Hyperfine interactions of paramagnetic radiation-induced defect centres in advanced ceramic breeder pebbles

Abstract

Advanced ceramic breeder (ACB) pebbles consisting of 65 mol% lithium orthosilicate (Li4SiO4) and 35 mol% lithium metatitanate (Li2TiO3) are developed for tritium breeding in the European Union’s (EU) helium cooled pebble bed (HCPB) breeder blanket concept of the demonstration fusion power plant (DEMO). Electron-nuclear double resonance (ENDOR) spectroscopy is a powerful and widely used magnetic resonance technique that combines aspects of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). In the present work, ENDOR spectroscopy was applied to investigate hyperfine (HF) interactions of paramagnetic radiation-induced defect centres (containing unpaired electrons) in the biphasic ACB pebbles after irradiation with 10 MeV accelerated electrons. To separate individual EPR signals of the formed and accumulated paramagnetic centres, isochronal annealing of the irradiated ACB pebbles followed by EPR spectra measurements at X- and Q-microwave frequency bands at room and low temperature were performed. Afterwards, X-band ENDOR spectra at low temperature were recorded for selected EPR signals to reveal HF interactions between the unpaired electron spins with the spins of magnetic 7Li and 1H isotope nuclei. The g-factor and HF coupling values determined from EPR and ENDOR spectra simulations provide novel insights into the local structure of paramagnetic hole- and electron-type centres in the irradiated ACB pebbles.