A tapered transition guide with irregular octagonal cross-section

We present the results of simulations to optimize a tapered neutron guide, with an initially rectangular cross-section that smoothly transitions to an irregular octagon. This guide serves as an adapter section between an existing cold-neutron guide (rectangular cross-section 120 x 60 mm(2), with m = 1.2 coating), and a 3 m long circular guide (76 mm inner diameter, with m >= 2.5). The circular guide is filled with superfluid He-4 to convert neutrons with wavelengths around 8.9 angstrom into ultracold neutrons (UCN) by inelastic scattering; it serves as the centerpiece of the ILL's new UCN source SuperSUN. The octagonal guide exit provides a better match than a rectangle to the circular aperture, and the beam divergence is transformed so that a larger fraction of the 8.9 angstrom neutrons can be guided through the converter. The required edge-reflectivity for the circular guide is also reduced: simulations show that, with a converter coating of m = 2.5, 85% of the 8.9 A neutrons from the primary, rectangular beam can be made available for UCN production. Due to the aspect ratio of the initial rectangle, an irregular octagonal cross section is found to perform better than a regular one. The corresponding guide has been fabricated and delivered, with first neutron measurements expected in 2019.