Intrinsic decay lengths of quasimonochromatic phonons in a glass below 1 K

The temperature dependence and magnitudes of the reciprocal decay length l^-1 of low-intensity longitudinal and transverse phonon pulses propagating in fused silica glass between 0.02 and 0.5 K for frequencies near 0.5 GHz are reported. The temperature dependence of l^-1 is in good agreement with resonant scattering from two-level systems, and the magnitudes yield thermal conductivities in good agreement with published data.