Investigation of Ultrasound Attenuation in Impurity-Helium Solids Containing Liquid Helium

The attenuation of ultrasound in Impurity-Helium (Im-He) solids created upon introduction of impurity atoms or molecules (D ₂ , N ₂ , Kr) into a volume of superfluid helium has been investigated. The observed features of attenuation show that a porous substance consisting of a loosely interconnected continuous network is created in superfluid helium. This network is formed by impurity particles encapsulated in solidified helium. Analysis of attenuation allows us to conclude that Im-He solid samples have a wide distribution of pores from 8 nm to 800 nm. It was established that the character of attenuation in D ₂ -He samples is considerably different from that in heavier Im-He solids, for which two maxima of attenuation were sometimes observed. A sharp peak was observed at T_c0 very close to the bulk helium lambda transition temperature and a second broad peak occurred at T_cc0 . This behavior is similar to that predicted theoretically for liquid helium in restricted fractal porous media.