Radiation effect and optical properties of MgO

MgO single crystals containing transition metal impurities, such as Cr and Mn, are irradiated with neutrons, and the effect of the irradiation is investigated with optical methods. Immediately after the irradiation, impurity-added MgO single crystals show increased forbidden transitions. When annealing is performed after the irradiation, part of the allowed transitions, as well as the spin forbidden transition of the d electrons, are also increased. The increase in the d electron transition in transition metal impurities in a MgO single crystal is compared with that in crystals of alkaline earth metal fluoride. A study is also made on the increase in spin allowed transition during annealing, which appears to be peculiar to MgO single crystals. Impurity-added MgO crystals irradiated with 0.7 - 3.8 x 1018 n/cm2 of fast neutrons are annealed at different temperature, followed by light absorption analysis at the liquid nitrogen temperature. The transition energy of the d electrons of the Cr3+ and Mn2+ ion is largely different from that expected from the Oh symmetric position in a MgO single crystal irradiated with neutrons. These crystals may contain a large number of Frenkel defects. If these defects are trapped near the impurity ions, their d electron transitions will undergo large perturbation. The increased transitions are attributed to the spin coupling and complex formation involving F centers. (N.K.)