Investigations of ultrasound propagation in porous impurity-helium solids

The velocity and attenuation of ultrasound passing through porous impurity- helium solids immersed in liquid He-4 have been measured in the temperature range 1.1-2.3 K. These solids were formed by injecting a mixture of impuri ty (e.g. D-2, Ne, N-2 or Kr) and helium gases into superfluid He-4. The sou nd signal seemed to propagate mainly in the helium contained in the pores, rather Man through the solid sample itself. We found that the speed of soun d at low temperatures is close to and decreases more rapidly with temperatu re than first sound in bulk helium, similar to behavior observed in aerogel . The attenuation of sound in helium in the "compressed" impurity-helium so lids is bigger than in bulk helium and increases rapidly with temperature u p to similar to 1.65 K, after which a crossover to a much weak er temperatu re dependence was observed.