PHONON BEAMS WITH A NARROW ANGULAR WIDTH CREATED BY GOLD FILM HEATER

The transmission of phonons across an interface between two materials is usually described by the acoustic mismatch model. This predicts a c one of phonons in the material with the smaller sound velocities. Howe ver, for a thin metal film submerged in liquid He-4, no such cone has been found. Indeed the distribution has been found to be more like cos theta. In this paper, we show that for heater pulse lengths of 10 mus , this is the case. But, for very short pulse lengths of < 0.2 mus, th e phonons from a metal film heater are radiated into a narrow cone in liquid He-4. The high frequency phonon cone is narrower than the low f requency phonon cone.