Band Gap Transition Studies of U:ThO2 Using Cathodoluminescence

The Department of Defense has an interest in thorium dioxide (ThO$_2$) and uranium dioxide (UO$_2$) as possible candidates for use in designing neutron detectors. Three U$_x$:Th$_{1-x}$O$_2$ (x= 0.00, 0.01, 0.22) hydrothermally grown single crystals were examined using cathodoluminescence to interrogate the changing electronic properties of ThO$_2$ as it became an alloy. Both depth-resolved and temperature-dependent cathodoluminescence studies were performed to examine the crystal structure and the defects present. An ultra-high vacuum system operating at 10$^{-8}$ Torr was used with electron beam energies ranging from 2 to 14 keV. Spectra were taken on all three samples before and after a proprietary chemical cleaning process involving a crown ether/picric acid solution was applied to the crystals to remove surface contaminants. Spectral deconvolution of the spectra showed evidence of both direct and indirect gap photon transitions from the O~2p to Th~6d at 4.2 eV and 4.8 eV respectively. Uranium-doped spectra showed evidence of the midgap O 2p to U 5f quadrupole transition and O~2p to U~6d transition at 5 eV.