Thermoluminescence of Thorium Oxide Single Crystals

Abstract
The report contains the results of measurements of the thermoluminescence between liquid nitrogen temperature and 300C of a large number of ThO2 single crystals. These results indicate that the same traps occur commonly in crystals grown by different methods and which differ considerably in purity, although there are substantial differences in apparent trapping densities. The most important glow peaks have maxima at about -160C, -120 to - 100C, -14C and 150C. The maxima of the thermoluminescence excitation spectra and the ultraviolet cutoff are shown to coincide, indicating that some trap excitation involves a band gap transition and that the band gap is 5.75 eV. The thermoluminescence emission spectra observed in this investigation are quite broad in some cases but in other cases they are the sharp line spectra of particular rare earths. The fluorescence emission for some of these ThO2 crystals appears to be identical to the thermoluminescent emission. Dosage studies on these crystals indicate relative trap densities and show cases of linear and quadratic growth of thermoluminescence with dose. There are strong indications that the trapping states are due to intrinsic defects involving oxygen vacancies, and several models are proposed and correlated with the experimental results.

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