Systematic performance study of common neutron guide geometries

In this report, we present the results from a systematic benchmarking of four different long neutron guide geometries: elliptic, parabolic, ballistic (piecewise linearly focusing/defocusing), and straight, for various wavelength, divergence restriction, and guide length settings. In this work, we mapped relevant parts of the neutron phase space to show where advanced guide geometries have significant transport advantages over simple guide geometries. The primary findings are that the elliptic and parabolic geometries perform almost equally well, and they are considerably superior to the other geometries, except for low-divergence, cold neutrons. In addition, it was observed that transporting thermal neutrons more than 100 m using elliptic guides was possible with only a 10% loss in the phase space density for divergences up to ±0.5°, which enables the construction of very long thermal neutron instruments. Our work will allow instrument designers to use tabulated, standard geometries as a starting point for optimising the guide required for the particular instrument.