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

Abstract

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.