Optical absorption in irradiated quartz and fused silica∗

Abstract

Optical absorption studies over the range from 1850 Å to 26,000 Å have been performed on crystalline quartz and fused silica after exposure to both fast neutrons in the reactor and Co 60 γ-rays. The prominent band near 2150 Å is produced by γ-rays and neutrons in fused silica but fast neutron exposures in excess of 5 × 10 18 cm −2 are necessary to cause appreciable development of the corresponding band in crystalline quartz. With shorter neutron exposures and γ-rays the absorption in quartz crystals is characterized by the broad visible bands and continuous absorption which rises almost linearly toward shorter wavelength below 3000 Å. This continuous absorption probably corresponds to the long wavelength tail of one of the bands observed by Mitchell and Paige in the vacuum ultraviolet region. These results do not seem to agree with their interpretation of the nature of the 2200 Å band in quartz. On annealing neutron-irradiated quartz (5 × 10 18 neutrons cm 2 ) at 250°C a new band at 2000 Å develops. In fused silica, besides the 2150 Å band, the shoulder at 2400 Å (resolved by appropriate optical bleaching as a band at 2570 Å) which has previously been reported, was observed with both γ-rays and fast neutrons. The rates of coloration of quartz and fused silica as well as studies of optical and thermal bleaching are discussed. In the fused materials a mechanism of photolytic coloration which appears to be consistent with observations provides a suitable model which is based on the rupture of Si-O bonds with the formation of free radicals as the principal product. These and other possible defects are discussed in the light of magnetic susceptibility and electron spin resonance studies.