Neutron Brillouin Scattering Study of Collective Dynamics in a Dense He-Ne Gaseous Mixture

The dynamic structure factor $S(k,\omega )$ of a $77\%$ He and $23\%$ Ne gaseous mixture at $T=39.3\mathrm{K}$ and total number density $n=15.8{\mathrm{nm}}^{-3\mathrm{}}$ has been measured by inelastic neutron scattering at small angles. In the range of wave vectors studied, $0.7<k/{\mathrm{nm}}^{-1\mathrm{}}<1.8\mathrm{}$, we find clear side peaks at frequencies following the hydrodynamic sound dispersion $ck$ of the binary mixture (with $c$ the adiabatic sound speed in the mixture). Surprisingly, neither fast modes nor a transition from normal to fast sound are observed in the above low- $k$ region, indicating that hydrodynamic behavior persists up to $k$ values which are larger than those predicted by theories and computer simulations.