EFFECT OF THE CONCENTRATION OF DIVALENT METAL IMPURITIES ON THE RATE OF RADIATION-INDUCED DESTRUCTION OF \(M\)-CENTERS

S. M. MINAEV

The effect of the concentration of divalent impurities \(Ca^{++}\), \(Sr^{++}\), and \(Pb^{++}\) in alkali-halide crystals on the process of radiation-induced destruction of \(M\)-centers is investigated, and the conclusion is drawn that delocalized holes take a substantial part in their destruction.

Introduction

It was shown by the EPR method [1] that in \(KCl + SrCl_2\) impurity hole color centers are associates of a divalent metal ion and a cation vacancy \(Me^{2+}V_k\).

G. K. Vale [2] found an increase in the number of \(Pb^{++}\) and \(Sr^{++}\) ions in crystals of \(NaCl—Sn\), \(KCl—Sn\), and \(KCl—Pb\) x-rayed at \(100^\circ K\), after illuminating them with light from the \(F\)-band. This effect is interpreted as a manifestation of hole capture by defects of the type \(Pb^{2+}V_k\) and \(Sn^{2+}V_k\).

Depending on the conditions, traps of the type \(Me^{2+}V_k\) can serve as capture centers both for holes and for electrons.

G. K. Vale, G. K. Zolotarev, T. A. Kuketaev, N. E. Lushchik, and Ch. B. Lushchik [3] are inclined to believe that at low temperatures, for such a dipole, the effective capture cross section for a hole is larger than the effective capture cross section for an electron.

At room and higher temperatures, the possibility of capturing a larger number of holes upon the introduction of \(Ca^{++}\), \(Sr^{++}\) impurities is noted in a number of works [4, 5].

Experimental Procedure and Results

In order to reduce the role of the “hole component” in the process of radiation-induced destruction of \(M\)-centers, based on the use of the dependence of the volume of their coagulation on temperature, crystals with different concentrations of hole-acceptor impurity were investigated.

Salts of \(NaCl\) and \(KCl\) of “chemically pure” grade were used as the base; \(CaCl_2\), \(SrCl_2\), and \(PbCl_2\) of “chemically pure for analysis” grade were used as impurities. The crystals were grown from the melt by the Kyropoulos method.

The impurity concentration was determined by a complexometric method [6—8]. (The activating impurity was additionally monitored before excitation by absorption in the activator band, and the nonactivating impurity by the hole absorption band near 350 nm, which appears after excitation.)

Crystals of the same host, but with different impurity contents, were loaded into the cryostat in batches of 5 and irradiated with protons simultaneously. The temperature of the crystal mount was measured with a copper–constantan thermocouple, was controlled by changing the flow rate of liquid nitrogen (in the region below room temperature), and was stabilized to an accuracy no worse than \(\pm 2^\circ\mathrm{K}\).

To measure the spectra of additional absorption, an SF-4A instrument was used as the monochromator, with a recording attachment developed by us [9], consisting of an FEU-53, 83-27 unit and an ÉTsPV-3 digital-printing voltmeter. The measurement of the absorption spectra was based on the comparison method.

The initial concentration of \(M\)-centers up to \(10^{17}\ \mathrm{cm}^{-3}\) was accumulated at room temperature. Then rapid cooling to the required temperature was carried out, and the nonequilibrium concentration of \(M\)-centers was measured. Subsequent irradiation reduced the concentration of \(M\)-centers.

The character of the radiation-induced destruction of \(M\)-centers in NaCl—CaCl\(_2\), KCl—SrCl\(_2\), and KCl—PbCl\(_2\) with different impurity concentrations is presented in Fig. 1.

Fig. 1. Radiation-induced destruction of \(M\)-centers under the action of protons with intensity
\(I = 2.08 \cdot 10^{16}\) protons/\(\mathrm{cm}^{2}\,\mathrm{s}\) and energy 6.8 MeV at low temperatures in crystals:

a) NaCl—CaCl\(_2\): ○○○ — 0.0026 M %, ××× — 0.0066 M %, △△△ — 0.0186 M %;

b) KCl—SrCl\(_2\): ○○○ — 0.0029 M %, ××× — 0.0069 M %, △△△ — 0.0116 M %;

c) KCl—PbCl\(_2\): ○○○ — “pure,” ××× — 0.0006 M %, △△△ — 0.0008 M %.

From Fig. 1 (a, b, c) it is seen:

1) that, with an increase in the concentration of the impurities $\mathrm{Ca}^{++}$, $\mathrm{Sr}^{++}$, and $\mathrm{Pb}^{++}$, which at low temperatures exhibit hole-acceptor properties, the intensity of destruction decreases for each of the fixed temperatures;

2) the intensity of radiation-induced destruction of $M$-centers increases sharply in the region of hole delocalization.

Consequently, it may be supposed that delocalized holes make a substantial contribution to the process of radiation-induced destruction of $M$-centers.

REFERENCES

1. W. Hayes, G. Nickols. Phys. Rev., 117, 993 (1960).
2. G. K. Vale. Tr. IFA AN ESSR, No. 23, 133 (1963).
3. G. K. Vale, G. K. Zolotarev, G. A. Kuketaev, N. E. Lushchik and Ch. B. Lushchik. Izv. AN SSSR, ser. fiz., vol. XXX, No. 4 (1966).
4. Ch. B. Lushchik. Investigation of Capture Centers in Alkali-Halide Crystals, Tartu, 1955.
5. F. N. Zaitov, V. Ya. Kark. Tr. IFA AN ESSR, No. 6, 82 (1957).
6. G. Schwarzenbach. Complexometry. Goskhimizdat, 1958.
7. R. Prishibil. Complexes in Chemical Analysis. Moscow, IL, 1960.
8. M. L. Ananievskaya, L. G. Shchekaturana. Manual of Chemical Analysis of Water. Novocherkassk, 1960.
9. S. Minaev, N. Terent'ev, V. Shkatov. Izv. vuzov, Physics, Proceedings of the Interuniversity Conference on Radiation Physics, 1967 (in press).