Radiation hardening of rocksalt

This report gives indentation hardness results for irradiated (60Co γ-irradiated and ion implanted) natural and synthetic NaCl for irradiation temperatures less than 950C. Both rocksalt and synthetic NaCl increase in indentation hardness (by a factor of about 1.7) when exposed to γ-ray doses in excess of 1010 rads at temperatures less than 950C. The large optical densities produced make it very difficult to assess the importance, if any, of colloidal Na upon the change in hardness. This difficulty has been circumvented by using ion implantation to get high concentrations of defects in a thin surface layer. Annealing of such implanted layers produces colloidal Na. Increases in hardness by a factor of about 2.1 have been found by this technique when only the colloidal Na band is present in the optical absorption. It is important to note that the colloidal band disappears in a matter of hours at 1700C and thus the changes in hardness we have noted will depend critically upon the interface temperature in the repository, the accumulated dose, and the time of exposure at a given temperature