The presence of vacancies created by high-energy radiation in sodium chloride single crystals has been detected by comparing the $F$-center coloration produced by ultraviolet light after irradiation and optical bleaching to that obtained before irradiation. The behavior of the $F$-center coloration as a function of time of irradiation under ultraviolet light after exposure of the crystals to high-energy radiation is different for synthetic and natural sodium chloride. This difference is attributed to the presence of hydroxyl ions in the synthetic crystals which are inadvertently introduced into the lattice during the growth of the crystals in air. The results obtained are the same whether 40-kvp x-rays, 2-Mev electrons, or ${\mathrm{Co}}^{60}$ $\gamma$ rays are used to create the $F$ centers.

• Received 16 June 1959

DOI:https://doi.org/10.1103/PhysRev.116.885