Gamma-Induced Divalent Dysprosium in Calcium Fluoride

Experiments dealing with photochemical and thermal reactions of dysprosium in calcium fluoride are described. Trivalent dysprosium ions doped in calcium fluoride have been reduced by gamma irradiation at room temperature. The efficiency of reduction of Dy3+ in CaF2 is described in terms of the dosage of gamma irradiation and concentration of the dopant ions. The optimum conditions for the photoreduction are given. The absorption band responsible for the reoxidation of the gamma-reduced dysprosium has been determined. It has been established that high densities of gamma-induced defect sites are necessary for the conversion of the majority of divalent ions to the trivalent state, which may be indicative of the tunneling of Dy2+ electrons to hole centers via the gamma-induced defect centers. The tunneling process is also believed to be responsible for the saturation of local and macroscopic concentrations of Dy2+ ions. The thermal reoxidation of Dy2+ ions is accompanied by bright luminescence, the spectrum of which resembles that of the photoluminescence of Dy3+ ions. The decay of the thermoluminescent emission bands is nonexponential, and the reaction kinetics fit the description of a second-order reaction. The evaluated activation energy (0.33 eV) is indistinguishable from the energy required for the thermal ionization of holes from V1 centers in alkali halides.