Vacancy migration and void formation in γ-irradiated ice

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Abstract
Positron annihilation techniques (PAT) have been used to study the effects in ice of γ irradiation at −196°C and of subsequent heating, both in poly‐ and monocrystalline samples. The main effects of irradiation are (1) the appearance in positron lifetime spectra of two long‐lived components (1.2 and 2.3 nsec) ascribed to ortho‐positronium (ortho‐Ps) trapping in radiation created mono‐ and divacancies, (2) a strong decrease of the intensity of a short‐lived component in the spectra, associated with para‐Ps, and (3) an increase in the width of Doppler broadening spectra. The two latter effects are ascribed to Ps inhibition and conversion by radiation created OH radicals. Heating above −165°C makes both effects disappear in agreement with radiation chemistry results on OH. Heating also increases the longest lifetime up to 11 nsec at −130°C. This is explained as vacancy migration leading to void formation. A vacancy migration energy is obtained, Em=0.34±0.07 eV, around three times higher than the previously assumed value. The advantage in this kind of study of using PAT, which are specifically sensitive to vacancy type defects, is pointed out.