Creation and decay of high-energy phonons in anisotropic systems of low-energy phonons in superfluid helium

The creation and decay rates of high-energy phonons (h-phonons) in anisotropic systems of low-energy phonons (l-phonons), are calculated using the exact distribution function for anisotropic phonon systems in superfluid helium. The angular distribution of created h-phonons is then found, and it is shown that the solid angle occupied by created h-phonons is always less than the solid angle occupied by l-phonons, as is found experimentally. The evolution of the l-phonon pulse, due to h-phonon creation, is analyzed. This leads to an explanation of the measured dependence of the l-phonon signal on heater power.