Excitations Radiated from a Thermal Source in Helium II below 0.3K

Results are presented on the propagation of excitations generated by a small (∼ 1 mm square) pulsed heater immersed in a large (∼ 300 cc) sample of He II maintained at temperatures below 300 mK. Small carbon-film detectors placed at 1, 2, 3, and 4 cm from the heater are used to measure the flux of radiation over a wide range of heater-power densities ($W_H$). The observed signals are free from wall reflections. At the lowest temperatures and small $W_H$, the fastest excitations propagate without dispersion or attenuation at a velocity of 234 ± 4 m/sec. Spatial attenuation which is observed at higher temperatures corresponds to effective mean free paths for large-angle scattering of 1, 2, and 3 cm at 306 ± 6, 272 ± 6, and 254 ± 8 mK, respectively. When $W_H$ exceeds 0.4 W/${\mathrm{cm}}^2$, signal shapes reflecting appreciable interactions between radiated excitations are observed. For $W_H>2.9\mathrm{}$ W/${\mathrm{cm}}^2$, the observed signal acquires another component which propagates without appreciable dispersion at a velocity of 200 ± 10 m/sec. In these experiments, no evidence has been found of any excitations associated with any upward bend in the He II phonon-dispersion curve resulting in signal velocities in excess of the first-sound velocity.