Comparison of Neutron Transport Concepts for the Single-Crystal Neutron Diffractometer PIONEER at the Second Target Station of the Spallation Neutron Source

PIONEER is a single-crystal neutron diffractometer for measuring tiny volume single-crystals and epitaxial films at the Second Target Station (STS), Spallation Neutron Source. To take advantage of the high source brilliance and the advancements in neutron optics, we have performed Monte-Carlo ray tracing simulations and geometry optimizations for multiple neutron transport concepts, including Montel mirrors, curved guides using a bender, and straight and kinked beamlines using elliptical guides. Our priorities include signal-to-background ratio and operational reliability and flexibility. The optimization metrics count for high brilliance transfer, high spatial and angular beam homogeneity within the phase-space region of interest (ROI), and low flux outside the ROI. We found that the straight beamline delivers the best performance, with the brightest beam but with more flux outside the spatial ROI. Montel mirrors deliver fewer neutrons outside the ROI but at a high cost of thermal neutron flux. The bender and kink options deliver less bright and less uniform beams. Our studies also show that the straight beamline displays a moderate flux loss to engineering imperfections, including mirror waviness and misalignment. Further optimization on the straight beamline has identified effective solutions for beam size and divergence control using slit packages.