Neutron Brillouin scattering study of collective dynamics in a dense He-Ne gaseous mixture

The dynamic structure factor S(k,?) of a 77% He and 23% Ne gaseous mixture at T = 39.3 K and total number density n = 15.8 nm-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 c k 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.