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×${10}^{17}$ to 1.8×${10}^{18}$/${\mathrm{cm}}^3$, 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.