Fast neutron radiation on single-crystal thorium dioxide. An optical absorption spectroscopy study of the Fo Colour centre

Abstract

The effects of fast neutrons have been recorded for the first time on the optical absorption spectra of arc-fused single crystals of thorium dioxide in the region 40 000–5000 cm^–1(5.0–0.6 eV). The crystals were subjected to anneals at 600 or 1000 °C in oxygen or at 1000 °C in hydrogen prior to irradiation with neutrons of mean energy 2 MeV. Doses ranged from (1 × 10¹⁶) to (5 × 10¹⁷) neutrons cm^–2 and the crystals became blue owing to three main bands generated in the region 13 000–20 000 cm^–1(1.61–2.48 eV) from the formation of colour centres. The spectra also exhibited a well pronounced shoulder, 26 000–38 000 cm^–1(3.223–4.711 eV), containing several bands, on the fundamental absorption edge in the UV region. After the induced radioactivity had decayed, the dark decay, at room temperature, of the radiation-induced absorption was first studied. The crystals were then given a series of isochronal anneals in oxygen or hydrogen (to correspond with their pre-irradiation anneal atmosphere) and their spectra recorded before and after a saturating dose of ⁶⁰Co γ radiation. The induced profiles were resolved into their component Gaussian bands. Our results on the colour-centre bands are compared with the similar spectra arising from proton and ¹⁶O ion bombardment studies by Childs et al., but our conclusions are different. The induced profiles arise from defect centres consequent upon displacement of oxygen ions via collision processes. Electron paramagnetic resonance (EPR) studies showed that the colour centres were not paramagnetic and therefore were most likely F^o centres, with two spin-paired electrons occupying the oxygen vacancy. This is the first time that F^o centres have been identified in crystals having the fluorite lattice. Other potential centres were examined but considered less likely. Post-neutron irradiation anneals in hydrogen indicated that, whether or not the defects involved trapped electrons, all were shallow traps since the profiles were optically bleached by light below 17 000 cm^–1(2.13 eV) and also showed thermal decay at room temperature. The bands produced in the shoulder region were attributed to transitions involving impurity ions.