Measurements and a detailed analysis were made of the thermal annealing of the $\alpha$ band in KBr x rayed at temperatures below 20°K. Four distinct annealing stages at 11, 17, 19, and 21°K were resolved, the first three stages corresponding to first order reactions and the fourth stage to a second order reaction. The activation energies for the annealing stages were about 0.015, 0.03, 0.04, and 0.06 eV, respectively. The first three stages were interpreted as a correlated recombination of close pairs of bromide vacancies and interstitials, and the fourth stage as the recombination of more distant defect pairs through a free migration of an interstitialcy. The activation energy 0.06 eV is interpreted as the free migration energy of a bromide-ion interstitialcy. Interaction energies between a vacancy and an interstitial ion in several configurations were calculated and possible annealing sequences for the first three annealing stages are proposed. The observed low pre-exponential factor for the first stage is also discussed.

• Received 9 June 1964

DOI:https://doi.org/10.1103/PhysRev.137.A1010