X-Ray-Induced First-Stage Coloring of NaCl

Abstract

The initial or first-stage $F$-center coloring of NaCl, irradiated with x rays at room temperature, has been carefully studied. Both Korth and Harshaw samples were used. The first-stage coloring is modified by changing the experimental conditions. These include altering the dose rate; subjecting the crystals to plastic deformation before irradiation; and cycles of coloring, bleaching, and recoloring. The observed curves of $F$-center concentration versus dose can be resolved into one linear and three additional components. Two of these can be considered saturating exponentials. The third, which does not occur in every curve, may also be a saturating exponential, but is too weak to characterize precisely. Changing the experimental conditions alters one or two of the exponential components but leaves the others unchanged. This suggests that each component is related to a separate process and that the resolution into components is not fortuitous. The linear component is unaffected by all the above-mentioned changes in experimental conditions. The exponential component which varies slowest, as a function of irradiation time, is independent of dose rate and prior coloring and bleaching. Its saturation level is increased by plastic deformation prior to irradiation, but only after the strain has exceeded a threshold value. Beyond this threshold strain, the saturation level increases linearly with strain. The saturation level of the next slowest component is unaffected by strain. However, it is increased by a coloring-bleaching-recoloring cycle and is a function of dose rate. This dose rate dependence is given by the expression: $C_1+C_2{I}^2$, where $C_1$ and $C_2$ are constants and $I$ is proportional to x-ray intensity. The saturation level of the third and fastest-changing component is always less than 10% of the total exponential contribution and is too small to study in detail. In the 185- to 350-mμ region both the absorption spectrum and the behavior under the various experimental conditions differ considerably in the two types of crystals. Also, the coloring curves seem to be strongly dependent on impurity content in a way that is also different in the two types of crystals.