ULTRASONIC STUDY OF HELIUM MIXTURES IN AEROGEL

We have made ultrasonic measurements of He-3-He-4 mixtures confined in silica aerogel. By measuring changes in the ultrasonic attenuation an d velocity, the phase diagram for the system has been determined. The transition from the normal to the superfluid state is accompanied by a rapid increase in the ultrasonic velocity due to decoupling of the su perfluid fraction. A critical attenuation peak, which increases with f requency, is also seen at this transition temperature (Tc). For He-4 r ich mixtures, the beginning of phase separation (at Ts) is marked by a nother change in the velocity. Phase separation also results in hyster esis in the velocity and attenuation, but only at temperatures somewha t below the onset temperature Ts.