Optimization of a Bonse–Hart Ultra-Small-Angle Neutron Scattering Facility by Elimination of the Rocking-Curve Wings

The ORNL Ultra-Small-Angle Neutron Scattering (USANS) facility at the High Flux Isotope Reactor (HIFR) has been recently upgraded, using the Bonse–Hart technique. Si(111) triple-bounce channel-cut single crystals have been used for both the monochromator and analyzer. The total width of the rocking curve of the analyzer is about 1.6′′ and the wavelength of the primary neutron beam is 2.59 Å. It has been demonstrated that, owing to the low neutron absorption of silicon, the wings of the rocking curve are generally contaminated by neutrons propagating and diffracting inside the walls of channel-cut crystals. This parasitic intensity has been eliminated by the cutting of a groove in the long wall and the insertion of a cadmium absorber (0.6 mm thick). This modification effectively suppresses the wings of the rocking curve by over two orders of magnitude and thus dramatically improves the sensitivity of the diffractometer. The upgraded facility has been tested with several samples, including a polystyrene latex with a radius of 2.50 × 10⁴ Å as determined by optical microscopy. The average radius calculated from USANS data is 2.48 × 10⁴ Å, in excellent agreement with independently determined dimensions. The minimum accessible scattering vector of the upgraded USANS facility is Q_min ≃ 2 × 10⁻⁵ Å⁻¹, which corresponds to a maximum resolvable real-space dimension of 2π/Q_min ≃ 3 × 10⁵ Å (30 μm).