Relaxation study of pre-densified silica glasses under 2.5 MeV electron irradiation

We examined the "relaxation properties" of pre-densified synthetic fused silica glass under 2.5 MeV electron irradiation. The densification of the glass was either obtained by hot compression (5 GPa-350 °C and 5 GPa-1000 °C) or via a thermal treatment increasing its fictive temperature (T_f = 1050, 1250 and 1400 °C). Under irradiation, the pre-densified silica glasses exhibit a relaxation of their macroscopic density with increasing integrated dose. Density was reduced for hot compressed silica and increased for T_f samples with different relaxation rates but it is remarkable that all sample densities follow a trend towards the same equilibrium value around 2.26 for a dose larger than 10 GGy despite a different final topology. After irradiation of hot compressed silica, the Raman spectra display a significant increment of 4 and almost 3-membered rings whereas they exhibit a glass density reduction; demonstrating that a D₂ band increase cannot be considered as an absolute marker of the glass compaction. The correlation between density and D₂ intensity remains valid until silica density remains lower than 2.26. In contrast, the FWHM of the main band peaking at 440 cm^-1 appears to remain correlated to the silica glass density for all investigated samples.