Volume, index-of-refraction, and stress changes in electron-irradiated vitreous silica

The volume, index-of-refraction, and stress changes induced in vitreous silica by irradiating with 18-keV electrons have been measured over the dose range from 2.5×1010 to 3×1012 rad. The volume and index measurements were performed using a new technique based on producing a spatial modulation of irradiated and unirradiated areas which is used as an optical phase grating. Comparison of the volume change and stress measurements shows there are two components to the compaction; one component, which dominates below doses of 2×1011 rad, is associated with stress generation, while the other component, which dominates at higher doses, proceeds without generating stress. In addition, the relationship between the index change and volume change observed for electron irradiation is different from that obtained by neutron irradiation and hydrostatic compression. The index-vs-volume change data can be interpreted using the Lorentz-Lorenz formalism and Neuman strain-optical constants, and provides further evidence of the two different compaction processes. The measured stress saturates at 1.5×108 N/m2 at approximately 6×1011 rad (0.24% volume change) and decreases with subsequent irradiation. In contrast the volume and index changes saturate at approximately 2×1012 rad (1.4% volume change).