Simulations of complex electron paramagnetic resonance spectra for radiation-induced defect centres in advanced ceramic breeder pebbles

Advanced ceramic breeder (ACB) pebbles consisting of lithium orthosilicate (Li4SiO4) and the strengthening phase lithium metatitanate (Li2TiO3) are accepted as the European Union's reference solid-state material for tritium breeding in thermonuclear fusion reactors. Previously, the influence of various radiation types on Li4SiO4-based ceramic breeder materials has already been investigated and described by several groups of researchers. Electron paramagnetic resonance (EPR) spectroscopy is one of the most frequently used analytical and non-destructive techniques that can be selectively applied for qualitative and quantitative research of radiationinduced defect centres with paramagnetic properties (electron spin S =/ 0). In the present work, individual signals of paramagnetic radiation-induced defect centres in the ACB pebbles after irradiation with X-rays were separated by EPR spectra simulations for the first time. Multiple signals of spin S = 1/2 systems with distinctive symmetries and g-factor (g) values were identified and characterised. Individual signal decay at room temperature was monitored and isochronal annealing of the irradiated ACB pebbles was performed in order to evaluate the stability of accumulated radiation-induced defect centres. Based on the obtained results, it was determined that radiation-induced defect centres with EPR signals in the g > 2.00 region are similar to irradiated single-phase Li4SiO4 ceramic materials, while additions of Li2TiO3 as the second phase in the ACB pebbles also stimulates the formation of several titanium-related electron centres with signals in the g < 2.00 region and different thermal properties.