A comparison of Monte-Carlo simulation programs with experiment: the effect of a focusing guide on resolution

Two Monte-Carlo neutron instrument simulation programs, RESTRAX and McSTAS, were used to determine the effect of using a converging supermirror guide between a monochromator and sample on the divergence of the incident beam. The results are compared with the test results on implementing such a focusing guide on the IN14 cold neutron spectrometer, Institut Laue–Langevin. The measured non-trivial incident beam divergence in both real and reciprocal space is reproduced by both the programs, giving confidence in the accuracy of the calculations and highlighting the dangers of using such devices on high-resolution instruments.