The interpretation of polycrystalline coherent inelastic neutron scattering from aluminium

A new approach to the interpretation and analysis of coherent inelastic neutron scatteringfrom polycrystals (poly-CINS) is presented. Here we describe a simulation of the one-phonon coherent inelastic scattering from a lattice model of an arbitrary crystal system. The one-phonon component is characterized by sharp features e.g. determined by boundaries of the (Q, omega) regions where one-phonon scattering is allowed. These features may be identified with the same features apparent in the measured total coherent inelastic cross-section, the other components of which(multiphonon or multiple scattering) show no sharp features. The parameters of the model can then be relaxed to improve the fit between model and experiment. This method is of particular interest whereno single crystals are available. To test the approach, we have measured the poly-CINS for polycrystalline aluminium using the MARI spectrometer (ISIS) because both lattice dynamical models and measured dispersion curves are available for this material. The models used include asimple Lennard-Jones model fitted to the elastic constants of this material plus a number of Embedded Atom Method (EAM) force fields. The agreement obtained suggests that the method demonstrated should be effective in developing models for other materials where single crystal dispersion curves are not available