Perfect Crystal Diffractometer for Ultra Small-Angle Neutron Scattering
Instrument Characteristics:
Q-range: 0.00003 Å-1 to ~ 0.01 Å-1 Size Regime: 0.1 to 20 microns Flux at Sample: <17,000 n/cm2-sec Sample Size: Up to 5 cm wide x 5 cm high The PCD is a Bonse-Hart -type instrument with large triple-bounce, channel-cut silicon (220) crystals as monochromator and analyzer. The perfect crystals provide high angular resolution while the multiple reflections suppress the "wings" of the beam profile, improving the signal-to-noise ratio to values comparable to that obtained with pinhole instruments. This technique, widely utilized for x-rays for many years, has only recently been successfully adapted for neutrons [1], as dynamical diffraction effects arising from the deep penetration of neutrons in thick perfect crystals have become understood. The design of the NCNR's PCD [2,3] successfully eliminates these undesirable effects, resulting in a signal-to-noise ratio of 105 at a minimum scattering vector Q = 0.0005 Å-1.
The measurement range of the PCD overlaps that of the NCNR's 30-m SANS instruments. Together they probe structure in materials over four orders of magnitude, from ~1 nm to 104 nm. Combined measurements on these instruments will enable hierarchical and highly anisotropic microstructures in materials, for example in fiber or clay impregnated nanocomposites, to be more fully characterized. The PCD is part of the NIST/NSF Center for High Resolution Neutron Scattering (CHRNS) with up to two-thirds of the available beam time to be allocated by the NCNR's Beam Time Allocation Committee to scientists and engineers who submit proposals for peer review.
[1] "Optimization of a Bonse-Hart Ultra-Small-Angle Neutron Scattering Facility by Elimination of the Rocking-Curve Wings" Agamalian M, et al. J. Appl. Cryst. 30(3), 345 (1997)
[2] "Design and performance of a thermal-neutron double-crystal diffractometer for USANS at NIST" Barker JG, et al. J Appl. Cryst. 38(6), 1004 (2005)
[3] "Development of a thermal-neutron double-crystal diffractometer for USANS at NIST" Drews AR, et al. Physica B 241-243, 189 (1998)
USANS Beam Optics Schematic:
The USANS instrument has a pyrolytic graphite premonochromator to diffract 2.4 Å neutrons, with a wavelength spread (~ 6 %) and a wavelength-angle correlation that is nearly optimal for Bragg scattering by the triple-bounce, Si (220) monochromator. With no sample, the matching analyzer crystal must be parallel to the monochromator, to within ~ 1 arc second, in order to triply-Bragg-scatter the beam into the detector. With a sample in place, rotating ('rocking') the analyzer by an angle, q, will orient the analyzer to scatter only those neutrons that have been scattered by the sample by the same angle q.
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Last modified 01-March-2007
