The angular distribution of phonons radiated from a heated solid surface into liquid⁴He. I. Measurement principles and techniques

A detailed description of a newly developed experimental technique for measuring the angular distribution of phonons radiated from the surface of a solid, heated to a few K, into liquid helium 4 at T<or approximately=0.1K is given. The relevance of such measurements to the Kapitza boundary conductance problem is discussed and examples of measured angular distributions from cleaved (100) faces of sodium fluoride crystals and from gold films evaporated on glass substrates are presented to illustrate the possibilities of the technique.