New neutron small-angle diffraction instrument at the Brookhaven High Flux Beam Reactor

This instrument utilizes cold neutrons emerging from a series of straight neutron guides. A multilayered monochromator is used in combination with a short collimator to obtain a monochromatized beam with a wavelength between 4 and 10 A and a wavelength spread of about 10%. The flux at 5 A exceeds 106 ns-1 cm-2 in a typical beam of 6-mm diameter at the sample. The spectrometer itself incorporates provisions for computer-controlled positioning of samples and a two-dimensional detector. At a sample-detector distance between 50 and 200 cm the detector can be centered at scattering angles of up to 450. The beam-defining components, the monochromator, the collimator, and various slits, are easily accessible and exchangeable for alternative devices. These features make the instrument modular and give it flexibility approaching that of standard x-ray equipment. The facility in its first year on line was used most of the time for solution-scattering experiments. These included experiments to investigate the structure of plasma lipoproteins, to further the triangulation of the small ribosomal subunit, and to characterize micellar microemulsions. The spectrometer is equally suited to measuring diffraction at higher angles on oriented specimens. In this paper, the components of the instrument are described, its performance is discussed, and characteristic parameters are compiled. 25 references, 14 figures, 3 tables