Influence of Lead Impurity on the Low-Temperature Color-Center Production in KC1

The growth of optical absorption bands in the visible and ultraviolet as a function of 1.5-MeV electron irradiation at liquid-nitrogen temperature has been measured for high-purity and lead-doped KCl. Dissolved lead causes the coloration near 360 nm to be enhanced; the rate of growth increases monotonically with Pb concentration, in agreement with previous studies of trapped holes. The $F$ coloration is also enhanced by small amounts of impurity; however, the $F$-growth enhancement saturates at impurity levels of 20-100 ppm lead. An actual lowering of the $F$-center production rate is observed when samples doped with more than 100 ppm lead are used. This lowering is shown to be due to an increase in the $\alpha$-center-to-$F$-center ratio in highly doped KCl. An explanation is proposed for the increased $F$-center coloration in terms of interstitial-$F$-center recombination and interstitial stabilization by the impurities.