Optical Absorption and Thermoluminescence in Natural Beryl

Abstract

Abstract Ultraviolet (UV) illumination of colourless beryl (goshenite) produces a single photo-induced TL peak centred at 150 oC which becomes prominent for illumination wavelengths between 230 and 240 nm. Optical absorption (OA) spectra have been measured from 350 to 190 nm. The effect of thermal treatments in the range 600 to 900 oC was monitored for both thermoluminescence (TL) and OA. In the OA spectra a general decrease in the absorption at all wavelengths was observed at the beginning of the treatments, with no further changes occurring for longer periods of time. Similar behaviour was observed for the TL curves. The sensitvity of the 150 oC goshenite glow peak is strongly enhanced when the sample, which was previously heated to 900 oC for less than 30 min, is exposed to UV light. For periods of time of more than one hour, the TL signal, for all the thermal treatments performed before UV illumination, becomes equally low. It is known from electron paramagnetic resonance (EPR) measurements that atomic hydrogen in interstitial sites is produced in UV-illuminated goshenite from the dissociation of OH- ions. The free hydrogen diffuses through the lattice, being stabilised by traps. When the material is heated the trapped hydrogen atoms become free to move and are captured by oxygen atoms, which act as recombination centres. In this work an attempt is made to correlate the Ho EPR line and the TL peak. OA and TL measurements have also been carried out on pink beryl (morganite). Different TL and OA spectra from the goshenites are observed since these samples contain different amounts and types of impurities.