Some features of the long-pulse propagation mode of a phonon sheet in superfluid 4He

The angular distribution of the energy density of an anisotropic “phonon sheet” was studied during its propagation in superfluid 4He at pulse duration up to 300 ns. The presence of a flat segment on the angular dependencies of phonon signal is explained by the formation of a “hot” area in the central region of the sheet and by the process of generation of high-energy phonons in this central region. The saturation of width of a flat section at pulse durations near 300 ns is explained by implementation of the long-pulsed beam propagation mode.