Optimization of monochromator and ToF powder diffractometers at a long-pulse spallation source using Monte Carlo simulations

Abstract

Recent development of the program package RESTRAX permitted us to carry out numerical optimizations of two generic types of powder diffractometers for the envisaged long-pulse structure of the European Spallation Source (ESS), namely a constant wavelength instrument equipped with a crystal monochromator (CM) and a narrow band time-of-flight (ToF) diffractometer. The aim was to compare properties of these two conceptually different instrument types. Optimization using particle swarm algorithm was carried out in the space of 10-15 free parameters for each instrument, while keeping the source, beam extraction optics, sample and detector fixed at the same values for both setups. The optimum performance was defined by the figure of merit based on the evaluation of complete simulated α-SiO 2 diffractograms. As a result, we arrived at configurations yielding similar average resolution, but large difference (up to a factor of 40) in neutron intensity in favour of the ToF concept. The CM option is however competitive with ToF in terms of spectral flux density and instantaneous flux, which may find use in stroboscopic measurements or in-situ experiments under pulsed sample environments with millisecond duty times.