Point Defects Aggregation in Lithium Fluoride Crystals After Irradiation

Abstract

Lithium fluoride crystals (LiF) with radiation-induced color centers are widely used and studied. These crystals are well known materials for ionizing radiation dosimetry. LiF crystal is a model crystal to study radiation influence on solids.

Samples were irradiated at liquid nitrogen temperature, which is below the temperature of anion vacancy mobility (T_v). We measured the aggregation kinetics for the color centers at different annealing temperatures T which are above the temperature of anion vacancy mobility T > T_v.

We determine time constants and activation energies for the various processes involving growth and fall in the concentrations of aggregate color centers. The values of the lifetimes were found for vacancies and F ⁺₂ centers for different doses of irradiation. We show that a lifetime of vacancies decreases while that of F ⁺₂ centers rises with radiation doses increasing. We demonstrate that vacancies on the long-term stage of aggregation are created in the result of a reaction F ⁺₂ + H → υ _a + fluoride ion at lattice site, in which F ⁺₂ centers participate. Therefore on the long-term stage vacancies υ _a and F ⁺₂ centers are involved in complicated process, consisting of two reactions, which lead to maintenance of vacancies in crystal during annealing until F ⁺₂ centers exist in crystal.

Both vacancies and F ⁺₂ centers take part in process of F ⁺₃ formation at the long-term stage. The vacancies take part in formation of F ⁺₂ , F₂ and F ⁺₃ centers at the initial stage of the aggregation kinetic. The fast stage of growth in the concentrations of F ⁺₂ , F₂ and F ⁺₃ centers are characterized by different time constants. This is due to different time, required by vacancies to reach their partners in reactions to form these centers. The presence of F ⁻₁ centers in samples after irradiation is proved by comparison of the vacancy activation energy with the activation energy of process, which describes the initial stage of the F₂ center concentration growth.