Brillouin scattering of neutrons from liquids

Abstract

Inelastic coherent neutron scattering is called Brillouin scattering, if the momentum transferred to the scattering system is entirely taken up by collective particle motions. With this scattering, single excited states of such motions can be observed in polycrystals and in liquids in a similar way as in single crystals, provided the measurements are not too much contaminated by multiple scattering processes combining Bragg and inelastic scattering. Liquid and solid lead are investigated in this way. The comparison of the measured dispersion laws shows that at 340°C, for frequencies between 6.4 × 10¹¹ and 2.3 × 10¹² sec^–1 and wavelengths between 7 and 21 Å, sound waves propagate in the liquid by the same mechanism as in the solid. The contaminating double scattering is predominantly determined by maxima in the distribution of frequencies of collective motions. The observed positions of such maxima indicate that transverse modes exist also in the liquid.