A long wavelength neutron monochromator for superthermal production of ultracold neutrons

Production of ultracold neutrons (UCN) by single-phonon downscattering of cold neutrons from superfluid helium (the "superthermal process") utilizes input neutrons only in a narrow wavelength band around 0.89 nm. Delivering a monochromatic 0.89 nm neutron beam to a superfluid helium target reduces backgrounds in the UCN production region with minimal loss in the UCN production rate. The design, construction, and testing of a 0.89 nm neutron monochromator is reported herein. This monochromator is constructed from nine tiled pieces of stage 2 potassium-intercalated graphite with mosaics between 1.1° and 2.1° and reflectivities of (73-91)% at 0.89 nm. In addition to stage 2 potassium-intercalated graphite, fluorophlogopite and stage 1 potassium-intercalated graphite are also characterized.