Comparative study of radiation-induced optical behavior of quartz glasses and sapphire crystals

Full text: Different parameters of plasma in accelerators and fusion reactors, such as density, temperature, dynamics of relaxation, are often estimated by emission in the optical spectral range. So that optics of detecting devices (windows, lenses, etc.) should be transparent in the range of 170-1200nm. Their parameters, however, appear to be changed under radiation influence (fast neutrons, gamma-quanta etc). It may lead to misrepresenting of useful signal due to both transparency decreasing and undesirable radiation stimulated luminescence of optical material. In this connection comparative study of the radiation damage of optical absorption and luminescent behaviors of KU-1 quartz glass and sapphire crystals was undertaken with the aim to predict to what extent the properties of these materials may degrade in the field of radiation. The samples were bombarded with neutrons of the fluencies 1017-1020 sm-2 (neutron flux 1013 sm-2s-1, E>0.1MeV) and gamma-irradiated in the dose interval of 106-1010 Rad (60Co, E=1.25MeV, gamma dose rate 670R/s). It should be noted, however, that such a huge fluencies and dose couldn't be reached in real conditions. In the gamma dose interval mentioned sapphire crystals do not show any additional absorption bands. On the contrary KU-1 quartz glass exhibits the radiation-induced absorption band peaked at 215nm, the intensity of which is increased with gamma dose up to 5.108R with following saturation. At the same time the transparency in the spectral range of 300-900nm doesn't decrease significantly. Sapphire crystals show gamma-stimulated luminescence in the range 300-330nm while quartz glasses do not emit under gamma excitation at room temperature. Neutron bombardment leads to deep coloration of sapphire crystals but does not change the transparency of quartz glasses in the spectral range of 300-600 nm. At the same time neutron irradiation causes the appearance of gamma-luminescence in both materials. Quartz glasses show two luminescence bands at 470 and 650nm, intensity of which is increased with neutron fluence and saturated at the fluence 1019cm-2. Two gamma-luminescence bands at 330nm (F+-centre) and 420nm (F-centre) appear in sapphire crystals after neutron irradiation. Their intensity is increased with neutron fluence up to 1018 cm-2 and then decreased. It should be noted that gamma-luminescence intensity in sapphire crystals is ten times larger in magnitude than in quartz glasses is. From the date obtained one can conclude that both materials may be applied to design optical elements for nuclear sets with neutron flux less than 1012cm-2s-1. In these conditions the rate of accumulation of intrinsic radiation defects would be less and, consequently, the extent of degradation of transparency and luminescence appearance would depend on initial disordering of material. (author)