Development of a Thermal-Neutron Double-Crystal Diffractometer for USANS at NIST. A.R. Drews, J.G. Barker and C.J. Glinka, Reactor Radiation Division, National Institute of Standards and Technology, Gaithersburg, MD 20899 and M. Agamalian, Solid State Division, Oak Ridge National Laboratory, Oak Ridge TN 37831.

We will describe the design and optimization of a double-crystal diffractometer for ultra-high resolution SANS (USANS). This instrument is being constructed at a dedicated thermal beam port at NIST's 20 MW research reactor. Our design utilizes two perfect silicon (220) triple-bounce channel-cut crystals at 2.4Å. Results of crystal tests show theoretical rocking-curve widths with 93% reflectivity per reflection and a peak-to-background ratio of 10⁴. We will discuss strategies for reaching our proposed peak-to-background ratio of 10⁸ and present preliminary data from USANS test-applications of the crystals.