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NCNR PUTS INTO OPERATION NEW PERFECT CRYSTAL DIFFRACTOMETER FOR ULTRA-HIGH RESOLUTION SANS.

A perfect crystal diffractometer (PCD) for ultra-high resolution small-angle neutron scattering (USANS) measurements is now operational at NIST's Center for Neutron Research (NCNR). The PCD increases the maximum size of features accessible with the NCNR's 30 m long, pinhole collimation SANS instruments by nearly two orders of magnitude, from [approximate][10.sup.2] nm to [10.sup.4] nm.

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, only recently has been adapted successfully for neutrons, as dynamical diffraction effects arising from the deep penetration of neutrons in thick perfect crystals have become understood. The design of the NCNRs PCD successfully eliminates these undesirable effects, resulting in a signal-to-noise ratio of 105 at a minimum scattering vector Q = 0.0004 [nm.sup.-1]. The performance of the NIST instrument is either superior or comparable to that of any USANS instrument currently in operation worldwide.

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 [approximate]1 nm to [10.sup.4] 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 Program Advisory Committee to scientists and engineers who submit proposals for peer review.
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Publication:Journal of Research of the National Institute of Standards and Technology
Article Type:Brief Article
Geographic Code:1USA
Date:Sep 1, 2000
Words:298
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