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

The dynamic structure factor S(k,omega) of a 77% He and 23% Ne gaseous mixt ure at T = 39.3 K and total number density n = 15.8 nm(-3) has been measure d by inelastic neutron scattering at small angles. In the range of wave vec tors 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 (wit h the adiabatic sound speed in the mixture). Surprisingly, neither fast mod es nor a transition from normal to fast sound are observed in the above low -k region, indicating that hydrodynamic behavior persists up k, k values wh ich are larger than those predicted by theories and computer simulations.