A four‐circle neutron diffractometer with a new multi‐wafer 331 Si monochromator has been installed and commissioned on a thermal beamline at the High Flux Isotope Reactor at Oak Ridge National Laboratory. The instrument is well suited to studies of nuclear and magnetic structures as a function of composition and temperature, resolving symmetry changes (lattice distortions and local structural changes), mapping the evolution of complex magnetic phases, determining hydrogen bonding, analyzing nuclear and spin densities, mapping diffuse scattering, and exploring fiber diffraction. Three incident wavelengths are available, 1.000, 1.536 and 2.540 Å, with intensities of 2.5 × 106, 2.2 × 107 and 8.0 × 106 neutrons cm−2 s−1, respectively. Either high‐resolution or high‐intensity modes are possible by horizontal bending of the monochromator. With increased bending of the monochromator, the incident flux on the sample passes through a maximum, increasing by ×2.0 for 1.000 Å, by ×3.5 for 1.536 Å and by ×3.5 for 2.540 Å, as compared to the flat condition. The flux increases because the lattice strain in the silicon crystals increases. The ω‐scan peak width increases with monochromator curvature and this width versus scattering angle flattens. Given these effects, the monochromator bending can be adjusted to deliver high intensity primarily for crystal structure refinements or high resolution for resolving symmetry changes. In addition to the traditional step‐scanning mode, a more efficient continuous‐scanning mode was developed, and both these are implemented through a LabView‐based control program, i.e. a modified version of the SPICE software package. A 4 K closed‐cycle helium refrigerator is permanently mounted on the χ‐circle of the goniometer to provide temperature control between 4 and 450 K.

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The optics of curved-crystal three-axis neutron spectrometers is investigated analytically. A program for computing resolution and intensity characteristics, which includes finding conditions for intensityresolution optimization, is presented. Ways of improving spectrometer performances are discussed. The program's predictions are supported by experimental results obtained on a spectrometer with pneumatically curved perfect Si crystals.

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The neutron reflectivity of deformed perfect crystals is examined for different types of uniform and nonuniform bending. The peculiarities of the pneumatic bending are discussed. Check measurements with bent perfect silicon crystals are presented.

The optics of successive Bragg reflection by two bent crystals is considered in the lamellar approximation. The optimal curvatures ensuring minimal rockingcurve widths and good reflection efficiencies are determined. The conditions under which a rocking curve reproduces the reflectivity curve of a bent crystal are indicated. Analytical formulae for the rocking-curve width and peak intensity are derived for three simple limiting cases. The computations are supported by experimental results obtained with bent perfect silicon crystals.

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