Thermostimulated Luminescence, Exoelectron Emission and X-Ray Induced Luminescence Studies in CaSO4 : Eu and CaSO4: Eu, Na

Abstract

X- and UV radiation induced thermostimulated luminescence (TSL), thermostimulated exoelectron emission (TSEE) and X-ray induced luminescence (XIL) studies in CaSO4 : Eu and CaSO4 : Eu, Na reveal that the X-ray induced TSL peaks exhibit Eu2+ (blue) and Eu3+ (red) emissions — but their relative ratio varies with the peak. In contrast, UV induced TSL peaks exhibit only Eu3+ emission. Even in the CaSO4 : Eu sample, which contains both Eu2+ as well as Eu3+, no Eu2+ emission is seen in all the TSL peaks produced by UV irradiation. These results confirm that the X-ray induced TSL in these phosphors is best explained by the energy transfer model while UV induced TSL is best explained by the charge transfer model. In CaSO4 : Eu, the X-ray induced TSEE peak coincides exactly with the 400 K TSL peak whereas the UV induced TSEE is negligible. This indicates an Auger-like decay of electrons following the self-absorption of 400 nm Eu2+ emission due to the 400 K TSL peak. Intense XIL in CaSO4 : Eu occurs mainly due to Eu2+ emission which decreases with increasing irradiation temperature from 100 K perhaps due to thermal quenching. But the significant variation in XIL intensity in the temperature interval 350 to 450 K is attributed to Eu2+ → Eu3+ oxidation. XIL in CaSO4: Eu increases with X-ray intensity but is nearly independent of X-ray exposure time. These results suggest that XIL emission is probably caused by metastable Eu2+ centers formed during X-irradiation.