Photoluminescence studies of γ-irradiated CaF2:Dy:Pb:Na single crystals

The optical absorption (OA) and photoluminescence (hereafter referred to as luminescence) studies were made on CaF2:Dy:Pb:Na single crystals (Dy-0.005 at%, Pb-0.188 at% and Na-0.007 at%) before and after γ-irradiation. The unirradiated crystal exhibited a strong OA band around 6.36 eV attributed to the 'A' band absorption of Pb2+ ions. The γ-irradiated crystal exhibited OA bands around 2.06, 3.28, 3.75 (broad shoulder) and 2.48 eV. The first three bands could be tentatively attributed to MNa centre when compared with that of the coloured CaF2:Na. The origin of 2.48 eV band was not explicitly known. Luminescence emission and excitation of Pb2+ and Dy3+ ions were negligible in the unirradiated crystal. Irradiated crystal exhibited a strong excitation spectrum with overlapping bands, due to different colour centres, in the UV-vis region for the 2.15 eV emission characteristic of Dy3+ ion. When excited, the absorbed energy (may be a part) was transferred from a colour centre to nearby Dy3+ ions and Dy3+ characteristic emission was observed. Exciting the irradiated crystal around 3.28 eV yielded emission at 2.56, 2.15 and 1.76 eV. The first two emission bands were due to Dy3+ ions. The excitation spectrum for the 1.76 eV emission showed two prominent bands around 2.02 and 3.08 eV and hence the emission was attributed to the MNa centre. The luminescence mechanism was described.