In the wonderland of ultra-parallel neutron beams

Abstract

Bragg reflections from single crystals yield angular widths of a few arcsec for thermal neutron beams. The Bonse-Hart proposal to attain a sharp, nearly rectangular profile by Bragg reflecting neutrons multiply from a channel-cut single crystal, was realized in its totality three and a half decades later by achieving the corresponding Darwin reflection curves for 5.23 Å neutrons. This facilitated SUSANS (Super USANS) measurements in the Q ∼ 10⁻⁵ Å⁻¹ range. The polarized neutron option was introduced into the SUSANS set-up by separating the up- and down-spin neutron beams by ∼10 arcsec with a magnetic (air) prism. The neutron angular width has recently been reduced further by an order of magnitude to ∼0.6 arcsec by diffracting 5.3 Å neutrons from a judiciously optimized Bragg prism. This constitutes the most parallel monochromatic neutron beam produced to date. I present the first SUSANS spectra probing the Q ∼ 10⁻⁶ Å⁻¹ domain, recorded with this beam.