Thermal bleaching of optical absorption and photoluminescence spectra of γ-irradiated CaF2:Dy:Pb:Na single crystals

The variation of the optical absorption (OA) and photoluminescence (PL) spectra with temperature was studied on γ-irradiated CaF2:Dy:Pb:Na single crystals. The OA spectrum showed bands around 2.05, 3.20, 3.82 and 6.20 eV which could be attributed to different sodium associated (SA) colour centres (CCs) such as MNa and RA+. Heating the crystal indicated the annihilation and formation of different SACCs. The excitation spectrum for the characteristic Dy3+ emission at 2.14 eV immediately after irradiation was very intense in the region 2.2-5.4 eV. The broad bands could be attributed to different SACCs. The absorbed energy was partly transferred to the Dy3+ ions and hence the characteristic Dy3+ emission was observed. Thus it became possible to excite the Dy3+ ions effectively after irradiation via energy transfer from the excited CCs to the Dy3+ ions (CC induced sensitization luminescence). The thermal bleaching of the excitation spectra for the characteristic MNa and RA+ emission were different. The excitation spectrum for the broad emission around 2.54 eV had bands around 3.05 and 4.21 eV which could be attributed to a new CC something similar to R4+ with a different configuration. However, the actual microscopic structure was unknown.