FANS - The Design
The new Filter Analyzer Neutron Spectrometer is being built in two phases. In phase I, which has been completed, the smaller wedge was constructed and placed on the moving arm of the old BT-4 instrument. After a testing period, during which the triple-axis part of the instrument will remain operational, the second, larger wedge will be built and placed with the first one on an air pad equipped undercarriage. At that time the sophisticated double focusing monochromators will also be installed.
The sample table and the pie-shaped filter-detector assembly pivot about the monochromator axis as the neutron scattering angle is scanned between 40 and 115 degrees. In phase-II the filter-detector assembly will consist of two wedges spanning the total of 230 degrees. Thus, neutrons scattered by the sample will be detected in a much larger solid angle (compared to the old BT-4 spectrometer), resulting in a substantial increase of instrument sensitivity. Sophisticated focusing monochromators maintain high neutron intensity for all incident energies.
The side view cross-section through the filter-detector assembly shows blocks of polycrystalline beryllium and graphite which diffract all neutrons with energies above 1.8 meV, as well as the removable bismuth filter. A significant improvement in background was achieved by placing a second Be filter after the graphite one. All filters are cooled to 77 K to reduce thermal diffuse scattering. Radial and vertical collimators are used to capture any neutrons not traveling on a line joining the sample with a detector. Filter sections are also separated by absorbing material, again intended to capture any Bragg diffracted neutrons.
The five principal components of the filter carriage: 1st beryllium section (nearest in view), graphite section, collimators, 2nd beryllium section, and the detector bank (with 48 He3 detectors).
The comparison between the old BT-4 instrument detector bank and the new FANS Phase I wedge
|Craig M. Brown
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