Luminescence studies in thorium dioxide

Thermoluminescence (TL) measurements in the temperature range 2950K-6730K indicate the existence of at least 5 trapping centres and 4 recombination centres for single crystals of nominally pure thorium (ThO2). Complementary fluroescence studies and TL emission spectra indicate that the main ''electric-hole'' recombination takes place at rare-earth ion impurities accidentally incorporated in the lattice in concentrations less than 5 ppm. Similar studies in the temperature range 770K-2950K reveal the existence of an additional 8 trapping centres. All recombination, however, takes place with the emission of only one broad band at 447 nm maximum. Furthermore the TL intensity of the lower temperature ''glow curves'' can be increased by up to 2 orders of magnitude if the crystals are exposed to a small dose of ultra-violet irradiation at 2950K before exposure at 770K. The relationship between the TL enhancement and the exact wave length of the irradiation at both 2950K and 770K is discussed. A band model mechanism is proposed to account for the experimental observations. (author)