Effect of Plastic Deformation on theγ-Ray Coloration of NaCl Crystals

Measurements are made at room temperature of the growth of the $F$-band in NaCl crystals, under gamma irradiation in a ${\mathrm{Co}}^{60}$ source of constant intensity. Crystals of different origins, heat treatments, and states of deformation, are examined. While deformation has only a relatively small effect on the colorability in the early stage of $F$-band growth, the effect on the later stage is quite striking. The annealed crystals appear to saturate sharply at a value close to ${10}^{17}$ $F$-centers/${\mathrm{cm}}^3$, except for a small linear increase with time, while the deformed crystals continue to show a high growth rate in the later stage of irradiation. Upon optical bleaching, the deformed crystals show $R$-bands, while undeformed crystals irradiated for the same time show no absorption maxima in the visible range. These and other observations confirm the concept advanced earlier that the first stage is due to vacancies and vacancy clusters present in the crystal prior to irradiation (which are determined by the impurity content of the crystal) while the second stage involves the generation of new vacancies at dislocations.