Computer simulation of coloring curves in alkali halides by using a heterogeneous nucleation model

A heterogeneous interstitial nucleation model for the radiation-induced F-coloring in alkali halides is here presented. It applies to the temperature region where H centers are mobile, although comparison with ex periments is primarily restricted to the behavior near room temperature. The model assumes the creation of F and H pairs as the primary event, as well as secondary thermally-activated interstitial processes. These include recombination with F centers as well as capture by empty traps (very likely impurities) and by all clusters formed after successive interstitial trapping. Therefore, a heterogeneous nucleation model is assumed for the interstitial-aggregation. As the key point of the model it is proposed that one of the interstitial centers (that containing two trapped interstitials) formed in the process of clustering is highly unstable. The model is able to account for the major features of the coloring in most alkali halides (such as NaCl and KC1): The three-stages structure of the coloring curve, and the inhibition in the occurrence of the late-stage by lowering the dose-rate or by impurity doping.