We have studied (ω,ω) backward-wave phonon echoes at 9.3 GHz in quartz crystals which are induced by neutron irradiation, electrodiffusion in vacuum, and heating in vacuum. It is argued that the backward-wave phonon echoes in the electrodiffused and annealed samples are due to impurity atoms present in the quartz crystal and which undergo a migration by this treatment. In the neutron-irradiated samples experimental evidence is found that there are two mechanisms present for the echo generation: two-level systems and impurity atoms, the latter being displaced by the irradiation.

• Received 6 May 1987

DOI:https://doi.org/10.1103/PhysRevB.37.9079