Radiation damage in NaCl. I. Optical-absorption experiments on heavily irradiated samples

Results of optical-absorption experiments on heavily irradiated NaCl single crystals are presented. The dose rates were between 4 and 250 Mrad/h; the doses between 1 and 7 Grad. The irradiation temperatures were in the range of 20–150 °C. Because of the intense optical bands, the thickness of the samples had to be reduced to values between 2 and 100 μm using a thinning device. With this device we are able to investigate samples containing about 1 mol % Na and ${\mathrm{Cl}}_2$. The optical spectra of the samples, irradiated at low temperatures, show intense F, M, R, and N bands. The positions of the optical bands of the electron excess centers are in agreement with the literature. With increasing irradiation temperature, the contribution of these centers to the total amount of radiation damage decreases due to coagulation to colloidal Na particles. The colloid band, however, is much wider than the literature values; in addition, it is located at longer wavelengths. These observations can be explained by assuming that we are dealing with extremely small colloids, showing appreciable quantum size effects.