Optical absorption and thermoluminescence of MgAl2O4 spinel crystals implanted with Xe++ ions

We have studied changes in optical absorption of MgAl2O4 spinel crystals implanted with 340 keV Xe++ ions at about 120 K, to fluences ranging from 1015-1021 ions/m2. With increasing ion fluence, we observe an increase in optical absorption, especially in the vicinity of two absorption bands: one centered at 5.3 eV, the other at 6.9 eV. The absorption band at 5.3 eV, caused by F-centers, saturates at a fluence between 1·1018 and 1·1019 ions/m2. This is the same dose range in which formation of a metastable phase of spinel has been reported previously. The band at 6.9 eV grows with increasing dose and saturates at 1·1020-3·1020 ions/m2. Previous studies have shown that spinel is amorphized by Xe ion irradiation in this dose range. Annealing studies were also conducted on the Xe ion implanted spinel crystals. By optical absorption, F-centers were found to anneal at 500 deg. C. Thermoluminescence measurements revealed a temperature dependence of luminescence that correlates well with the optical absorption