Design and performance of a thermal-neutron double-crystal diffractometer for USANS at NIST

An ultra-high-resolution small-angle neutron scattering (USANS) double-crystal diffractometer (DCD) is now in operation at the NIST Center for Neutron Research (NCNR). The instrument uses multiple reflections from large silicon (220) perfect single crystals, before and after the sample, to produce both high beam intensity and a low instrument background suitable for small-angle scattering measurements. The minimum detector background to beam intensity ratio (noise-to-signal, N/S) for q >= 5 x 10(-4) angstrom(-1) is 4 x 10(-7). The instrument uses 2.38 A wavelength neutrons on a dedicated thermal neutron beam port, producing a peak flux on the sample of 17300 cm(-2) s(-1). The typical measurement range of the instrument extends from 3 x 10(-5) angstrom(-1) to 5 x 10(-3) angstrom(-1) in scattering wavevector (q), providing information on material structure over the size range from 0.1 mm to 20 mm. This paper describes the design and characteristics of the instrument, the mode of operation, and presents data that demonstrate the instrument's performance.