DESTRUCTION OF F-CENTERS IN ALKALI-HALIDE CRYSTALS BY X-RAYS

M. M. ARINSHTEIN, I. Ya. MELIK-GAIKAZYAN

As a result of the action of ionizing radiation on alkali-halide crystals, electron-hole processes of two types occur in them: the formation of color centers and the reverse process—their destruction. Taking into account the processes of destruction of color centers is necessary for a correct explanation of the kinetics of their accumulation. In [1] a kinetic equation was proposed that contains constants determining the rate of formation and destruction of \(F\)-centers:

\[ n_F = n^* \left(1 - e^{-b^* t}\right) + a^* t - \frac{a^*}{c^*}\left(1 - e^{-c^* t}\right), \tag{1} \]

where \(n_F\) is the density of \(F\)-centers,

\[ n^* = \frac{n_0 b}{b + \alpha} = \frac{n_0}{1 + \frac{\alpha}{b}}, \qquad b^* = b + \alpha, \]

\(n_0\) is the initial density of vacancies in the crystal;
\(b\) is the constant determining the rate of formation of \(F\)-centers;
\(\alpha\) is the bleaching constant,

\[ a^* = \frac{ac}{c + \beta}, \]

\(a\) is the rate of formation of new vacancies;
\(c\) is the rate of transformation of new vacancies into \(F\)-centers;
\(\beta\) is the constant of their bleaching.

To determine the unstarred quantities from experimental data, one more equation is necessary. In [1] it was assumed that

\[ \beta \ll c. \tag{2} \]

From the fact that \(a^* \sim I^2\), they concluded that \(a \sim I\). Abramson and Kaspari [2] showed experimentally that \(a \sim I\), and doubted the correctness of assumption (2). In [3], the condition \(a \sim I\) was chosen as an additional equation and, on the basis of a comparison of two kinetic ...

from the curves at different intensities, the value of the destruction constant \(\beta\) was calculated for crystals of different chemical composition.

In a number of works, a direct effect of the bleaching action of X-rays has been observed for atomic centers and \(F\)-centers generated by these same rays in alkali-halide crystals [4—12].

In considering the kinetics of relaxation processes in alkali-halide phosphors, Ch. B. Lushchik and his coworkers studied the distribution of electrons and holes over trapping levels.

It was found that complete filling of the activator trapping levels with electrons does not occur. If, after the excitation of the phosphor by X-rays has been switched off, it is illuminated in the \(F\)-band, then further filling of the trapping levels with electrons occurs. Repeated X-irradiation again decreases the degree of filling of the trapping levels with electrons. This experiment is direct evidence of the bleaching action of X-rays. The presence of a bleaching action of X-rays leads to the impossibility of complete filling of trapping levels with electrons. These data confirm Antonov-Romanovskii’s ideas concerning the role of the bleaching action of the exciting light.

It is clear that this applies not only to activator levels, but also to anion vacancies.

If, in X-irradiated crystals after the end of X-irradiation, or during it, a supersaturated concentration of certain absorption centers is created with the aid of an additional factor, then, after removal of the additional factor, continued X-irradiation will cause a decrease in the concentration of these centers until the concentration becomes the equilibrium one.

In the experiments carried out by Ch. B. Lushchik and his coworkers, a supersaturated concentration of \(F\)-centers was created owing to illumination in the \(F\)-, \(M\)-, and atomic bands during irradiation of the crystal. After the illumination was stopped, the concentration of these centers decreased in the course of subsequent irradiation by X-rays.

In Harten’s experiments, a supersaturated concentration of \(F\)-centers, as compared with the concentration at room temperature, was achieved by irradiating the crystal at a temperature of \(-20^\circ\mathrm{C}\). Heating the crystal to room temperature destroys part of the centers, but their concentration nevertheless remains higher than under irradiation at room temperature. Further X-irradiation of the crystal lowers the concentration of \(F\)-centers, driving it toward the equilibrium value corresponding to room temperature.

In Harrison’s work the bleaching action of X-rays was discovered upon decreasing the irradiation intensity. Harrison develops the idea of Belar [14] and Przibram [15] that the saturation level of the concentration of \(F\)-centers is the equilibrium level between the formation of \(F\)-centers and their destruction, dependent on the irradiation intensity. Then the magnitude of the bleaching action of X-rays upon decreasing the intensity from \(I_1\) to \(I_2\) can be predicted from the saturation level under irradiation with intensity \(I_2\). This effect appears at all stages of coloration after saturation has been reached.

We have attempted to trace the process of destruction of \(F\)-centers in the transition from soft to hard X-radiation.

In setting up our experiments we proceeded from the following considerations: as a result of the absorption of X-rays in a crystal, free electrons, holes, and excitons arise. Localization

electrons at trapping centers, predominantly at anion vacancies present in the crystals before irradiation as part of vacancy pairs, as well as at vacancies generated by irradiation, leads to the formation of \(F\)-centers. Simultaneously with the formation of \(F\)-centers, their destruction also occurs; moreover, under the given experimental conditions the excitation and de-excitation factors of a given crystal are a function of the hardness of the radiation. This may be due to the different dependence on quantum energy of the cross section for the formation of electrons, holes, and excitons and of the cross section for radiative destruction of \(F\)-centers. In addition, it may be caused by a change in the conditions for hole localization when the hardness of the X-radiation is changed, and also by differences in the exciton states of the crystal.

Experiments were carried out with pure and activated KCl crystals of grade KhCh at room temperature. The samples were cleaved to a thickness of 0.2–0.3 mm. The source of X-rays was a URS-70 apparatus with tubes having a copper anticathode (soft radiation), operating at 47 kV, 15 mA, and with a tungsten anticathode (hard radiation), operating at 63 kV, 8 mA. When the tungsten tube was used, the radiation was filtered by an aluminum filter 2.04 mm thick. The absorption spectrum was measured on an SF-4 spectrophotometer. The experiment shows that if a limiting concentration of \(F\)-centers has been attained in a crystal irradiated with soft X-rays, then exposure of this crystal to hard radiation will lead to their destruction (Fig. 1). To determine how a change in the intensity of irradiation affects the result under the conditions of our experiment, the intensity of the hard radiation was reduced by a factor of 9. The difference in the results for the exposure time used lies within the experimental error, which indicates a considerably stronger influence of the change in hardness. Keeping the crystals in the dark for 12 to 36 hours caused only slight bleaching in comparison with the effect described above.

Fig. 1. 1 — curve of accumulation of \(F\)-centers in KCl irradiated with hard X-rays; 2 — the same, with soft X-rays. 3 — destruction of \(F\)-centers by hard X-rays.

The bleaching action of X-rays depends to a considerable degree on the purity of the crystals. This effect increases in crystals activated by additives having high acceptor properties with respect to electrons (Fig. 2), and decreases in crystals activated by additives that improve the conditions for hole localization (Fig. 3) [16].

Thus, our experiment shows that the equilibrium level between the processes of generation and destruction of \(F\)-centers depends not only on the intensity [12] or temperature [11], but also on a number of other conditions, including the hardness of the radiation. The establishment of a mobile equilibrium between generation and bleaching of \(F\)-centers

centers will occur in any case, if at the initial moment this equilibrium is disturbed in any way. Whether the establishment of equilibrium will lead to coloring or bleaching of the crystal depends on the direction in which the nonequilibrium concentration of \(F\)-centers differed from the equilibrium one.

This process can be described by means of the differential equation [1]

\[ \frac{dn_F}{dt}=bn_i-\alpha n_F, \tag{3} \]

where \(n_i\) is the density of vacancies that can be transformed into \(F\)-centers. In all the experiments considered, the irradiation conditions are varied after equilibrium has been reached between the number of \(F\)-centers created from primary vacancies and the \(F\)-centers destroyed by the radiation. In addition, let us consider the process over a time interval sufficiently short that the generation of vacancies may be neglected. Then

Fig. 2. 1 — accumulation curve of \(F\)-centers in KCl·Sr irradiated with hard X-rays; 2 — the same, with soft X-rays; 3 — destruction of \(F\)-centers by hard X-rays.

\[ n_i=n_0-n_F \]

and equation (3) takes the form

\[ \frac{dn_F}{dt}=b\,(n_0-n_F)-an_F. \tag{4} \]

The solution of this equation is

\[ n_F=n^*-\left[n^*-n_F(0)\right]e^{-b^*t}, \tag{5} \]

where \(n_F(0)\) is the initial density of \(F\)-centers.

The equilibrium density of \(F\)-centers \(n^*\) is reached after a sufficiently long irradiation time. It depends on the ratio of the bleaching and generation constants of the \(F\)-centers. Changing the irradiation conditions changes these constants, which leads to a change in \(n^*\) and in the rate of the process \(b^*\).

Fig. 3. 1 — accumulation curve of \(F\)-centers in KCl·Pb irradiated with hard X-rays; 2 — the same, with soft X-rays; 3 — destruction of \(F\)-centers by hard X-rays.

The initial condition \(n_F(0)\) for a new stage of the process will be the final \(n_F\) of the preceding stage. For \(n^*>n_F(0)\), equation (5) describes the growth kinetics of the density of \(F\)-centers; for \(n^*<n_F(0)\), the kinetics of their destruction.

If one discards the simplification that we made in passing from (3) to (4), the result will be more cumbersome, but qualitatively the same.

In our experimental arrangement both the intensity and the hardness of the radiation were varied; this led to a change in the parameter \(b\), proportional to the radiation intensity [2], and also to an increase in the parameter \(\alpha\), which is independent of the intensity.

At present it is difficult to understand the reason for the increase in the constant of radiation decay with increasing quantum energy. One can only suppose that, as the quantum energy increases, the fraction of energy expended by the radiation on the excitation process increases in comparison with the ionization process. On the other hand, excitons generated upon excitation of lattice ions may, by interacting with \(F\)-centers, ionize the latter, which will lead to an intensification of the process of radiation destruction of \(F\)-centers.

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