Search

An extensive neutron diffraction investigation on low-density (n < 4.35 nm[sup -3]) states of helium gas along the 6-K isotherm has been performed by means of both wideand small-angle experiments, allowing for the extraction of the zero- and first-order density coefficients of c(k), the Fourier transform of the direct correlation function, in a very wide wavevector region extending from k = 1 nm[sup -1] to k = 160 nm[sup -1]. The two independent measurements provide quantitatively consistent results, and the experimental quantities show a good agreement with the thermodynamic (i.e. k = 0) data. The comparison of the experimental pure two-body correlation with the corresponding result of a classical calculation clearly indicates the need of a quantum-mechanical approach. [ABSTRACT FROM AUTHOR]

The presence of a fast-sound mode in the microscopic dynamics of the rare-gas mixture He-Ne, predicted by theoretical studies and molecular-dynamics simulations, was demonstrated by an inelastic neutron scattering experiment. In order to study the transition between the fast and the normal acoustic modes in the hydrodynamic regime, k values lower by about one order of magnitude than in the usual experiments have to be probed. We describe here the results of the first neutron Brillouin scattering experiment performed with this purpose on the same system already investigated at larger k. The results of both experiments, together with those of a new molecular-dynamics simulation, provide a complete and consistent description, still missing so far, of the onset of fast-sound propagation in a binary mixture. [ABSTRACT FROM AUTHOR]