EXCITATIONS IN A DENSE HELIUM-NEON GAS-MIXTURE

We determine by inelastic neutron scattering the dynamic structure fac tor S(q,w) for a mixture of 65 at.% He-4 and 35 at.% Ne at T=44.8 K an d p=136 bars, for momentum transfers q=5 and 8 nm(-1). We confirm the existence of a fast-sound mode (directly visible as a broad shoulder), with an intrinsic width considerably higher than inferred from a prev ious study. We find no evidence for ultrafast-sound or interdiffusion modes.