Effect of temperature on X-ray excited luminescence of nutural CaF₂ and its implication in thermoluminescence

Abstract

A number of glow peaks have been observed in the thermoluminescence (TL) of natural and doped CaF₂ crystals from room temperature to 650°C. The object of the present work is to find out the effect of temperature on the luminescence efficiency at the various glow peaks. X-ray excited luminescence (XEL) intensity is recorded at various elevated temperatures for this purpose. The experimental results lead to the following conclusions. (1) A continuous fall in the intensity of XEL occurs in the temperature range 320°C to 500°C. (2) The effect of temperature on emissions from different impurity ions is not the same. Main emissions observed in the TL and XEL spectra are from four lanthanide impurities, namely Ce, Gd, Tb and Dy in the wavelength range 200–500 nm. Gd emission decreases sharply with temperature right from room temperature. Dy emission increases till about 200°C and then decreases. The emission from Ce and Tb impurities shows a decrease from room temperature to 100°C, an increase from 100°C to 320°C and a continuous decrease thereafter. These observations are contrary to the general behaviour known as thermal quenching in the luminescence phenomena. The changes are explained qualitatively on the basis of the population of the emitting centres, their conversion to non-emitting species by the irradiation and restoration by the heat.