Effect of space charge on F centers near the stopping region of monoenergetic protons

Single crystals of sodium chloride were bombarded with 1--2-MeV protons from a Van de Graaff accelerator at low temperature (77 K) and at room temperature. In the case of low temperature, a layered structure was found to be developed within the irradiated part of the crystal. This was indicated by the color-center formation. The color-center distribution in the irradiated area along the particles' trajectories was studied using thin sections under a petrographic microscope and etched crystal surfaces in a scanning electron microscope. Three zones are visible: (1) strong color-center development in the upper region, followed by (2) a colorless layer, and (3) the end of the particles' path is marked by a deeply colored zone caused mainly by severe lattice damage. The relative degree of lattice damage throughout the irradiated region is clearly visible in scanning electron microscope photographs. During the irradiation, the terminal layer constitutes a region of positive charge. This space charge gives rise to a second-order Stark effect resulting in the annihilation of F centers immediately above this zone and is, therefore, responsible for the observed decoloration