FERMI-CHOPPER TIME-OF-FLIGHT SPECTROMETER

The energy resolution of this instrument, the first of two time-of-flight spectrometers to be operating in the NCNR, lies in the range from 60 to 1000 micro-eV. With incident neutron energies from 2.2 to 15 meV, the spectrometer allows a broad range of quasielastic scattering experiments on diffusional motions in solids and liquids, and inelastic scattering experiments on magnetic and vibrational excitations.

The spectrometer is located on neutron guide NG-6, which has a 150 by 60 mm (height x width) cross section. The pulsed monoenergetic neutron beam is produced by a combination of monochromator crystals and a Fermi chopper. The double monochromator consists of two PG crystals, one of which can be curved vertically to focus neutrons onto the sample position. The curvature can be varied automatically to adjust for changes in monochomator-sample distances as the incident energy is varied. Two Fermi choppers with different blade curvatures are available. An oscillating radial collimator between the sample and detectors eliminates scattering from cryostat and furnace shields around the sample position.

Contact:
Terry Udovic, (301) 975-6241; udovic@nist.gov
Craig Brown, (301) 975-5134; craig.brown@nist.gov

Specifications:

• Maximum Beam Size: 100 mm x 25 mm
• Monochromators (double): Pyrolytic graphite PG(002)
• 1st Monochromator (hxw/type): 160 x 100 mm/flat
• 2nd Monochromator (hxw/type): 184 mm x 100 mm/vertically focussing
• Monochromator Mosaic (v/h): 25'/50'
• Intermonochromator collimation: 60'
• Beam Filters: PG or cooled (77K) Be Sample-Detector Distance: 2.286 m
• Scattering Angle Range: 5 to 140 deg. post-sample
• Radial Collimation (oscillating)
• Blade Separation: 3 degrees
• Inner/Outer Radius: 203/305 mm
• Detectors (tubular, 3He-filled)
• Fill Pressure: 0.4 MPa
• Diameter: 25 mm
• Angular Separation: 1.273 degrees
• Quantity/Active Height: 12/229 mm from 5 to 20 deg.
  91/457 mm from 20 to 140 deg.
  
• Incident Wavelength Range: 0.23 < lambda < 0.61 nm (x0.5 for PG(004)
• Incident Energy Range: 2.2 < E < 15.5 meV (x4 for PG(004)
• Elastic Momentum Transfer Range: 1 < Q < 50 nm-1 (x2 for PG(004))
• Flux on sample at lambda = 4 A : 2.4 x 10**8 n/m2/s with H2 cold source.

Last modified 07-March-2002 by website owner: NCNR (attn: )