The use of transmission spectroscopy revealed that X-rays generated a controllable number of F-, M- and R-centers, in single crystals, that were stable at up to 325K. Color center production was studied for various irradiation doses. It was found that post-irradiation annealing removed color centers but did not lead to the colloid formation which was commonly observed in the case of electron irradiation. The distribution depth of the present centers was 2 orders of magnitude larger, and their diffusion range was too small, to yield any aggregation at room temperature. This difference in the responses to X-irradiation and electron irradiation was attributed to the much greater penetration depth of X-rays, for a given energy.

E.Stenzel, N.Bouchaala, S.Gogoll, T.Klotzbücher, M.Reichling, E.Matthias: Materials Science Forum, 1997, 239-241, 591-4