We have measured the angular distribution of phonons radiated from a cleaved (100) face of a sodium fluoride crystal, pulse heated to $T\sim 0.5$ K, into liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at $T<\mathrm{}0.1\mathrm{}$ K. The distribution shows a sharp peak superimposed on a low-amplitude background extending to ∼35° to the normal. The peak width is consistent with a critical cone ${\theta }_c={4.25}^{\circ }$ calculated under the assumption of conservation of phonon wave vector parallel to the interface.

• Received 17 July 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.1299