Paramagnetic radiation-induced defect centres and their correlation with the optical properties of irradiated advanced ceramic breeder pebbles

Advanced ceramic breeder (ACB) pebbles, primarily composed of lithium orthosilicate (Li₄SiO₄) with lithium metatitanate (Li₂TiO₃) as a second phase, are currently under development and testing as the European Union's reference material for tritium breeding in future thermonuclear fusion reactors. In the present work, the formation and accumulation of paramagnetic radiation-induced defect centres is investigated and compared for the first time in the untreated and thermally pre-treated ACB pebbles under exposure to different types of ionising radiation. Electron paramagnetic resonance (EPR) spectroscopy is employed, with particular focus on correlating the detected EPR signals with the optical properties of the irradiated pebbles. The stability of the radiation-induced optical absorption bands and the positions of the main peaks in the thermally stimulated luminescence (TSL) glow curves are correlated to the annealing of the EPR signals at g = 2.04 and g = 2.00. Within the same temperature range, transformations occur among various radiation-induced electron-type centres, originating from structurally related sites formed in the bulk of the material. The annealing of these electron-type centres proceeds in multiple stages up to 350 °C, involving recombination with hole-type centres that exhibit different stabilities. The obtained results highlight the important role of paramagnetic centres in determining the optical properties of the irradiated ACB pebbles.