ON THE QUESTION OF THE SO-CALLED OPTIMAL TEMPERATURE OF RADIATION COLORING OF ALKALI METAL HALIDES

B. I. Pankrats, E. M. Uskov

(Presented by the scientific seminar of the Department of Radiation Chemistry)

When comparing the kinetic curves for the accumulation of $F$-centers of color in alkali metal halides under the action of ionizing radiation at different temperatures, obtained by various authors [1, 2, 3], the difference in their form is striking.

In specimens X-rayed at low temperatures—for example, the boiling points of liquid helium and liquid nitrogen—the concentration of $F$-centers of color is directly proportional to the dose absorbed by the specimen, whereas at room temperature, in specimens of the same chemical composition and at the same absorbed dose, the concentration of $F$-centers of color reaches saturation.

Bauer and Gordon [4] observed that both the rate of accumulation and the finally attained value of the density of $F$-centers of color at saturation depend very strongly on temperature, and that there exists a temperature at which a given concentration of $F$-centers of color is reached at the minimum absorbed dose.

We investigated alkali metal halides (NaCl, KCl, RbCl, KBr, and KI) irradiated with X-rays in the temperature range $78$–$300^\circ\mathrm{K}$. Special cryostats were designed and fabricated which made it possible to carry out X-irradiation and optical studies of specimens on an SF-4 spectrophotometer, and also to study the processes of optical and thermal destruction of color centers induced by irradiation. X-irradiation and optical measurements of the specimens were performed coaxially. The vacuum in the cryostat—not worse than $1 \cdot 10^{-3}$ mm Hg—was maintained continuously. The absorbed energy was calculated from the ratio of the mass absorption coefficients in the irradiated crystal and in air, and was also determined experimentally with an X-ray meter.

As Riehl [5] has shown, by changing the thickness of the specimen—as well as by changing the energy of the incident X-ray beam—significant changes in the spectral distribution of photoelectrons in the crystal can be produced. Fröhlich [6], in order to ensure uniform coloring of crystals, used very thin specimens ($\sim 0.2$ mm).

We used specimens $0.25$–$0.30$ mm thick, cleaved from a single crystal grown by the Kyropoulos method.

In work [3] it is noted that the soft component of X-ray radiation causes surface effects. The use only of the hard component of penetrating radiation makes it possible to avoid

large surface concentrations of color centers. To filter the soft component of the X-ray radiation, we used an aluminum filter 2.4 mm thick.

Irradiation was carried out on a URS-70 X-ray unit (BSV-1 W tube). The concentration of \(F\)-centers was calculated by Smakula’s formula.

Fig. 1. Growth curves of the optical density of KCl in the \(F\)-band as a function of irradiation time at different temperatures: 1 — dependence of the optical density in the \(F\)-band at \(78^\circ\mathrm{K}\); 2 — at \(123^\circ\mathrm{K}\); 3 — at \(163^\circ\mathrm{K}\); 4 — at \(290^\circ\mathrm{K}\); 5 — at \(198^\circ\mathrm{K}\); 6 — at \(273^\circ\mathrm{K}\); \(X\) — optical density of the X-rayed specimen, \(X_0\) — background, \(\sigma\) — specimen thickness.

In the crystals studied, irradiation at intermediate temperatures induces considerably higher concentrations of \(F\)-color centers. As can be seen from Fig. 1, the initial rate of accumulation of \(F\)-centers in KCl, for example, increases with increasing temperature, but then, however, decreases if irradiation is carried out at a temperature above \(273^\circ\mathrm{K}\).

Thus, it may be asserted that in the temperature interval \(78\text{–}300^\circ\mathrm{K}\) there exists a temperature at which a given concentration of \(F\)-centers is generated with the minimum absorbed dose.

It has been established by several investigators [2, 5] that the efficiency of accumulation of \(F\)-centers at low temperatures (for example, the boiling temperature of liquid helium) does not depend on the initial defectiveness, but is an intrinsic property of the given crystal and depends only on its chemical composition. The curves of the coloring rate of crystals at the temperature of liquid helium are analogous to the curves of the coloring rate at the temperature of liquid nitrogen (with the exception of the curves for KCl, for which, at \(5^\circ\mathrm{K}\), Ritz observed an initial nonlinearity that could not be explained). The similarity of the curves indicates that at these temperatures essentially the same processes take place.

It is reasonable to suppose that at sufficiently low temperatures the processes of migration of ions and vacancies are strongly suppressed, and the accumulation of \(F\)-centers occurs at the expense of vacancies generated by irradiation. Processes of radiation annealing at these temperatures also practically do not occur.

With increasing temperature, however, the specimen becomes more and more structure-sensitive. The concentration of coloration \(F\)-centers arising at post-radiation vacancies makes an increasingly appreciable contribution at one and the same absorbed dose.

Fig. 2. Temperature dependence of the concentration of coloration \(F\)-centers \(n_F\) in alkali-metal halides at equal-dose absorption: 1—NaCl; 2—KCl; 3—RbCl; 4—KBr; 5—KI.

Thus, at a certain temperature (the latter depends on the chemical composition of the specimen under study), a dynamic equilibrium is observed between the processes of formation and annealing of coloration \(F\)-centers.

Frenkel, Landau, and Pekar showed that at low temperatures self-localization of holes is possible. Delbecq, Hayes, and Yuster [7] believe that self-localization of holes occurs in the temperature interval \(100\text{--}250^\circ\mathrm{K}\) for crystals of different chemical composition. Elango established [8] that in the temperature interval \(300\text{--}450^\circ\mathrm{K}\) intensive ionic processes begin in crystals.

The experimental results obtained by us confirm the idea [9] that, for the effective creation of \(F\)-centers in ionic crystals at the first stage of x-ray coloration, the presence of mobile holes is necessary.

The efficiency of accumulation of coloration \(F\)-centers at the optimal temperatures (Fig. 2) decreases in the series
\[ \mathrm{NaCl}\to\mathrm{KCl}\to\mathrm{RbCl}\to\mathrm{KBr}\to\mathrm{KI}. \]
Consequently, the radiation yield depends on the magnitude of the free volume of the elementary cell of the crystal, representing the difference between the volume of the elementary cell in the crystal lattice and the volume of the ions forming it.

LITERATURE

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