Radiation-induced color centers were studied by applying spectroscopic techniques to 8 crystals of various degrees of purity. The crystals were irradiated with 7MeV electrons, to doses of between 104 and 3 x 106rad, at liquid N temperatures. It was found that the absorption spectra at 250 to 750nm, recorded at the irradiation temperature, varied from crystal to crystal with regard to the value of the absorption coefficient (17 to 5/cm) and the number and form of the absorption bands. Step-by-step annealing up to room temperature produced new absorption bands. Between 200 and 240K, three well-resolved bands appeared - as in the case of crystals which were irradiated at room temperature.

E.Apostol, I.Dafinei, G.Mitroaica, T.Tanase, V.Topa: Radiation Effects and Defects in Solids, 1995, 134, 361-6