Thermoluminescent spectra and optical absorption in CaSO4:Dy

The CaSO 4 :Dy TL emission spectrum between 400 nm and 800 nm is studied using real-time spectroscopy. The emission spectrum is found to be independent of exposure between 460 R and 10 4 R, and in the temperature range between room temperature and 400 degrees C the TL emission spectrum is solely the result of characteristic trivalent Dy transitions. Optical absorption bands observed in CaSO 4 :Dy are correlated with optical transition of the Dy 2+ and Dy 3+ ions to determine that Dy 3+ is photo-reduced during irradiation and oxidised during subsequent heating. The effect of thermal treatments on the behaviour of the SO 3 - and SO 4 - radicals produced by ionising radiation, as determined in independent ESR results, is discussed in terms of the concurrent changes in the Dy impurity that results in the thermally stimulated release of luminescence. It is concluded that the model of the TL process most consistent with the various experimental results is one in which the Dy impurity acts as an electron trap, with charge recombination at the radical sites resulting in energy transfer to Dy 3+ ions which de-excite by luminescent emission.