Influence of Impurities on the Room-Temperature Coloration Processes in Alkali Halides

The $F$-center growth curves for undoped KCl and KCl crystals doped with Ba, Co, and Cd have been obtained by x irradiation at room temperature. These growth curves are fitted to an equation with two exponential terms and a linear term, and the parameters occuring in the equation are evaluated. Two exponential terms have been found to be adequate to describe the first-stage coloration, with which this paper is mainly concerned. The analysis shows that the slower exponential component is strongly influenced by the presence of impurities; specifically, the rate of coloration during the process responsible for this component decreases, and the saturation level for the process increases, on account of the impurities. The faster exponential component appears to be relatively insensitive to the presence of impurities. These results are discussed with reference to certain mechanisms that have been put forward to account for the first stage.