Effect of X-Ray Intensity on the Production of Color Centers in NaCl and KCl

The results of a study of the effect of irradiation intensity on the coloration of NaCl and KCl are reported. Three observations stand out: (1) The equilibrium established between $F$ and complex centers shifts to favor $F$ centers with increasing intensity of irradiation. Cycling of the irradiation intensity between low and high values produces reversible shifts in the relative density of $F$ and complex centers. (2) The density of $F$ centers produced by uv irradiation of a crystal following optical bleaching of a prior x irradiation increases with the intensity of the prior x irradiation. The density of uv-produced $F$ centers could also be shifted in a reversible manner on cycling the intensity of x irradiation. (3) The intensity dependence of the $F$-center growth curves is present only at irradiation temperatures above 200°K. It is proposed that the intensity dependence of the $F$-center coloration arises from the establishment of a balance between two competing processes. It is proposed that the primary effect of x-ray photons is to disperse single vacancies in the crystal. Competing with this dispersal is the tendency of the anion vacancies to reaggregate by the diffusion of mobile vacancy units.