A McStas simulation of the incident neutron beam on the VESUVIO spectrometer

Abstract

We present a Monte Carlo simulation of the incident neutron beam on the VESUVIO spectrometer at the ISIS Facility using the McStas code. As VESUVIO allows for concurrent measurements of neutron diffraction, neutron transmission, and deep inelastic neutron scattering, both incident and transmitted beams are characterized by a broad energy range, spanning over several orders of magnitude from fractions of meV to tens of keV. A transport simulation in the case of the VESUVIO spectrometer is a challenging task, for the McStas code has been traditionally applied to cold and thermal neutrons, and never used in the modelling of electron-volt neutron spectrometers, to the best of our knowledge. In this simulation study, we discuss the modelling of the collimation stages along the primary flight path so as to reproduce the absolute intensity of the incident neutron beam and its shape, both recently characterized experimentally. Finally, we show some preliminary results employing incoherent scattering samples so as to compare the epithermal component of the simulated backscattering spectra to experimental results from Pb.