Backward-wave phonon echoes in irradiated quartz

We have observed parametric ω-ω phonon echoes in neutron- and electron- irradiated quartz at 9 GHz and at 1.5 K. Power dependences of the echo amplitude can be explained by saturation of two-level systems and the similarity between the results for the two materials suggest that point defects rather than amorphous-like regions, which are not created in electron-irradiated quartz, are the responsible centres. (author)