Measurement of the Sound-Velocity Anisotropy in Oriented Single Crystals of bcc Helium-3

The longitudinal and transverse sound velocities have been measured in x-ray-oriented single crystals of bcc ${\mathrm{He}}^3$ at a molar volume of 22.6 ${\mathrm{cm}}^3$. General agreement with the predicted anisotropy is found; however, the transverse velocities fall significantly below the expected values. A ${\theta }_{\mathrm{D}}$ of 23.7°K calculated using the elastic moduli is in excellent agreement with the value extrapolated from specific-heat measurements.