Effect of neutron irradiation on optical spectra of sapphire crystals

We have studied the effects of neutron irradiation (fluences from 5 × 10 15 to 5 × 10 19 cm −2 ) and thermal annealing on the optical spectra of sapphire crystals and the generation and annealing of visible luminescence and absorption centers in the crystals. The radiation-induced color centers with λ = 460, 570, 620, and 780 nm have been shown to be annealed in steps at temperatures from 70 to 1150°C. The process can be represented by two exponentials with activation energies of 0.05 and 0.30 eV. We have revealed antiannealing of the 460-, 570-, and 620-nm color centers at 200–300 and 300–450°C, and evaluated the thermal ionization potential U of the 400-and 460-nm color centers. The results suggest the possibility of charge transfer from the former center ( U = 0.5 eV) to the latter ( U = 0.2 eV). Increasing the neutron fluence from 2 × 10 18 to 5 × 10 19 cm −2 is shown to increase the color center concentration by one order of magnitude. We have analyzed the nature of the radiation-induced centers and have shown that the concentrations of 460-nm absorption and 540-nm emission centers are power law functions of neutron fluence, with exponents of 0.79 and 0.81, respectively. These findings strongly suggest that the same mechanism underlies the generation of these centers.