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Exposure of single crystals of MgA1204 to fast neutrons and to Van de Graaff electrons with energies in excess of 0.35 MeV introduces an optical-absorption band at 5.3 eV with a 1-eV half-width. This band can be partially bleached at temperatures as low as 40 K and a shoulder at 4.75 eV develops concurrently. This bleaching treatment also partially destroys a previously reported V-type absorption centered at 3.2 eV. Subsequent exposure to ionizing radiation destroys the 4.75-eV band and restores both the 5.3and 3.2-eV bands to their original intensities. Since this behavior is analogous to the interconversion of F to F+ centers in Al, O" it is concluded that the 5.3-eV band is the principal optical transition of the F center (two electrons trapped at an oxide-ion vacancy) and the 4.75-eV band is attributed to absorption by the F+ center (one electron trapped at an oxide-ion vacancy). In electron-irradiated crystals the 5.3-eV absorption begins to anneal near 110'C and is about 90% destroyed upon isochronal annealing (10-min pulses) up to 355'C. Neutron-irradiated crystals behave similarly. Measurement of the threshold energy for damage by electrons at 77 K yields a displacement energy for the creation of 0 interstitial-vacancy pairs of 59 eV.The defect yield drops off substantially with increasing temperature, and at room temperature the apparent 0' displacement energy is 130 eV. Possible reasons for this strong temperature effect are discussed.

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Based on the studies of the thermal stability and the oxidizing annealing at 1350 °C, it is concluded that the one‐hole center observed in a 300 K γ‐irradiated Al2O3 crystal is predominantly the VOH‐ center. The OH− ions can be removed from the Al2O3 crystal by prolonged annealing in air at 1350 °C. The emission spectra show that the γ‐ray induced thermoluminescence peaks at 390, 420, 440, 500, and 550 K peaks observed in oxidized and non‐oxidized Al2O3 crystal are all related to the R‐lines emission from Cr3+ in Al2O3. The 390, 420, and 500 K peaks result from thermal destruction of the hole centers by hole release. The 550 K peak is caused by the restoration of unperturbed, substitutional Cr3+ ions.