STORED ENERGY IN IRRADIATED SODIUM CHLORIDE

Irradiated NaCl crystals are heated from room temperature to 400 °C, and the energy released is measured by calorimetry F-center densities range from 3×1017 to 1.8×1018/cm3, which are low enough to be measured directly, rather than inferred, as in some previous work. There are three definite peaks in energy release: centered at 135 °, 250 °, and 350 °. The 250 °peak is the largest, and is associated with almost all of the coloring changes. The total stored energy is 12.4 eV per F center, of which 8.4 eV is released in the 250 °peak and 2.2 eV is released in the 135 °peak. Assignment of thermal energies to specific atomic processes is difficult, because there are so many processes but only three experimental peaks. Evidence is good that the 250 °peak is mainly a superposition of two unresolved peaks, of which the lower is due to the annealing of the F-center electrons, and the upper to the annealing of vacancies. We suggest that the 350 °peak comes from the annealing of M-center divacancies. We do not have good evidence with which to explain the large amount of energy in the 135 °peak. A calcium-doped crystal also releases energy in three peaks centered at these temperatures, but here most of the energy, and all of the bleaching, occur under the lowest-temperature peak. © 1969 The American Physical Society.