A comparison of the doppler-broadened positron annihilation spectra of neutron irradiated Al2O3 and MgAl2O3

Radiation damage studies of oxides and ceramics have become of increasing importance due to the projected use of these materials in thermonuclear fusion reactors as electronic insulators and first wall materials. In addition these materials are important in RAD waste disposal. As part of a study of the defect structure in radiation damaged ceramics Doppler-broadened positron annihilation spectra have been obtained for a series of single crystal sapphire (α-Al2O3) and polycrystal (1:1) and (1:2) magnesium aluminate spinel (MgO·Al2O3 and MgO-2Al2O3) samples. These samples were irradiated in EBR-II to a fluence of 3 × 10²⁵ n/m² (E > 0.1 MeV) at 740°C, and 2 × 10²⁶ n/m² (E > 0.1 MeV) at ~ 550°C respectively. Positron annihilation spectra lineshapes for the irradiated, annealed, and as-received samples of both materials were compared using S parameter analysis. These calculations were made on deconvoluted gamma ray spectra that were free of any instrumental broadening effects. In this way, absolute S parameter changes could be calculated. The observed changes in the S parameter are consistent with independent volume swelling measurements for both the α-A12O3 and the (1:2) MgAl2O4 samples. However, the change in S parameter measured for the (1:1) spinel is contrary to the measured volume change. This apparent anomaly indicates a predominence of interstitial as opposed to vacancy type defects in this material. © 1985.