Optical absorption studies were made of heavily irradiated single crystals after using dose rates of between 4 and 250Mrad/h, and doses of between 1 and 7Grad at 20 to 150C. Because of the intense optical bands, the thickness of the samples had to be reduced to between 2 and 100 by thinning. In this way, it was possible to investigate samples which contained about 1mol% of Na and Cl2. The optical spectra of samples which were irradiated at low temperatures exhibited intense F, M, R and N bands. The positions of the optical bands of the electron excess centers agreed with published data. With increasing irradiation temperature, the contribution of these centers to the total degree of radiation damage decreased; due to coagulation into colloidal Na particles. However, the colloid band was much wider than the literature values, and it was located at longer wavelengths. The observations could be explained by assuming that the colloids were extremely small and thus gave rise to appreciable quantum effects.

J.R.W.Weerkamp, J.C.Groote, J.Seinen, H.W.Den Hartog: Physical Review B, 1994, 50[14], 9781-6