The growth of colloidal centres in irradiated alkali halides

The alkali halides are very susceptible to radiation damage in the halogen sublattice when exposed to ionizing radiation, the primary products being F-centres and interstitial halogen atoms (H-centres). Experimental observations have shown that clustering of the H-centres leads to the formation and growth of perfect interstitial loops. Further recent studies by Hobbs (1975) on the growth of colloidal Na centres by F-centre aggregation in NaCl irradiated to high doses have shown several features in common with the growth of voids in heavily irradiated metals. In this paper we present a theoretical model suggested by this analogy and by other knowledge of defects and radiation damage in the alkali halides. Though some of the parameters needed for quantitative predictions are not known accurately enough, the properties of the model in general are in good agreement with the available, rather limited experimental data on colloid growth in NaCl. For example, the predicted dependence of colloid growth upon temperature and dose are correct. Also the predicted increase of the F-centre concentration with time in the early stages of irradiation appears to provide an explanation of some otherwise puzzling features of earlier observations of F-centre production at temperatures where these defects are mobile. More stringent tests of the theory would nevertheless be provided by more extensive measurements of the growth of colloids and of the accompanying F-centre concentrations.