Multiple reflections in elliptic neutron guide tubes

Neutron guide tubes are widely used to transport neutron beams over long distances. The neutron mirrors used to line the guide tubes have imperfect reflectivity and, in long conventional guides, the average number of reflections for neutron rays becomes large thus reducing the transmission. This issue is extremely important for modern spallation sources, especially for the proposed long pulse European Spallation Source to be constructed in Lund, Sweden, where technical constraints require many instruments to be far from the source. Several solutions to the problem of transporting neutrons over long distances have been proposed and currently the most favored model is that of guides with elliptic shapes. It is widely believed that elliptic guides transport neutron rays from source to sample with a single bounce, a near perfect solution for long neutron guides, and a view which is true in ideal circumstances. This article uses computed Monte Carlo ray tracing simulations (VITESS) and other techniques to demonstrate that transport of neutrons by realistic elliptic guides usually involves many reflections, contrary to the usual expectations. These multiple reflections explain the irregular divergence distributions observed in computer simulations of transmission by some elliptic guides.