High resolution neutron scattering with bent monochromators made of commercial silicon wafers

Commercial thin silicon wafers are shown to be superb focusing crystals in three-axis spectrometers. With fully open beams, they allow resolutions that in conventional flat-crystal arrangements can only be achieved with Soller collimators of 2min of arc. At given resolution, intensities are much higher than in conventional arrangements with vertically focusing pyrolytic graphite crystals. Resolution ellipsoids were measured by diffraction from powder samples. The ellipsoid orientation is controlled by the crystal curvatures and can be set to any specified value, in particular turned by 90" to pass from high resolution in energy transfer to high resolution in wavevector transfer. At neutron energies on the peak of cold sources spectra the focusing technique gives energy transfer resolutions around 10 pV. The technique fills the resolution gap between back-scattering instruments and conventional three-axis spectrometers.