High-dose neutron irradiation of MgAl2O4 spinel: effects of post-irradiation thermal annealing on EPR and optical absorption

Electron paramagnetic resonance (EPR) and optical absorption spectra were measured during thermal annealing of stoichiometric MgAl2O4 spinel that was previously irradiated in the Materials Open Test Assembly in the Fast Flux Test Facility (FFTF/MOTA) at ≈680 K to ≈50 dpa. Both F and F+ centres are to persist up to very high temperatures (over 1000 K) suggesting the operation of an annealing mechanism controlled by the thermal stability of extended defects. Using X-ray irradiation following the different annealing steps it was shown that an optical absorption band at 37 000 cm−1 is related to a sharp EPR band at g = 2.0005 and that the defect causing these effects is the F+ centre.