American Physical Society
We have observed the ballistic-phonon-focusing pattern along the  axis of a 1-mm-thick silicon crystal using α-particle bombardment as the phonon source. These experiments on phonon-mediated particle detection are performed in vacuum at about 400 mK and use titanium-superconducting-transition-edge phonon sensors on the crystal surfaces. The ballistic time of flight is confirmed in one experiment and the focusing patterns are spatially resolved in another. These data indicate that about 1/3 of the phonon energy striking the back face during the first μsec is ballistic.
Young, B. A., Cabrera, B., & Lee, A. T. (1990). Observation of ballistic phonons in silicon crystals induced by alpha particles. Physical Review Letters, 64(23), 2795–2798. https://doi.org/10.1103/PhysRevLett.64.2795