Radiation-induced defects in CaF2: An electron-spin resonance and dielectric constant investigation

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Abstract
CaF2 single crystals have been irradiated at T=21 K and at 0 °C with electrons of 0.5 and 1 MeV energy. The low‐temperature irradiated samples exhibit electron‐spin‐resonance (ESR) spectra with signals due to F‐centers and an unchanged real component of the dielectric constant ε′. The room temperature irradiated crystals, on the other hand, present ESR‐spectra due to VF‐, U‐, and, possibly, to metallic‐colloid (g=2.003) centers as well as an increased (dose dependent) ε′. The introduced defects and the radiation‐induced Δε′ are investigated in a thermal annealing treatment up to 900 °C. Their recovery behaviour is complex and can be interpreted as a competition between growth and annihilation processes of metallic clusters estimated to have initially a radius of ∼200 Å. Thus, the appearance of a new ESR‐line near g=2.000 after an anneal at 200 °C indicates the possible transformation of colloids to larger units before vanishing. The annealing spectrum of Δε′ and the dose dependence of its substages support the ESR findings.