Radiation Defect Production and Annihilation in KCl near Room Temperature

Extended $\gamma$ irradiation of KCl near room temperature causes the $F$-center concentration to saturate between ${10}^{18}$ and ${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The saturation concentration is a function of the dose rate and the temperature, and is the same whether approached by production of defects in a fresh sample or by destruction of an excess number of defects in a sample that had been preirradiated under different conditions of temperature or dose rate. The temperature dependence of the saturation level can be described by an empirical activation energy of 0.4 eV between 10 and 50°C. The approach to saturation is approximately 50 times slower than the approach to the aggregation steady state, suggesting that mobile vacancies must travel much further to annihilate than to form $F$-aggregate centers. Observations that the approach to saturation cannot be described by simple kinetics and that the shape of decay curves depends upon previous irradiation conditions support the suggestion that the size of the centers with which vacancies annihilate depends upon previous irradiation conditions.