Brillouin scattering with neutrons and recent results from the investigation of compressed gases

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Abstract
Brillouin scattering experiments with neutrons are discussed and recent instrumental developments are presented. The most recent results of such experiments performed on compressed Ar and N2 gases at temperatures well above the critical temperature Tc are summarized. The results are discussed within the framework of hydrodynamic theory and it is shown that thermodynamic conditions can be found under which the newly developed experimental techniques allow to intrude into the region of pure hydrodynamics, up to those now reserved for light scattering techniques. For atomic and molecular fluids a common limit between the pure hydrodynamic and the generalized hydrodynamic regimes is found between 0.25 and 0.3 lEQ, where lE is the mean free path between atomic collisions in the fluid and h(cross)Q is the momentum transfer. This experimentally determined limit is about a factor of two to three higher than the values of lEQ predicted by generalized Enskog theories for hard-sphere fluids.