Master thesis defense by Henrik Carlsen – Niels Bohr Institute - University of Copenhagen

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Master thesis defense by Henrik Carlsen

Simulation of inelastic neutron quasi-particle scattering

Many neutron scattering instruments use pyrolytic graphite (PG) for monochromators and analysers. However PG has a non-negligible inelastic scattering due to phonons leading to a background signal. This effect can be minimized by cooling the graphite or by optimizing the geometry of the instrument. To assess this background-creating scattering, a model capable of realistically simulating coherent inelastic neutron scattering was implemented in McStas. This model approximate the calculated phonon dispersion bands by piecewise cubic splines as this allows for calculating an interpolated value of the possible neutron energy transfers along any particular scattering direction.

The simulation results using our model were shown to be in agreement with energy-integrated phonon scattering data taken on PG at the DMC powder diffractometer at PSI [J.O. Birk et al, in preparation]. From these simulation it was concluded that cooling the sample will not remove the contribution to the background from PG at 5 meV, partly due to the quadratic nature of the ZA phonon band. This model can be included when simulating instrument geometries, in this way minimizing this background the contribution to the scattering from phonons in the monochromator- or analyser material.