NG3 30-m Small Angle Neutron Scattering Instrument
Sponsored by the National Science Foundation as part of the Center for High Resolution Neutron Scattering (CHRNS), this instrument is installed on a dedicated neutron guide, NG-3. Designed to cover a wide Q-range, from 0.015 to nearly 6 nm-1, it is suitable for examining structural features in materials ranging from roughly 1 to 500 nm. At least 75% of the instrument time will be allocated to guest researchers by the Beam Time Allocation Committee.
The layout of the CHRNS-SANS instrument is shown in the figure below. Neutrons from the cold source pass through a multidisk mechanical velocity selector with variable speed and pitch, enabling both the mean wavelength and wave-length resolution to be varied over a wide range.
The monochromatic beam is collimated by circular pinhole irises in a 15 m long evacuated pre-sample flight path. The source iris can be preceded by up to eight 1.5 m guide sections that can be easily shifted in or out of the beam. The beam divergence and flux on the sample can thus be varied by changing the effective source-to-sample distance from 4 m to 16 m in 1.5 m increments.
A polarized beam option is available on this instrument. The polarizer is based on a transmission (Mezei) cavity which reflects one spin state and transmits the other spin state. The 1.2 m long polarizing device consists of silicon wafers coated with Fe/Si supermirror aligned to form a V inside the neutron guide section coated with copper. This device polarizes large cross section neutron beams (4cmx5cm) over a wavelength range from 5 Å to 15 Å. Immediately following the polarizer a spin flipper is available to rotate the neutron spin orientation by 90° (up to down). Permanent magnets magnetize the iron layers of the supermirror coating and provide a guide field to maintain the polarization up to the sample position. Test measurements at a typical wavelength of 8 Å yielded a beam polarization of 89% and a flipping efficiency of 94% of the spin flipper. The test results gave flipping ratios (ratio of beam intensity with flipper current off to that with current on) between 10-15 depending on wavelength and sample size.
There is a choice of two sample locations: (i) a permanently installed evacuable sample chamber with a computer-controlled translation stage, and (ii) a 600 mm diam sample table, between the pre-sample flight path and the main sample chamber, to accommodate large pieces of apparatus, such as electromagnets, cryostats, furnaces, or shear cells. Multi-specimen sample holders with temperature control (-10°C to 200°C) are available for use in the sample chamber. The sample table consists of a Huber stage with a computer-controlled rotation and transmission stages for alignment.
The post-sample flight path consists of a long cylindrical section that forms a vacuum enclosure for a large 2D position-sensitive detector. The area detector (64x64 cm2 with a 0.5 cm fwhm spatial resolution) moves along rails (that are parallel to the neutron beam) inside the cylindrical vessel to vary the sample-to-detector distance from 1.3 m to 13.2 m. In addition, the detector moves transversely to the beam direction (by up to 30 cm) to extend the Q range covered at a given detector distance.
Encoded data from the area detector are stored in a set of histogramming memories. Data flows freely through flat ribbon cables from a "router" module to "histogramming" modules. The router module is capable of performing "time slicing" measurements whereby the 2D data are routed to different memory modules for each preset time (that can be chosen between 100 microsec to 100 sec). For the time-slicing option to work, the sample environment must send a TTL signal on a periodic basis to initiate the router rescan. A dedicated MicroVax is used both for data acquisition and data reduction. Data are automatically transferred to a dedicated color graphics terminal (Macintosh) where thay are viewed in real time.
A dedicated computer for data acquisition and instrument control is operated through a user-friendly menu-driven interface. A color monitor displays a continuously updated image of the data being collected. Completed data sets can be reduced on-line and transferred electronically to a remote node, or to PC diskettes in ASCII format.
For more information, check out the SANS Manuals.
[1] C.J. Glinka, J. Barker, B. Hammouda, S. Krueger, J. Moyer and W. Orts, “The 30m SANS Instruments at NIST”, J. Appl. Cryst. 31, 43 0-445 (1998)
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Last modified 01-March-2007