Electron irradiation damage in quartz, SiO2

Crystallographically orientated samples of synthetic optical-grade colourless quartz with high chemical purity and low dislocation density together with synthetic gem-grade amethyst with high Fe-concentration and ca. 250 H/10⁶ Si (“dry”) or 600 H/10⁶ Si (“wet”) and with very high dislocation densities were irradiated using TEM. Samples of cuts perpendicular (<c>-cuts) and parallel (<X>-cuts) to the c-axis, that were as-grown or pretreated for 5 days at 820 K on air or under p(H₂O)=10⁸ Pa were prepared. Characterization methods used include AAS, FTIR, Raman-spectroscopy, X-ray-topography, REM, TEM in SAED and bright-field mode and polarized light microscopy. Radiolysis was carried out in TEM from 10 to 300 K with 100 kV and from 70–850 K (low-high-transition temperature of quartz) with 200 kV. Irradiation damage was investigated by decay of Kikuchi-lines or of Bragg reflections in SAED and in bright-field mode by development of strain contrast centres and of noncrystalline volume areas. Special preparates where the irradiation damage was of microscopic dimensions were investigated using Raman-spectroscopy. Radiolysis of quartz is able to proceed at 10 K with measurable velocity. The required electron dose for a standardized irradiation damage decreases with increasing temperature. At ca. 500 K it goes through a minimum and then increases steadily up to ca. 700 K. From there the increase is steep until ca. 820 K where it culminates sharply, showing strong fluctuations until 850 K. The <X>-cuts in the as-grown state show significantly higher irradiation damage sensitivity than <E1>-cuts. Dry or hydrothermal preheating increases the overall sensitivity of irradiation damage and levels out the orientation differences. The high Fe-concentrations in amethyst in comparison with very pure quartz have no detectable influence on the damage sensitivity. This is also true for different water concentrations independently from the ratio of silanole-group to molecular water. Sample thinning by ion etching with different gun currents produces differences in irradiation sensitivity. Thinning by crushing produces samples with sensitivities comparable with ion-etching at low gun current.