Density Change of Sodium Chloride Produced by Proton Irradiation and Its Thermal Annealing

The density change of sodium chloride crystals upon irradiation with 350 Mev protons with a total flux of 1×${10}^{16}$ protons/${\mathrm{cm}}^2$ has been measured and found to be -8.3×${10}^{-4\mathrm{}}$ g/${\mathrm{cm}}^3$. The number of negative-ion vacancies calculated from this density change is of the same order as the number of $F$ centers estimated from the stored-energy measurements. The recovery of the density change with thermal annealing has also been studied. A relation between the behavior of the density change and of color centers is discussed.