Observation of Ballistic Phonons in Crystalline Silicon Induced by Gamma-Ray and Neutron Interactions

Document Type

Conference Proceeding

Publication Date





There has recently been much interest in the propagation of phonons in semiconductors at cryogenic temperatures, not only because of the solid state physics involved, but also because of its relevance to a new generation of semiconductor-based phonon-mediated particle detectors [1,2]. Phonons from particle interactions start out near the Debye frequency (~13.4 THz Si) where the scattering and decay rates are high. Phonons that are down-converted sufficiently for their mean free path to be comparable to the size of the crystal can reach the crystal surface with few or no additional scatterings and are termed ballistic. If the final down-conversion occurs sufficiently close to the original particle interaction, a ballistic phonon will contribute to a phonon focusing pattern on the surface of the crystal. It is necessary to understand the level at which particle interactions produce ballistic phonons to fully understand the response of a phonon-mediated detector. Furthermore, ballistic phonons can be exploited to improve both spatial and temporal resolution in a detector which leads to improved background rejection. One of the main goals of the group at Stanford is to build a detector capable of detecting non-baryonic dark matter. The improvement of background rejection is one of the most important considerations in developing a phonon-mediated detector for dark matter searches.

Chapter of

Phonon Scattering in Condensed Matter VII

Part of

Springer Series in Solid-State Sciences


Michael Meissner
Robert O. Pohl


Proceedings of the Seventh International Conference, Cornell University, Ithaca, New York, August 3–7, 1992