Optimisation of guide exits by combining MC simulations and optimising routines

Abstract

In many neutron scattering instruments a convergent guide exit is used to increase the flux on the sample. But at the same time, the divergence of the neutrons increases. The resultant flux on the sample can increase or decrease depending on its distance to the guide exit and the sample size (relative to the guide size). An optimum can be expected for an intermediate convergence. It was our aim to find this optimum for different distances and samples sizes. Therefore, we have performed MC simulations with the software package VITESS. We combined it with a numerical method to find the optimal values. Different kinds of shapes were tested. The influences of starting values and criteria for the optimal exit were checked. For long wavelengths and great distances between exit and sample, the maximum was obtained by diverging exits and not by converging exits. Funnels with a kind of elliptical shape perform better than linearly converging ones. Significant gains can be reached by adapting exits to a certain wavelength range—at the cost of losses in other ranges.