Optical spectra of MgAl2O4 crystals exposed to ionizing radiation

Exposure of MgAl2O4 crystals to 137Cs γ rays produces a broad absorption complex centered near 3 eV and at the same time destroys absorption at 4.8 and 6.4 eV which is attributed to charge transfer transitions on octahedral Fe3+ impurity ions. Thermal and optical bleaching of the 3-eV absorption complex in crystals with different Fe3+ concentration reveal that this complex is composed of absorption bands located at 2.8, 3.1, 3.6, and possibly at 3.9 eV. Analysis of the intense, persistent afterglow and conductivity after γ-ray exposure and thermally stimulated conductivity discloses a very high concentration of shallow electron traps with a prominent set of these traps having a thermal depth of ∼0.9 eV. It is proposed that antistructure disorder, e.g., Al3+ ions on tetrahedral sites, is responsible for a substantial portion of these electron traps.