New neutron-guide concepts and simulation results for the POWTEX instrument

The POWTEX instrument will be a high-intensity time-of-flight diffractometer at a continuous reactor source. Its design utilizes several new concepts and is suited for small samples. Herein, the optimization of the guide system is presented. It is based on two coupled elliptic neutron guides intersected by the pulse-chopper as an eye of a needle to increase the time resolution and to reduce the background transported from the source. The optical properties of two coupled ellipses are investigated with respect to their design constraints in terms of geometry and regarding the choice of the super-mirror coating. It is shown that low-divergent neutrons are preferentially transported near the pulse-chopper. Accordingly, a numerical optimization is utilized to tailor the coating to the instrument requirements using the back-tracing method that has been developed for the VITESS Monte-Carlo simulation package for virtual instrumentation. The procedure allows for a cost and background optimization because the mc-value can be kept as low as needed. For a diffractometer, the homogeneity of the divergence profile is crucial. Herein, the new development of neutron guides with octagonal cross sections is investigated. Appendix A shows a step-by-step description of the newly developed optimization features within VITESS as it was used for POWTEX.