Structure Sensitivity of the X-Ray Coloration of NaCl Crystals

Room temperature measurements of the rate of coloring of NaCl crystals by x-rays at different depths below the irradiated surface and for different states of deformation and heat treatment are reported. From the results it is concluded that two mechanisms of coloring operate in these crystals. The first, or "rapidtype" coloring, approaches a saturation density of $F$-centers of the order of ${10}^{17}$/${\mathrm{cm}}^3$, and appears to result from the generation of color centers from vacancies already present in the unirradiated crystal. The second, or "slow-type" coloring, takes place at a constant rate until $F$-centers in excess of ${10}^{18}$/${\mathrm{cm}}^3$ are formed. This type of coloring, which is usually observed only near the irradiated surface, is due to the generation of $F$-centers at dislocations, and is responsible for the hardness and density changes produced by the x-rays. Rapid-type coloring is found to occur at essentially the same rate in deformed crystals and in carefully annealed crystals; recovery at low temperatures after deformation, however, decreases the colorability. These results indicate that the principal effects of deformation and heat treatment on colorability may be related to the state of dispersion of impurities.