Perfect-Crystal Diffractometer (PCD) for Ultra-High Resolution Small-Angle Neutron Scattering (USANS)

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.

Instrument Layout at BT-5: