Optimization of a double bent-crystal diffractometer for neutron small-angle scattering experiments

The optical properties of a double bent-crystal diffractometer utilized in medium-resolution small-angle neutron-scattering experiments have been studied in detail. The expressions for the angular resolution of this device, represented by a statistical dispersion of its instrumental curve, were derived under the assumption of a simple beam collimation. In contrast to the conventional step-by-step measurements in Bragg reflection geometry, the use of fully asymmetric analyzer geometry with a position-sensitive detector, proposed previously, makes the totally nondispersive setting possible for any angular resolution set by the monochromator bending. These conclusions are supported by Monte-Carlo simulations and experimental data.