Four-circle single-crystal neutron diffractometer at the High Flux Isotope Reactor

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 angstrom, with intensities of 2.5 x 10(6), 2.2 x 10(7) and 8.0 x 10(6) 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 x2.0 for 1.000 angstrom, by x3.5 for 1.536 angstrom and by x3.5 for 2.540 angstrom, as compared to the flat condition. The flux increases because the lattice strain in the silicon crystals increases. The omega-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 chi-circle of the goniometer to provide temperature control between 4 and 450 K.