Biology Department Brookhaven National Laboratory

Neutron Small-Angle Scattering A User Facility at the HFBR

H9B

Contact:   Dieter K. Schneider Biology Department, 463 Brookhaven National Laboratory Upton, NY 11973-5000	tel: (516) 344-3423 fax: (516) 344-3407 schneider@bnl.gov
The H9B Group: H9B Beamline (516) 344-4440 Vito Graziano (516) 344-4738 John Lara (516) 344-4440	graziano@bnl.gov jlara@bnl.gov

The H9B spectrometer is a general purpose small-angle neutron scattering instrument used primarily for structure determinations of macromolecules of biological interest. Neutrons from the cold source travel through a liquid nitrogen cooled beryllium filter, and are delivered to the monochromator vessel by eight meters of Ni-coated neutron guide. Monochromatization is achieved by Bragg diffraction off NiTi multilayers. The resulting monochromatic neutron beam is collimated in a helium-filled tube by circular cadmium apertures, and is directed to the sample housed inside a temperature controlled sample changer. Scattered neutrons are detected by a He-3 filled high-resolution area detector that can be moved in a range of sample-to-detector distances between 50 to 200 cm. The instrument is interfaced to a MicroVAX 3500 computer for instrument control and data collection. Data analysis can be performed online or on a number of available networked computers.

Spectrometer Description:

Cold Neutron Source - To provide intense beams of long wavelength neutrons (5-10Å), a liquid-hydrogen moderator has been installed in the beam thimble of H9. Shown at left is the H9 plug and moderator before insertion into the H9 beam hole. The cold source contains 1.4 liters of liquid hydrogen in an oblate ellipsoidal chamber with a diameter of 20 cm and a maximal thickness of 7 cm. Circulating cold helium gas from an external refrigerator maintains the hydrogen at 19°K.

Neutron Flux Spectrum - The gain in neutron flux due to the cold source is plotted as a function of wavelength (left). The full size figure shows the flux spectrum in the apparent source on a relative scale. The upper spectrum is that of the liquid hydrogen moderated neutron beam (H2 at 20°K), and the lower spectrum was measured on a beam through the evacuated moderator chamber (empty). The full line superimposed on the cold moderator spectrum is a Maxwelliam spectrum calculated for an apparent temperature of 35°K. The shaded bar at 3.96Å represents the wavelength cutoff of the Be filter.

Neutron Guides - Eight meters of straight neutron guides, installed in 1994, transport the white beam to the monochromator. The evacuated glass guides are coated with natural Ni and have a rectangular cross section of 15 by 45 mm² (width x height).

Monochromator - Monochromatization up to wavelengths of 8Å is achieved by means of a single Ni-Ti multilayer that deflects the beam by about 4 degrees. Wavelengths are tuned either by rotation of the multilayer, or more simply, at fixed angles, by selecting a multilayer of suitable spacing. The monochromator consists of two linked rotary stages and can also be operated as a double multilayer device. The multilayers are fabricated in-house in A. Saxena's laboratory.

Collimator and Sample Changer - The collimator (black tube protruding from the right) contains three exchangeable circular cadmium apertures, equally spaced by 68 cm in a helium filled tube. The last collimating aperture is part of a telescopic extension that can be brought into contact with the sample changer (in center of picture). Typically, the primary beam has a diameter of 6 mm at the sample and a wavelength spread of 10%. The flux at the sample usually exceeds 106 n/s-cm² at the current reactor power of 30MW.

Sample Axis - A linear sample changer provides the repetitive handling capabilities required for solution scattering experiments (see picture above). It shuffles up to 30 standard spectroscopy cuvettes in racks of 5 (shown here) at controlled temperatures between -10° and 40° C. With an insert, temperatures to 100° C can be attained. Alternatively, the sample axis may be equipped with the various devices available at the HFBR, including cryostats and magnets.

Detector - Scattering patterns are recorded with a high-resolution multiwire area detector of 50 x 50 cm² on a 128 by 128 pixel grid. This wire chamber (iside the white conical shield at left) was developed and built by the Brookhaven Instrumentation Division. The sample-to-detector distance is variable between about 50 to 200 cm and the detector can move about the sample axis on an arc of 45°. When positioned closest to the sample, it can intercept scattering angles to 30°. At the other extreme, at maximal detector distance and wavelength, it can resolve first order diffraction peaks of 750Å.

Data Analysis - For data reduction and analysis, both text-based and graphical packages are available as outlined in the software guide. Several commercial packages are available on node BNLH9B for display and inspection of raw data images. The colormap shown here is the raw scattered intensity of a chaperonin GroEL solution in D2O measured for an hour and displayed with SAOimage.

Beamlines Parameters:

Additional Information: Biology Neutron Staff H3B Membrane Neutron Diffractometer Neutron Scattering WWW Pages Neutron scattering lengths and cross-sections from Neutron News Neutron Scattering Lengths and Cross Sections compiled by V. Sears of Chalk River are available here along with connections to various Web Elements. Teaching Guide for X-ray and Neutron Diffraction World Directory of SANS Instruments

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