LUMINESCENCE OBSERVATION FROM LITHIUM-OXIDE UNDER NEUTRON-IRRADIATION

It is well recognized that neutron irradiation influences tritium migr ation processes and the chemical form of tritium. Point defects such a s F-0 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 f ast-neutron research reactor YAYOI at the University of Tokyo. Single- crystal and polycrystalline Li2O samples were irradiated at room tempe rature with fast neutrons of about 1 MeV, or thermal neutrons, obtaine d from thermalization using polyethylene. The luminescence spectrum sh owed a broad peak which is composed of four major peaks at 330, 380, 4 30 and 490 nm. The peak at 330 nm was considered to be from F-0 center , the 380 and 430 peaks from F+ centers and the 490 nm peak from impur ities. The luminescence intensity was found to trace the stepwise powe r change; however, the intensity gradually decreased with increasing i rradiation time. This suggests that the luminescence is emitted when t he point defects are generated. Luminescence obtained by thermal neutr on irradiation was found to be stronger by more than one order of magn itude than obtained by fast neutron irradiation at the same flux. Lumi nescence was measured for a neutron-irradiated sample after annealing at various temperatures. It was found that the F-0 centers recovered a t about 670 K when compared with the reported recovery behavior of the F+ centers.