CROSSOVER TEMPERATURE FROM TUNNELING TO THERMAL-ACTIVATION IN NEUTRON-IRRADIATED QUARTZ

The low-temperature internal friction data in neutron-irradiated quart z and amorphous SiO2 are compared. The region of the tunneling plateau in irradiated quartz extends to much higher temperatures in compariso n with amorphous SiO2. It is interpreted within the soft-potential mod el that two-level systems in irradiated quartz have a higher crossover temperature T-c from tunneling to thermal activation than in the glas sy state. The characteristic energies W of the particles in the double -well potentials are estimated. Their difference is ascribed to higher effective masses of the tunneling centers inside the amorphous networ k than in the distorted crystalline regions of irradiated quartz. [S01 63-1829(98)01542-2].