Abstract
The optical absorption (OA) and photoluminescence (hereafter referred to as luminescence) studies were made on CaF 2: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 Pb 2+ 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 M Na centre when compared with that of the coloured CaF 2:Na. The origin of 2.48 eV band was not explicitly known. Luminescence emission and excitation of Pb 2+ and Dy 3+ 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 Dy 3+ ion. When excited, the absorbed energy (may be a part) was transferred from a colour centre to nearby Dy 3+ ions and Dy 3+ 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 Dy 3+ 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 M Na centre. The luminescence mechanism was described.
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