals after annealing at different temperatures.

is accompanied by the release of the main part of the stored energy (about 90%). Annealing for 6 hours leads to complete destruction of the radiation defects.

From the comparison presented above of the destruction of color centers and the release of energy at an annealing temperature of \(200^\circ\mathrm{C}\), it is evident that electron centers are responsible for the storage of energy in \(\mathrm{NaCl}\) crystals.

A comparison of the annealing of the defects responsible for the change in microhardness with the annealing of the defects responsible for the stored energy and optical absorption makes it possible to conclude that the increase in microhardness after irradiation is associated with defects that do not make a substantial contribution to the stored energy. Such defects may be either vacancy clusters of a definite size or hole centers.

From the obtained regularities of annealing of radiation defects (Fig. 1) we shall determine the activation energy and the order of the annealing reaction. The curves presented can be described by the kinetic equation [4]

\[ \frac{dn}{d\tau}=An^x \exp\left(-\frac{E_A}{\kappa T}\right), \tag{1} \]

where \(n\) is the concentration of defects in the crystal,

\[ \frac{dn}{d\tau} \]

is the rate of decrease of defects,

\(E_A\) is the activation energy for the displacement of defects,

\(x\) is the reaction order, and \(A\) is the frequency factor.

Since the stored energy is proportional to the defect concentration, let us replace \(dn/d\tau\) in equation 1 by the experimentally obtained decrease in stored energy. After substitution, integration, and taking logarithms, we have:

\[ \ln \int_{E_0}^{E_t}\frac{dE}{E^x}=\ln A+\ln \tau-E_A/\kappa T. \tag{2} \]

From the isothermal annealing curves for different temperatures one can find the activation energy from the following expression:

\[ E_A=0.862\cdot 10^{-4}\, \frac{\ln \dfrac{\tau_2}{\tau_1}} {\dfrac{1}{T_2}-\dfrac{1}{T_1}}. \tag{3} \]

As is seen from expression (3), the activation energy does not depend on the reaction order; however, for its calculation it is necessary that, in the given temperature interval, the activation energy be constant. To determine the reaction order, we plot \(\ln \ln E_t/E_0\) versus \(\ln \tau\). The linear dependence obtained indicates that, in the temperature range \(100\text{–}200^\circ\mathrm{C}\), the annealing of radiation defects is described by a first-order equation. The activation energy calculated from the isothermal annealing data in the temperature interval \(100\text{–}150^\circ\mathrm{C}\) (Table 1) is close to the activation energy of a cation vacancy, obtained experimentally in studies of electrical conductivity [5].

Table 1

Activation energy of defects in NaCl irradiated with protons

Unlike metals, in alkali-halide crystals it is impossible, from the activation energy, to determine the type of defects that contribute to the stored energy.

energy. Annealing of defects in these compounds is due to the migration of cation vacancies and is accompanied both by the destruction and by the coalescence of defects into larger aggregates.

Fig. 2. Change in the absorption spectra of sodium chloride crystals irradiated with protons (dose \(6.8\cdot 10^{14}\) protons/cm\(^2\)) after annealing at a temperature of 200°C for different times.

2. Nonisothermal annealing

Nonisothermal annealing of irradiated NaCl crystals was carried out at a heating rate of about \(1.4^\circ\ \mathrm{min}^{-1}\). Upon reaching a given temperature, several crystals were removed from the furnace, and the stored energy in them was determined by the dissolution method [6]. Figure 3 shows the dependence of \(E_t/E_0\) on the annealing temperature, where \(E_0\) is the energy stored in the crystals after irradiation (at a dose of \(6.8\cdot 10^{14}\) protons/cm\(^2\) it was equal to...

was about 8.5 J/g), \(E_t\) is the energy stored in the specimens after annealing to the indicated temperature. The annealing of radiation defects proceeds in three stages. The maxima of energy release are the same in different batches of specimens. The maximum of energy release at 218°C is poorly resolved and therefore is not shown in Fig. 3. In addition to measuring the stored energy during nonisothermal annealing, the change in the optical absorption of specimens at various annealing temperatures was investigated (Fig. 4). Because of the high concentration of \(F\)-centers, the maximum of the \(F\)-band did not shift up to 140°C. In this temperature range the annealing of \(F\)-centers may be characterized

Fig. 3. Dependence on annealing temperature of \(E_t/E_0\) (curve 1) and \(\dfrac{dE_t/E_0}{dt}\) (curve 2)

by the growth of the \(M\)- and more complex bands. Up to 150°C an increase in the \(M\)-band is observed, along with the appearance, on its long-wavelength slope, of a new band whose exact position could not be determined. In this same temperature interval the absorption in the region of 940–950 mµ increases. As the temperature rises, the absorption between the \(F\)- and \(M\)-bands increases. At temperatures of 150–180°C, intensive coagulation of \(F\)- and \(M\)-centers into complex centers occurs. A series of \(R\)-bands appears. Further heating of the specimens leads to the appearance of a band at 578 mµ, which anneals without further transformation. At a temperature of 220°C the crystal is completely decolorized.

Comparison of the change in stored energy as a function of annealing temperature with the change in the optical absorption spectra makes it possible to explain the two-stage character of stored-energy release in the following way. It is known that the form of the annealing curve of \(F\)-centers depends on the degree of imperfection of the crystal lattice [7]. The less perfect the crystal structure, the more complex the character of the annealing curve of \(F\)-centers. Therefore, in contrast to Kobayasi, we believe that the release of energy in the temperature range 100–160°C is due both to partial recombination of \(F\)-centers and to the aggregation of

them into more complex centers. The release of energy in the temperature range 160–200°C is associated with the destruction of \(F\)- and \(M\)-centers, which is accompanied—

Designations:

1 — \(t = 24^\circ\mathrm{C}\)
2 — \(t = 100^\circ\mathrm{C}\)
3 — \(t = 110^\circ\mathrm{C}\)
4 — \(t = 120^\circ\mathrm{C}\)
5 — \(t = 130^\circ\mathrm{C}\)
6 — \(t = 140^\circ\mathrm{C}\)
7 — \(t = 150^\circ\mathrm{C}\)
8 — \(t = 160^\circ\mathrm{C}\)
9 — \(t = 170^\circ\mathrm{C}\)
10 — \(t = 180^\circ\mathrm{C}\)
11 — \(t = 190^\circ\mathrm{C}\)
12 — \(t = 200^\circ\mathrm{C}\)
13 — \(t = 210^\circ\mathrm{C}\)
14 — \(t = 220^\circ\mathrm{C}\)

Vertical axis: optical density
Horizontal axis: wavelength, mµ

Fig. 4. Change in the absorption spectra of sodium chloride crystals irradiated with protons (exposure dose \(6.8 \cdot 10^{14}\) protons/cm\(^2\)) as a function of annealing temperature

—is accompanied by the formation of a band at 578 mµ. The destruction of the band at 578 mµ is accompanied by the release of energy with a maximum at 218°C.

The position of the energy-release maxima in the present work differs somewhat from the literature data [1, 2]. Apparently, this is connected with differences in the quality of the crystals studied, as well as in the rate at which they were heated.

Conclusions

Thus, as a result of the study of the thermal stability of defects in NaCl crystals irradiated with protons, the following conclusions may be drawn:

1. The low-temperature maximum of energy release is due to coagulation and partial recombination of \(F\)-centers.

2. The thermal destruction of radiation defects occurs as a result of ionic processes.

References

1. K. Kobayashi. Phys. Rev. 102, 348, 1956.
2. A. V. Kuz’mina. Dissertation. Tomsk Polytechnic Institute (1962).
3. E. K. Zavadovskaya, A. V. Kuz’mina. Izv. vuzov, Fizika, No. 1, (1965).
4. N. Hennen. Z. angew. Phys. 15, 1, (1963).
5. R. W. Dreyfus, A. S. Nowick. J. Appl. Phys. Supl. 33, 1, (1962).
6. E. K. Zavadovskaya, A. V. Kuz’mina, E. M. Golovchanskii. PTE* (in press).
7. V. I. Sidlyarenko et al. Izvestiya vuzov, Fizika, No. 5, (1965).