ULTRASONIC MEASUREMENTS AS A PROBE OF HE-3 HE-4 PHASE-SEPARATION IN AEROGELS/

We have used ultrasonic velocity and attenuation measurements to study the phase separation of He-3/He-4 mixtures confined in a silica aerog el with a porosity of 87%. We used both shear and longitudinal sound a nd varied the frequency between 4 and 20 MHz. The superfluid transitio n is accompanied by a velocity increase due to decoupling and by a cri tical attenuation peak which increases with frequency. At the phase se paration there are changes in the velocity and attenuation, and hyster esis on thermal cycling We show some recent results and discuss how th ey relate to the phase diagram inferred from torsional oscillator and heat capacity measurements on helium mixtures in aerogels.