We have measured the angular distribution of phonons propagating in liquid He-4 which are created by a planar heater. The angular distribution varies considerably with pressure in the range 0-20 bars. At small angles to the heater normal, the angular distribution shows a mesa shape at P=0. At 6 19 bars the overall angular distribution is cosinelike. The behavior is explained in terms of phonon interactions amongst the injected phonons, that take place in the liquid helium when there is anomalous phonon dispersion. The interactions change the energy of the phonons and this is detected by the phonon-energy sensitive bolometer. Modeling of the results indicates that at small angles to the heater normal, there is thermalization of the low energy phonons and the creation of high energy phonons, which decreases in the range 7.5 20 degrees, phonons with energy below the pressure-dependent critical energy only spontaneously decay. We find that the injected phonon spectrum can be approximately characterized by a Bose-Einstein spectrum with a temperature similar to 0.6 K, which is nearly independent of heater power.
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