Saturation of the density of states of tunneling systems in defective crystalline neutron-irradiated quartz

The variation of the longitudinal velocity of sound has been measured in a neutron-irradiated quartz sample exposed to a moderate dose of 4.7×${10}^{19}$ n/${\mathrm{cm}}^2$ (E≥0.3 MeV), at low temperatures for three different frequencies. A surprisingly high density of tunneling systems was found, equal to the value in neutron-irradiated vitreous silica. This result is especially interesting in view of the fact that this sample was found to be still to a large extent crystalline, whereas neutron-irradiated vitreous silica is fully amorphous. The observation of the saturation of the density of states of tunneling systems in a still largely crystalline sample is further related to studies of the damage induced by neutrons in quartz. It is suggested that the tunneling systems partly reside at the interfaces of the ${\alpha }_1$-${\alpha }_2$ domains present in neutron-irradiated quartz.