Color centers aggregation kinetics in lithium fluoride after gamma irradiation

Lithium fluoride crystals are irradiated at various doses by gamma rays at 77 K. The time evolution of photoluminescence signals from aggregated F2+, F2, F3+ and F3 color centers, and of the absorption intensity of primary F centers are measured at various annealing temperatures. The lifetimes of anionic vacancies υa and F2+ centers, the characteristic times of concentration growth of F2, F3+ and F3 centers, and also the activation energies of diffusion of vacancies and F2+ centers together with various processes of aggregation are determined. It is found that lifetime decreases for vacancies while increases for F2+ centers by increasing the irradiation dose. It is also shown that, after irradiation during annealing, vacancies are formed as a result of the reaction F2++H→υa+Fl−, where Fl− is a fluorine ion in a lattice site and H is a fluorine interstitial atom. Then these vacancies participate in color centers aggregation kinetics. The presence of F− centers in the irradiated crystal is established, and the processes which lead to the formation of F2, F3+ and F3 centers after irradiation, are unveiled.