Neutron-induced damage in sapphire (α-Al 2 O 3 ) and its thermal evolution were studied using positron annihilation lifetime (PAL) and optical-transmission (OT) spectroscopy. Six samples were irradiated with reactor neutrons to fluences of (1–6)×10 18 n/cm 2 ; one was subsequently annealed isochronally to 1500 °C. PAL revealed saturated positron trapping in Al-vacancy-type defects, with a mean lifetime of 193 ± 1 ps across all fluences. OT showed strong oxygen-vacancy-related absorption: high-energy F-center bands became optically saturated at the lowest fluence, while F 2 2+ and F 2 + divacancy bands increased monotonically with fluence. During annealing, vacancy-cluster formation became evident from 540 °C and clusters largely dissolved near 1500 °C. A phenomenological Arrhenius-type analysis gave E_eff ≈ 0.21 eV, interpreted as a characteristic recovery energy scale rather than a microscopic migration barrier. Oxygen-related optical centers were no longer resolved near 1500 °C, whereas residual Al-vacancy-type defects persisted at ~10 17 cm −3 .
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Israelashvili et al. (2026) studied this question.
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