A NEUTRON-DIFFRACTION AND MAS-NMR STUDY OF THE STRUCTURE OF FAST-NEUTRON IRRADIATED VITREOUS SILICA

Neutron diffraction and magic angle spinning (MAS)-NMR measurements have been performed on vitreous silica before and after irradiation to a dose of 2.8 x 10(20) fast neutrons cm-2. On fast neutron irradiation the first peak in the diffraction pattern is reduced in height, broadened and shifted to a slightly higher scattering vector, reflecting the increase in density of 2.4%. This may be interpreted as an enhancement of smaller scale structures, consistent with irradiation generating a higher proportion of three-membered rings as indicated by Raman data. In real space, there is a corresponding small increase in the Si-O bond length which is consistent with a reduction in the mean Si-O-Si bond angle of approximately 9.5-degrees as indicated by the MAS-NMR data. Fast neutron irradiation also leads to a peak in the difference correlation function at approximately 3.2 angstrom which has previously been ascribed to oxygen atom interstitials, but may be identified as the Si-(2)O distance within a three-membered ring.