We present evidence that nuclear recoils in silicon at cryogenic temperatures result in a larger ballistic component of phonons than do equal-energy electron recoils. A 300 μm thick crystal of pure silicon was instrumented on both sides with superconducting titanium transition-edge sensors, and the phonon ballistic energy fraction was determined by measuring phonon focusing effects. Comparison of the data with simulations indicates that this fraction is 1.${49}_{\mathrm{-}0.26}^{+0.34\mathrm{}}$ times higher (at 68% confidence level) for nuclear recoils than for electron recoils.

• Received 11 February 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1395