Neutron irradiation effects and structure of noncrystalline SiO2

The nature of the defects produced in noncrystalline SiO2 and α-quartz single crystals by fast neutron irradiation (E > 0.18 MeV) has been investigated by Raman and infrared spectroscopy, small-angle x-ray scattering, and electron microscopy. Measurements were made at 25°C on samples irradiated to integrated fluxes (exposures) from ∼1×1018 to 2×1020 neutrons/cm2 (n/cm2). It was observed that at 2×1020 n/cm2, the α-quartz sample had been transformed to an essentially noncrystalline material. However, at 9×1019 n/cm2, remnant crystallites of the original α-quartz having a radius of gyration of 47 Å were found. The spectra of the noncrystalline phase of these materials are interpreted in terms of local modes at defect centers. Simple models are considered which account for the vibrational spectra and extrapolated x-ray scattering in the forward direction. The post-irradiation annealing behavior of the α-quartz samples has been followed to 1000°C. It was found that the structure of the noncrystalline phase gradually transformed to that of unirradiated SiO2. In the α-quartz sample irradiated to 9×1019 n/cm2, the remnant microcrystallites were found to anneal out by 1000°C.