Impurities and Secondary Reactions in Radiation Defect Production at Liquid-Nitrogen Temperature in Alkali Halides

Abstract

The $F$-center production rate due to electron irradiation at 80°K has been measured in pure KCl and KBr, in KCl doped with Ag, Ca, Cd, Co, and Tl, and in KBr doped with Ca and Cl. In every case an enhancement of the defect production rate was observed in samples containing between 1 and 100 ppm impurity. Accompanying the enhancement of the $F$-center production rate was a saturating growth of ultraviolet absorption, usually in the $V_1$ region of the spectrum, and a temporary suppression of the growth of the intrinsic ultraviolet absorption band (240 nm for KCl and 275 nm for KBr, sometimes labeled $V_4$). In KCl samples containing more than 100 ppm impurity, the 240-nm band was observed to shift to shorter wavelengths. In addition, the stability of the defects produced in doped samples was found to be less upon warming or bleaching than that of pure samples. The $F$-center growth curves have been compared with predictions of a model in which free interstitials can either recombine with vacancies or be trapped by defects. The curve shapes suggest that the model in its simplest form is not valid, and that perhaps most of the interstitials and vacancies remain correlated.