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

The dynamic structure factor S(k,ω) of a $77%$ He and $23%$ Ne gaseous mixture at T0ex0ex=0ex0ex39.30ex0exK and total number density n0ex0ex=0ex0ex15.80ex0exnm^-3 has been measured by inelastic neutron scattering at small angles. In the range of wave vectors studied, 0.7<k/nm^-1<1.8, 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.