Effect of Impurities on the Hardening and Coloration of KCl Irradiated at Room Temperature

Measurements of flow stress and F-band coloration have been made at room temperature on a number of ``pure'' and impurity-doped KCl crystals as a function of gamma and electron irradiation. As has been observed, the flow stress increase, for late-stage coloration, is proportional to the square root of the F-band absorption. However, it is found that the early-stage coloration does not contribute to the radiation hardening. These results suggest that the late-stage production mechanism for F centers involves interstitial formation and clustering, whereas no stable interstitials are formed during early-stage coloration. Moreover, even though it is known that certain impurities enhance early-stage coloration, it is found that the concentration of early-stage F centers is only a small percentage of the total impurity concentration. This suggests that only isolated positive-ion vacancies are involved in the F-center production mechanism for this stage.