Low frequency elastic properties of neutron-irradiated quartz. Comparison with glasses

Neutron-irradiated quartz is a promising model system to learn more about t he low-energy excitations (tunneling states) in vitreous silica and in simi lar glasses. We present the first systematic study of the elastic propertie s of neutron-irradiated quartz at low frequencies and very low temperatures . Using the vibrating reed technique at frequencies of several kHz we have measured the sound velocity and internal friction of six quartz crystals ir radiated with different neutron doses over a wide range of temperatures (7 mK-300 K). The results are analyzed using the tunneling model and several e xtensions of this theory. Comparisons are made with recent low-frequency me asurements on normal and compacted vitreous silica and with ultrasound expe riments on neutron-irradiated quartz.