High-resolution neutron scattering with commercial thin silicon wafers as focusing monochromators

Quasielastic scattering measurements with commercial silicon wafers as focusing monochromator and analyzer in a three-axis spectrometer showed energy-transfer resolutions in the 10-100 μeVrange (fwhm). Resolutions were better and intensities higher than in a conventional arrangement with Soller collimators and pyrolytic graphite (PG) monochromator and analyzer. Resolution remained high when extended-plate samples were used in focusing orientation. At cold sources, this technique would give 10-20 μeV resolutions at neutron energies near the peak of the spectrum. Projections of resolution ellipsoids were determined by diffraction from powder samples. The orientations of the ellipsoids are controlled by horizontal curvatures and can be rotated 90° for high Q-resolution. A monochromator unit with remote control of horizontal curvature and fixed vertical curvature set to spatial focusing at the sample position was also tested. The strong vertical focusing gave a significant gain in integrated intensities, but worsened the resolution because of second-order aberrations.