Annealing of Dislocation Pinning Centers in the Temperature Range 120° to 180°K in Alkali Halide Crystals

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Abstract
The ultrasonic attenuation of several alkali halides has been studied at temperatures between 120° and 180°K and at frequencies of 10 and 30 Mc/sec. The effects of γ irradiation and light in pinning and unpinning dislocations are in agreement with the observations at lower frequencies of Bauer and Gordon. In addition a thermal annealing of the γ‐ray‐induced pinning has been found in NaCl, KCl, and, to a limited extent, in KI. The effect was not present to a measurable extent in NaBr or KBr. The annealing is analyzed in terms of a concentration of pinning points which appear to anneal with two activation energies in NaCl and in KCl, all in the range from 0.01 to 0.15 eV. Following the model of Bauer and Gordon, we suggest that the anneal is related to the changes occurring in this temperature range in the V bands and that the activation energies may correspond to thermal ionization of holes from V centers.