Investigation of ultrasound propagation in 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 1.2-2.3 K, These solids were formed by injecting a mixture of impuri ty (Ne or N-2) and helium gases into superfluid He-4. We found that the spe ed of sound at low temperatures is close to that of first sound in bulk hel ium but decreases more rapidly with increasing temperature, similar to beha vior observed in aerogel. The attenuation of sound in helium in the impurit y-helium solids is bigger than in bulk helium and increases rapidly with te mperature up to similar to 1.8 K, after which a crossover to a much weaker temperature dependence was observed. (C) 2000 Elsevier Science B.V. All rig hts reserved.