Dielectric Losses in KBr and KJ Crystals Irradiated with Protons and X-rays
Abstract Generated abstract
This paper investigates radiation-induced dielectric losses in KBr and KI crystals exposed to protons and X-rays, addressing discrepancies in earlier observations for alkali halides. Temperature dependences of the dielectric loss tangent were measured after irradiation over specified absorbed-dose ranges, with comparison to unirradiated crystals and to associated optical coloration. Both proton and X-ray irradiation produced similar relaxation maxima near 125 to 135°C, with dose-dependent saturation behavior and activation energies estimated at 2.0 eV for KBr and 2.8 eV for KI. The results suggest that the relevant polarizing defects are complex quasidipoles involving cation vacancies trapped by larger defect complexes, while hole color centers appear necessary but not sufficient for their formation.
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DIELECTRIC LOSSES IN KBr AND KJ CRYSTALS IRRADIATED WITH PROTONS AND X-RAYS
Yu. M. Annenkov
Studies of the optical properties of alkali halides have led to the conclusion that different types of ionizing radiation generate identical color centers.
No such unity is observed when the dielectric characteristics of alkali-halide crystals are considered. The result of the interaction of reactor radiation with KBr crystals is an intense maximum on the curve of the thermal variation of $\operatorname{tg}\delta$ [1].
The data of work [2] indicate a decrease in the dielectric-loss angle in the range of audio frequencies after the action of X-rays on KCl single crystals.
If, however, X-rayed crystals are illuminated with light in the F-band of absorption, then in the temperature dependence of the dielectric losses, measured at ultrahigh frequencies, a series of peaks appears [3, 4].
One reason for the discrepancy among the experimental data of many authors may be considered to be differences in experimental conditions, which do not make it possible to trace changes in the dielectric characteristics of alkali-halide crystals after exposure to different types of radiation.
The comparatively recently published results of the work of V. V. Krasnopevtsev [5] indicate the presence of relaxation dielectric losses in KBr crystals after electron bombardment, which manifest themselves in a maximum of $\operatorname{tg}\delta$ at temperatures above 100°C.
In explaining the dielectric relaxation in KBr irradiated with electrons, V. V. Krasnopevtsev assumes the existence, in interstitial sites, of positive and negative ions formed as a result of multiple ionization of ions of the anion sublattice. Following Varley’s modeling of color centers [6], the author proposes relaxators in the form of associated $V_3$ centers and interstitial cations. The view expressed concerning the configuration of quasidipoles that cause an increase in $\operatorname{tg}\delta$ in irradiated crystals has a number of vulnerable points. The $V_4$ absorption band, which Varley identifies with the presence of interstitial metal ions in the lattice, is observed experimentally only at low temperatures.
In addition, a number of authors [7] consider more probable the model of a \(V_3\) center in the form of a molecular halide ion of the type \(X_3^-\), oriented in the \([011]\) direction and occupying two anion and one cation vacancies. Chemically this is equivalent to an \(H\)-center plus a cation vacancy and a hole. Such a center is inefficient as a trap for positive charge.
It is possible that the study of dielectric losses in alkali-halide crystals disturbed by other types of ionizing radiation will make it possible to clarify the question under discussion.
We investigated the effect of X-ray and proton irradiation on the dielectric-loss angle in KBr and KJ crystals. The proton energy was \(4.1\) MeV, and the dose absorbed by the crystals varied from \(2 \cdot 10^{22}\ \text{eV}/\text{cm}^3\) to \(5 \cdot 10^{23}\ \text{eV}/\text{cm}^3\). Irradiation with X-rays was carried out on a RUP-2 apparatus; the absorbed energy varied within the limits \(5 \cdot 10^{19}\ \text{eV}/\text{cm}^3\)—\(2 \cdot 10^{21}\ \text{eV}/\text{cm}^3\).
With increasing temperature, \(\operatorname{tg}\delta\) in crystals of KBr and KJ undisturbed by radiation changes monotonically, increasing with temperature. Figure 1 presents the temperature dependences of the dielectric losses for KBr single crystals, measured before and after irradiation with ionizing radiation.
Fig. 1. Temperature dependences of \(\operatorname{tg}\delta\) at 1000 Hz for:
1 — unirradiated KBr crystals irradiated with X-rays:
2 — \(1.3 \cdot 10^{20}\ \text{eV}/\text{cm}^3\),
3 — \(5.2 \cdot 10^{20}\ \text{eV}/\text{cm}^3\),
4 — \(1.6 \cdot 10^{21}\ \text{eV}/\text{cm}^3\),
5 — irradiated with protons \((2.0 \cdot 10^{23}\ \text{eV}/\text{cm}^3)\).
Proton bombardment of the crystals under study leads to a decrease in \(\operatorname{tg}\delta\) in the low-temperature region and to the appearance of an intense maximum at \(130\text{–}135^\circ\text{C}\).
It should be emphasized that changing the integral proton flux under the selected irradiation conditions does not affect the properties under study of irradiated potassium bromide.
X-irradiation of KBr crystals produces a pattern of radiation-induced change in dielectric losses analogous to proton bombardment.
However, an increase in the dose of X-rays absorbed by the crystals stimulates growth in the amplitude of the maximum and a shift of the peak toward higher temperatures. At absorbed doses greater than \(1.4 \cdot 10^{21}\ \mathrm{eV/cm^3}\), the shift in the temperature of the maximum ceases, and its magnitude is comparable with the effect after proton irradiation.
The broad temperature peak of \(\operatorname{tg}\delta\), appearing after exposure of KJ crystals to protons, lies within \(125\text{--}135^\circ\mathrm{C}\) (Fig. 2). The amplitude of the maximum increases, reaching saturation at doses greater than \(1 \cdot 10^{23}\ \mathrm{eV/cm^3}\).
Fig. 2. Dependences of \(\operatorname{tg}\delta\) on temperature at \(100\ \mathrm{cps}\) for KJ crystals:
\(\bullet\!-\!\bullet\!-\!\bullet\!-\!\bullet\) — unirradiated crystals;
\(\circ\!-\!\circ\!-\!\circ\!-\!\circ\) — irradiated with X-rays, \(1.3 \cdot 10^{21}\ \mathrm{eV/cm^3}\);
\(\times\!-\!\times\!-\!\times\!-\!\times\) — \(2.6 \cdot 10^{21}\ \mathrm{eV/cm^3}\);
\(\triangle\!-\!\triangle\!-\!\triangle\!-\!\triangle\) — irradiated with protons \((2.5 \cdot 10^{23}\ \mathrm{eV/cm^3})\).
(The numbers indicate the values of the irradiation energy absorbed by the crystal.)
In the temperature interval \(20\text{--}70^\circ\mathrm{C}\), the loss angle in proton-irradiated crystals is smaller than in unirradiated ones.
X-irradiation of potassium iodide leads to a decrease in dielectric losses over a broad temperature range up to \(170^\circ\mathrm{C}\). Only when the crystal absorbs X-ray energy of \(1.3 \cdot 10^{21}\ \mathrm{eV/cm^3}\) is it possible to observe the appearance of a temperature maximum of \(\operatorname{tg}\delta\) in the same region of the curve where an intense peak of the loss angle is observed for KJ after proton irradiation.
The positions of the temperature maxima of \(\operatorname{tg}\delta\), observed in colored KBr and KJ crystals, shift toward higher tempe-
ature in accordance with the activation energy of the relaxation process, equal respectively to 2.0 eV and 2.8 eV.
X-ray irradiation leads mainly to the appearance of the \(V_3\) and \(F\) absorption bands, which become more intense in the case of proton bombardment. Heating KBr and KJ crystals to \(200^\circ\mathrm{C}\) completely eliminates the maxima of \(\operatorname{tg}\delta\) and the coloration.
As was indicated, the temperature maximum of \(\operatorname{tg}\delta\) arising after irradiation of KJ crystals with protons decreases at absorbed-energy values smaller than \(10 \cdot 10^{22}\ \text{eV}/\text{cm}^3\). The dose of X-rays varied from \(5 \cdot 10^{19}\ \text{eV}/\text{cm}^3\) to \(2 \cdot 10^{21}\ \text{eV}/\text{cm}^3\). Consequently, the maximum energy of X-radiation absorbed by the specimen is still insufficient to generate intensively the defects that give rise to the clearly expressed temperature maximum of \(\operatorname{tg}\delta\) observed after proton bombardment.
Absorption of X-ray energy of \(2.5 \cdot 10^{21}\ \text{eV}/\text{cm}^3\) for KBr is sufficient to saturate the process of formation of defects responsible for the increase in \(\operatorname{tg}\delta\).
Consequently, comparison of radiation-induced changes in the dielectric properties of ionic crystals of different chemical composition with the absorbed dose of radiation may lead to incorrect quantitative estimates of the irradiation effect.
The experimental results presented in Fig. 1 and Fig. 2 clearly show that proton bombardment and X-raying cause the appearance in alkali-halide crystals of identical defects in concentrations proportional to the stored energy.
The activation energies of the relaxation processes taking place in irradiated KBr and KJ crystals are considerable and are greater for single crystals with lower lattice energy (2.0 eV for KBr, 2.8 eV for KJ).
Comparison of the activation energy for the orientation of polarizing defects in irradiated KBr and KJ with a similar characteristic of the relaxation process caused by associated vacancy pairs [8] indicates a complication of the structure of the quasidipoles on going to crystals with weaker bonding forces between ions.
The decrease in the electrical conductivity of alkali-halide crystals after X-ray [9] and proton [10] irradiation permits the supposition that the main part of the relaxing quasidipoles consists of cation vacancies.
Most likely, the quasidipoles are cation vacancies captured by definite traps, the dimensions of which are larger in KJ than in KBr.
Our results on the study of dielectric losses in KJ crystals irradiated with protons and bleached with white light indicate an insignificant influence of electron color centers on the magnitude of the maximum of \(\operatorname{tg}\delta\).
However, it is hardly possible to represent a trap of cation vacancies (or interstitial ions, as follows from the work of Krasnotsev) in the form of \(V_2\) or \(V_3\)-centers.
In the case of NaCl single crystals irradiated with protons and X-rays, we did not observe a relaxation maximum of \(\operatorname{tg}\delta\) analogous to the temperature peak of dielectric losses for KBr and KJ crystals disturbed by radiation, despite the fact that the concentration of hole centers in sodium chloride was greater than in KBr and KJ [8].
It is possible that the presence of \(V\)-centers is necessary but not sufficient for the formation of complex quasidipoles in irradiated alkali-halide crystals. The additional conditions are probably satisfied only in more loosely packed crystals (KBr, KJ). A joint study of the temperature stability of hole color centers and of defects causing relaxation in irradiated KBr and KJ, as well as an investigation of dielectric losses in additively colored crystals, will make it possible to model the polarizing complexes with greater confidence.
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