Fast and slow sound in a dense gas mixture of helium and neon

For a dense gas equilibrium mixture of 65 at.% He and 35 at.% Ne at T=39.3 K and p=114 bar, we determine from neutron scattering the total dynamic structure factor S(k,ω) as a function of frequency ω for wave numbers 4<kck can be attributed to fast oscillations of the light He particles (fast sound) and ${\mathrm{\omega }}_{\mathit{s}}^{\mathrm{}\left(2\right)\mathrm{}}$(k)<ck to slow oscillations of the heavy Ne particles (slow sound), where c is the hydrodynamic (k→0) sound velocity of the total mixture. ${\mathrm{\omega }}_{\mathit{s}}^{\mathrm{}\left(1\right)\mathrm{}}$(k) is virtually indistinguishable from the (extended) sound dispersion measured in pure helium at T=39.3 K and p=114 bar. A kinetic model, consistent with our experiment, predicts that fast sound vanishes at k=0.7 ${\mathrm{nm}}^{\mathrm{-}1}$ and that slow sound merges into hydrodynamic sound then.