Coupling of neutron optics and shielding using Vitess and Geant4

Abstract: The European Spallation Source (ESS) will be a material research center built around a powerful neutron source. At ESS, researchers will be able to carry out precise measurements using neutron scattering. The quality of the measurements is determined by the signal-to-noise ratio. The signal consists of neutrons with energies that are useful for experiments, while neutrons with higher energies constitute background noise. Neutron optical systems transport the useful neutrons from the source to the experimental station, and shielding is used to block higher-energy neutrons. Neutrons will also create gamma rays when they interact with matter, and these need to be shielded for safety reasons. The behavior of the neutron beam is investigated using simulation software. Simulations of neutrons optics and shielding are typically carried out separately, using different types of software. The shielding is studied by simulating the paths of the high-energy neutrons from the source (E > 1 eV) using general particle transport software. Lower-energy neutrons are simulated using ray-tracing software that models the neutron optics. The latter do not model scattering of neutrons in the shielding around neutron guides. This could potentially be a problem, since in reality these neutrons might scatter in the guide material and end up as background noise in instruments. If the simulations do not take these neutrons into account, they will not be included in noise estimations. If this noise is not shielded properly it could have a negative effect on instrument performance. In this project, a method for coupling two software packages for neutron optics and neutron shielding was implemented. The software packages used were the ray-tracing software Vitess and the particle transport software Geant4. A method was added to the source code of Vitess to extract information about neutrons that are not transmitted through neutron optics. This information was passed to Geant4, allowing it to simulate the continued path of these neutrons. The coupling was tested on a simplified neutron guide with different curvatures. The results show that the coupling does have an impact on the background noise, suggesting that coupling effects should be accounted for in signal-to-noise calculations. Adding the coupling also increases the amount of gamma rays, which should be considered when determining radiation levels for safety purposes.