Luminescence observation from lithium oxide under neutron irradiation

It is well recognized that neutron irradiation influences tritium migration processes and the chemical form of tritium. Point defects such as F0 and F+ centers are considered to be important. As the first step to investigate the effect of irradiation defects on tritium release, luminescence observation was conducted at room temperature using the fast-neutron research reactor YAYOI at the University of Tokyo. Single-crystal and polycrystalline Li2O samples were irradiated at room temperature with fast neutrons of about 1 MeV, or thermal neutrons, obtained from thermalization using polyethylene. minescence spectrum showed a broad peak which is composed of four major peaks at 330, 380, 430 and 490 nm. The peak at 330 nm was considered to be from F0 center, the 380 and 430 peaks from F+ centers and the 490 nm peak from impurities. The luminescence intensity was found to trace the stepwise power change; however, the intensity gradually decreased with increasing irradiation time. This suggests that the luminescence is emitted when the point defects are generated. Luminescence obtained by thermal neutron irradiation was found to be stronger by more than one order of magnitude than obtained by fast neutron irradiation at the same flux. Luminescence was measured for a neutron-irradiated sample after annealing at various temperatures. It was found that the F0 centers recovered at about 670 K when compared with the reported recovery behavior of the F+ centers.