Electron irradiation of single crystal thorium dioxide and electron transfer reactions

This first study of electron irradiation (3 MeV) of single crystals of thoria has revealed that the crystals turn blue, as seen with neutron irradiation, but the absorption profiles in the 12 000–17 260 cm−1 (1.49–2.14 eV) region were similar but not identical. Calculations showed that the electrons transferred energy sufficient to displace lattice oxygen atoms, but not thorium atoms. The crystals were the purest currently available and supported the proposal that the fundamental absorption edge occurs close to 47 600 cm−1 (5.9 eV). Theoretically, thoria is colourless and cannot exhibit intervalence bands, but aggregates of impurity ions having more than one oxidation state can produce such bands, and some crystals showed pale amber or grey zones. The most likely impurity involved was lead. The effect of electron irradiation on defect generation after various oxidising and reducing anneals, up to 1000°C, was studied, together with the loss of the blue radiation-induced absorption on isochronal anneals at temperatures up to 400°C. The generated profile could be resolved equally well into three or four Gaussian bands and hence band assignments are not possible. It is concluded that electrons transfer energy to thoria to create defects of shallow energy, and electron transfer involving intervalence bands can occur in the presence of impurity ions.