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 impurity (e.g. D-2, Ne, N-2 or Kr) and helium gases into superfluid He-4. The sound 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 sound at low temperatures is close to and decreases more rapidly with temperature than first sound in bulk helium, similar to behavior observed in aerogel. The attenuation of sound in helium in the "compressed" impurity-helium solids is bigger than in bulk helium and increases rapidly with temperature up to similar to 1.65 K, after which a crossover to a much weak er temperature dependence was observed.