The angular spreading of phonon beams in liquid4He: upward phonon dispersion

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Abstract
Measurements have been made of the angular spreading of initially well defined beams of phonons of energies h(cross) Omega /kB4He at pressures between SVP and 24 bar. For bath temperatures T3He. The spreading increases with increasing source temperature and decreases with pressure, becoming undetectable (or=17 bar. The decay lengths of the processes giving rise to this spreading are found to be approximately 1 mm. Various scattering mechanisms are considered and it is concluded that the results are consistent with a spontaneous three-phonon decay process allowed when the phonon dispersion curve initially deviates upward from its asymptotic linear form.