High-Energy Pulses in Liquid HELIUM-4.

Available from UMI in association with The British Library. At temperatures below 0.1K the mean free path of rotons and phonons in liquid ^4He at pressures greater than 19bar is many millimetres. It has been shown previously that rotons and phonons are produced by thin film Joule heaters immersed in the helium, and that the phonons only can be detected by bolometers. At low input energies the detected signal can be understood in terms of the ballistic propagation of phonons. This thesis presents experimental data describing the signal detected from a higher input energy, single pulse. Two new features are described, the D-Feature and the M Peak. The D-Feature is composed of phonons which have been highly scattered within the first millimeter of their propagation, and thereafter undergo ballistic propagation to the detector. The behaviour of the M Peak shown to be unlike that expected of a feature composed of ballistic excitations. The D-feature and the M Peak are investigated by the interaction of two heater pulses, and two further features are shown to be produced by this interaction. These are named the E Signal and the Q Peak. It is suggested that the M and Q Peaks arise from the stimulated production, and consequent decay, of Surface Rotons at the helium-bolometer interface. A model based on this suggestion goes some way towards explaining why no ballistic rotons are detected, how two M Peaks interact to form a Q Peak, and why both features are very sensitive to heater-bolometer distance, pulse input energies and helium pressure. The E Signal is demonstrated to have much in common with the D-Feature, being composed of ballistic phonons for the latter part of its trajectory and scattered phonons for the initial part.