Development and extension of a monochromatic neutron beamline for neutron polarimetry device characterization at the Spallation Neutron Source

The precise manipulation and analysis of neutron spin states are foundational for a wide range of physics experiments, from fundamental symmetry tests to materials science. To enable systematic characterization of neutron polarimetry devices, we have constructed and extended a monochromatic neutron beamline at the Spallation Neutron Source, Oak Ridge National Laboratory. The beamline delivers monochromatic neutrons and provides a flexible platform for deploying and evaluating advanced neutron spin manipulation instruments. We describe the design and commissioning of the extended beamline and present a proof-of-concept neutron polarimetry study using three devices: a supermirror neutron polarizer, a Mezei spin flipper, and an in situ neutron 3He spin analyzer system. Performance metrics, optimization strategies, and systematic effects are discussed, demonstrating the beamline utility for neutron instrumentation testing. These results establish the extended monochromatic beamline as a useful resource for the development and validation of neutron polarimetry technologies.