Photoelastic Observations of the Expansion of Alkali Halides on Irradiation

Abstract

Alkali halide crystals subjected to a nonuniform irradiation by x-rays or high-energy deuterons are found to become anomalously birefringent. The effect has been studied in detail in lithium fluoride. It is shown that the birefringence is due to the photoelastic effect accompanying a nonuniform dilatation which is calculated from the measured birefringence with the aid of elasticity theory. The crystals are colored as a result of the irradiation; and from the absorption coefficients, the number of color centers per unit volume is calculated. It is found that the volume of vacancies per unit volume is approximately equal to the dilatation. A possible explanation for this result is that the vacancies associated with the color centers are generated at the jogs of edge dislocations during irradiation.