Thermal annealing of structural defects in fast-neutron-irradiated Al₂O₃ single crystals

Sufficient radiation resistance against prolonged irradiation is an important requirement seriously limiting the range of materials suitable for application in the harsh environment of future fusion reactors. Due to high tolerance to neutron irradiation as well as good optical and mechanical properties, aluminium oxide in the form of single crystals or transparent polycrystalline ceramics is a promising candidate for using as optical window material. The detailed understanding of the processes of primary lattice defect formation, aggregation and thermal annealing of radiation-induced damage is necessary to improve the radiation resistance of material. This paper presents a study of the process of annealing radiation damage in α-Al₂O₃ single crystals induced by fast fission neutrons (energy > 0.1 MeV at ~60 °C, fluence ~6.9 ×10¹⁸ n/cm²). The damage was analyzed via radiation-induced optical absorption (RIOA) in the 2–5.6 eV region containing several absorption bands related to oxygen vacancy dimers (F₂-type centers). The annealing of RIOA at 295–1100 K was registered in a stepwise regime: an irradiated sample was heated in highly pure argon atmosphere to a certain temperature T_i, kept at T_i for 10 min and rapidly cooled down to 295 K, at which all spectra were measured. For precise analysis of individual defect-related bands, decomposition of the RIOA spectra into elementary Gaussians was performed. Additionally, spectrally integrated (1.9–4.1 eV range) thermostimulated luminescence with heating rate 2 K/s was registered for temperatures up to ~850 K. The Gaussian components of the RIOA with the maxima at 4.08, 3.45 and 2.75 eV have been considered as a measure of the F₂, F₂⁺ and F₂²⁺ centers, respectively. Unlike F and F⁺ centers (an oxygen vacancy with two/one trapped electrons), the concentration of which continuously decreases at the sample heating up to 1100 K, the concentration of dimer defects with different charge states passes the increasing stages above 500 K starting from the F₂²⁺ centers (i.e., two spatially close F⁺ centers). The tentative mechanisms of such rise of the F₂²⁺ center concentration as well as the subsequent transformation/rise of dimer centers, F₂²⁺ → F₂⁺ → F₂ at 650-800 K are considered. The possible sources of carriers (electrons) needed for the recharging of dimer centers are also analyzed on the basis of thermally stimulated luminescence measurements. (author)