AGGREGATE COLOR CENTER FORMATION PROCESSES IN LITHIUM FLUORIDE CRYSTALS AFTER IRRADIATION

Lithium fluoride crystals were irradiated by different doses of gamma photons at a temperature of 77 K. We measured the aggregation kinetics for the color centers with different annealing temperatures above the temperature of anion vacancy mobility. We show that the lifetimes of the vacancies decrease while the lifetimes of the F-2(+) centers increase as the irradiation dose increases. We explain these types of dependences based on the aggregation processes for color centers in the post-radiation period. We determine the time constants and energies (analogous to activation energies in the Arrhenius equation) for the various processes involving rise and fall in the concentration of aggregate color centers. Based on the experimental data obtained, we have established the processes forming F-2 and F-3(+) centers in the post-radiation period. The F-2 centers are formed when vacancies v(a) add to F-1(-) centers. Vacancies arising during irradiation of the crystal participate in their creation in the first fast stage. In the long final stage, vacancies are used which appear in the post-radiation period on occurrence of the reaction F-2(+) + H -> v(a) + fluoride ion at the lattice site, where H is an interstitial fluorine atom. The F-3(+) centers are formed both by merging F-2(+) and F-1 centers and as a result of addition of vacancies to F-2 centers. In this case, vacancies are used that are generated not only during irradiation of the crystal but also in the post-radiation period. The rise in the concentration of F-3(+) centers occurs faster than the rise in the concentration of F-2 centers.