The Angular Distribution of a Pulse of Low Energy Phonons in Liquid

We have made time-resolved measurements the angular distribution of low energy phonons (l-phonons) that are created by a heater in liquid helium, at several powers and pulse lengths, in order to investigate the temporal and spatial development of such l-phonon pulses. The angular distributions show a mesa shape with a flat top in the direction normal to the heater. The width of the flat top increases with power and pulse length. We argue that the flat top is due to the creation of high energy phonons (h-phonons). These are created until the temperature of the central region of the l-phonon sheet reaches 0.7 K, when the creation of h-phonons essentially stops. Outside of the flat top, the angular distribution is due the lateral spreading of the l-phonon sheet. A model for this is described in the preceding paper [1], which gives a framework for considering these measurements.