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Photo-reversible charge transfer processes and thermoluminescence in CaF2:Ce

The optical absorption spectrum of room-temperature gamma -irradiated CaF2:Ce reveals absorption bands due to divalent cerium in cubic symmetry, to photochromic (PC) centres and to ionised photochromic centres (PC+). The photochromic centres consist of a Ce2+ ion with a NN F centre in a (111) direction. Thermoluminescence (TL) from room-temperature gamma -irradiated CaF2:Ce exhibits two major composite glow-peaks, one at approximately 130 degrees C and one at approximately 350 degrees C. The spectrum of the TL consists of approximately 290 nm emission, due to a (Vk+e-) type recombination, and an emission doublet, at approximately 320 nm and approximately 340 nm, due to Ce3+ luminescence. The authors propose that the 130 degrees C glow-peak is the consequence of the recombination between electrons, released from cubic Ce2+ ions, and perturbed Vk-centres. The doublet Ce3+ emission arises from the radiative energy transfer from this recombination to Ce3+ ions in C4v and cluster sites by the process of reabsorption. The authors further suggest that the higher-temperature TL (near 350 degrees C) is due to the recombination of PC centres and fluorine interstitial atoms, resulting again in 290 nm and Ce3+ luminescence.