Slow-Neutron Scattering and Collective Motions in Liquid Lead

Using the Materials-Testing-Reactor phased-chopper velocity selector, measurements have been made of the scattering law $S(\kappa ,\hslash \omega )$ for liquid lead at 352°C for energy transfers $\hslash \omega$ in the range 0-20 meV and for momentum transfer $\hslash \kappa$ in the range 1-4 ${\mathrm{\AA }}^{-1\mathrm{}}$. The scattering law exhibits a strong momentum-dependent structure at small energy transfer (corresponding to the diffraction maxima). Although for large values of the energy transfer this pronounced structure has disappeared, the $S(\kappa ,\hslash \omega )$ curves still show two very broad maxima. The "one-phonon" partial differential scattering cross section is extracted from the data after subtracting multiphonon and multiple-scattering contributions, and the results are compared with those calculated on the basis of a model proposed by one of us (K.S.S.). This comparison shows that the model not only has the correct qualitative features but also gives a semi-quantitative agreement. From the analysis of the data, we conclude that there exist "collective excitations" in liquid lead. Also, for times of the order of ${10}^{-12\mathrm{}}$ sec, there exist at least the first two somewhat diffuse Brillouin zones in the liquid.