THE TRICRITICAL REGION FOR HELIUM MIXTURES IN 0.87 POROSITY AEROGEL

We have used ultrasonic velocity measurements to study He-3-He-4 mixtu res in aerogel with a porosity 0.87. The phase diagram resembles that of bulk mixtures, with a single transition for He-3-rich mixtures, in contrast to the detached phase separation curve seen in 0.98 porosity aerogel. A kink in the lambda line at a He-3 concentration of X-C = 0. 51 suggests that the phase separation line meets it at a tricritical p oint. We have measured the amount of superfluid which decouples both a t low temperature and close to the superfluid transition as functions of He-3 concentration. Each showed a sudden change at the concentratio n where the kink appeared in the lambda line, suggesting an abrupt cha nge in the morphology of the superfluid phase in the mixtures. Similar measurements were made for pure He-4 films on the same aerogel. We di scuss the nature of He-3-rich mixtures in aerogels based on these expe riments.