The absorption of sound in dilute solutions of 3 He in liquid helium II

Abstract

The coefficient of the absorption of sound in dilute solutions of 3 He in liquid helium II has been measured in the temperature range down to 0.4 °K at a frequency of 14 Mc/s. Results are given for molar concentrations of 3 He of 0.32, 1.6 and 5.2%. In all cases the absorption is less than in pure helium II, and the peak in the absorption-temperature curve is shifted to somewhat lower temperatures. It appears that the 3 He speeds up the interactions between the thermal excitations in the liquid, and thus reduces the second viscosity which is responsible for most of the absorption in pure helium II at the higher temperatures (Khalatnikov 1950; Dransfeld, Newell & Wilks 1958). Our results are compared with a calculation of this effect due to Andreev (1961). At lower temperatures the absorption arises through a different mechanism, and a treatment taking into account the fact that all the relaxation times are long compared to the period of the sound wave has been given by Dransfeld (1958, 1962). Our results are consistent with, and give support to, Dransfeld’s treatment which involves the ‘bunching’ of the thermal phonons by the sound wave.