Sound absorption in weak 3He–4He solutions at high densities and low temperatures

The absorption coefficient for first sound has been measured below 0.9 °K at a frequency of 29.8 MHz in 3He–4He solutions with molar concentrations 0.5 and 0.05% 3He at constant densities of 0.153, 0.159, 0.165 and 0.170 g cm–3. Analysis of the experimental results showed that absorption in a 0.5% 3He mixture at all densities studied is produced by the viscosity of 3He and by scattering by thermal phonons, whose mean free path is limited by the 3He impurities. Absorption in a solution containing 0.05% 3He is qualitatively similar to that in pure 4He, indicating that for not too high densities the initial part of the phonon spectrum still shows anomalous dispersion on addition of 3He impurities. The transverse relaxation time for phonons in 4He at a pressure of 1.1 atm was determined, as was the relaxation time for phonons on interaction with 3He impurities.

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