Incident beam polarization system for the single crystal diffractometer Pioneer

Pioneer is a single-crystal, time-of-flight neutron diffractometer designed for the Second Target Station at Oak Ridge National Laboratory. It offers high flux across a broad wavelength band of 4.3 Å and is optimized for studying weak scattering signals across a volumetric reciprocal space, using versatile sample environments. To investigate complex magnetism, Pioneer will provide a polarized incident beam option to probe local magnetic susceptibility. The polarization system includes a translatable multichannel polarizing V-cavity, an RF adiabatic spin flipper for polarization reversal, and bar magnets to generate the required guide fields. The V-cavity is strategically positioned in a low-divergence region of the beamline, where the guide geometry is optimized to minimize the impact of large in-guide gaps. It is enclosed in a vacuum vessel with gate valves to maintain a window-free optical path during operation. When a non-polarized beam is needed, the V-cavity can be translated out of the beam path and replaced with an absorbing flight tube. The polarizer's alignment and motion drive systems are isolated from the vacuum vessel to ensure operational stability, with additional shielding panels inside the vessel and a shielding wall between the polarizer and the end station to reduce background. The V-cavity is optimized for a cutoff wavelength of 1.2 Å. Monte Carlo ray tracing simulations indicate that with m = 5 double-side coated FeSi polarizing mirrors, the V-cavity achieves a polarization (P) of ~90% for 1.2 Å neutrons and >95% for neutrons with wavelengths greater than 1.5 Å. Considering transmission (T), the design delivers a high P²T metric of >0.35 across the operating wavelength band of 1.2–5.5 Å. In comparison, ³He spin filters exhibit lower P²T over such a broad band due to strongly wavelength-dependent absorption. Furthermore, the V-cavity is simpler to operate, as ³He filters require a complex optical pumping system, making the V-cavity a better option for Pioneer.