Measured Displacement Energies of Oxygen Ions in Zirconolite and Rutile
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Measured Displacement Energies of Oxygen Ions in Zirconolite and Rutile Katherine L. Smith1, 2, Ronald Cooper3, Michael Colella1 and Eric R. Vance1 1 Materials Division, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia. 2 Contact author: [email protected] 3 Department of Chemistry, University of Melbourne, Parkville, Vic. 3052, Australia. ABSTRACT Optical emission spectra in the 300-700 nm range were collected from zirconolite and rutile specimens irradiated with a 3 µs pulsed electron beam using a Febetron 706 variable energy pulsed electron beam generator. The long-lived emissions (up to microseconds after the electron pulse) consist of broad (halfwidths ~ 100 nm) bands centred around ~400 nm. Over the range 0.2 MeV to 0.6 MeV, the emission intensity per unit dose versus electron beam energy data from the rutile sample showed a single stage dependence on electron beam energy, whereas the zirconolite data suggested a two stage dependence. Rutile has a threshold of 0.23 ± 0.02 MeV, which gives an Ed value of 39 ± 4 eV for oxygen. Zirconolite has a threshold of 0.26 ± 0.02 MeV, which gives an Ed value of 45 ± 4 eV for oxygen. These data are discussed in the context of previously measured and calculated Ed values for other oxides. INTRODUCTION Zirconolite (CaZrTi2O7) is constituent phase of various synroc [1] related titanate waste forms, designed to immobilize high-level radioactive waste (HLW). In synroc, zirconolite is one of the two primary actinide host phases. Over time, alpha decay of actinides incorporated in zirconolite causes lattice damage, primarily resulting from the movement of recoil nuclei (mass ~210-240 amu, energy ~ 80-100 keV). Approximately 1500 atoms are displaced per event [2]. Such damage has been evidenced: a) in x-ray diffraction studies of synroc doped with the shortlived actinide emitters Cm-244 [3, 4] and Pu-238 [5, 6] in natural samples [7-12] and c) in heavy-ion irradiated zirconolite [13-17]. Cooper et al. [18] recently used time resolved cathodoluminescence spectroscopy (TRCS) to measure the displacement energy (Ed) of oxygen ions in three titanate perovskites (CaTiO3, SrTiO3 and BaTiO3). In this study, TRCS was used to investigate both zirconolite and rutile. Rutile was included in this study for two reasons. Firstly, the structure of rutile is significantly different to those of both zirconolite and perovskite. Every Ti-O octahedron in rutile is both corner and edge linked, whereas in perovskite the Ti-O octahedra are only corner linked to one another. In zirconolite some of the Ti-O polyhedra are only corner linked and some are both corner and edge linked. Secondly, rutile is an ancillary phase in some HLW titanate ceramics. Consequently, the peripheries of rutile grains in such ceramics will suffer radiation damage due to the decay of actinide ions in adjacent actinide bearing phases.
TRCS involves irradiating the sample with pulsed beams of fast electrons (0.2 MeV to 0.6 MeV) and monitoring the ultraviolet/visible luminescence produced by s
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