Optimizing the Setup of the BRISP Spectrometer by Upgraded McStas Simulations

Here we report on new results obtained by ad hoc developed McStas simulations of the time-of-flight thermal neutron Brillouin spectrometer BRISP, presently under construction at the ILL. In such a spectrometer a double-curvature focusing monochromator is combined with a multi-beam converging collimator. This configuration, first exploited for BRISP, has been carefully analyzed by developing new specific McStas codes which enable a reliable simulation of the real components, including the detailed features of the monochromator, the background disk-chopper and the honeycomb converging collimator. The present McStas calculations are also used as a guide to refine technical details, such as the optimal arrangement of the crystals on the monochromator surface as a function of the measured mosaic spread. The simulation results are consistent with the expected estimates of both intensity and resolution, and show that BRISP can provide new opportunities for advanced investigations of the low-Q dynamics of disordered and magnetic systems.