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SPIN-POLARIZED TRIPLE-AXIS SPECTROMETER


This instrument is part of the Center for High Resolution Neutron Scattering (CHRNS) supported by the National Science Foundation, from which it derives half of its funding. Two-thirds of its beam time will be reserved for guest researcher experiments through the NCNR proposal system. Located on guide NG-5, it is currently operated in three different modes: a conventional triple axis mode, a horizontally focusing mode, and a polarized beam mode. A vertically focusing pyrolytic graphite (PG) monochromator produces beams with wavelengths from 2.2 to 6.1 Å. Energy resolution is in the range of 0.02 to 1 meV, depending on incident wavelength and collimation. Immediately following the monochromator, beam filters (PG or cooled Be) may be automatically inserted to remove higher order contamination. Cooled Be (cutoff energy of 5 meV) and a BeO (cutoff energy of 3.7 meV) filters are also available to be placed after sample.

In the horizontally focusing mode, a multi-crystal analyzer with 11 independently rotating 2cm × 15 cm (w×h) PG blades can be used to focus scattered neutrons of a particular energy onto a single detector (diameter of 2.54 cm and length of 15cm) and yield a signal increase of a factor of ~4 by relaxing Q resolution. The focusing is fully automated and controlled from the instrument computer. A two-dimensional position-sensitive detector (PSD) is also available. With the multi-crystal analyzer, the PSD (20 × 25 cm 2 in area) can simultaneously probe scattering events with different momentum and energy transfer, increasing sensitivity by up to a factor of 10 for many problems.

In the polarized beam mode of operation, supermirror transmission polarizers, consisting of a stack of single crystal Si plates with Fe/Si supermirror coatings, are inserted in the incident and scattered beams.

Contact:
C. F. Majkrzak, (301) 975-5251; majkrzak@nist.gov
Seung-Hun Lee, (301) 975-4257, shl@jazz.ncnr.nist.gov

Diagram of Instrument
Specifications:

  • Source: neutron guide NG-5, 50 mm x 120 mm
  • Monochromator: PG(002) vert. focusing, 30' FWHM, incident energies from 17 to 2 meV, wavelengths from 2.2 to 6.1 A
  • Analyzer: (i) PG(002) flat, mosaic 30' FWHM (ii) PG(002) horiz. focusing
  • Detector: (i)3He detector, w/ diameter 2.54 cm and length 15.09 cm (ii) 3He PSD w/ 20 × 25 cm 2 area.
  • Beam filters: Before sample: PG or cooled Be; After sample: cooled Be or BeO.
  • Collimation: source-monochromator:open guide
    monochromator-sample:10',20',40',80'
    sample-analyzer:10',20',40',80'
    analyzer-detector:20',40',80'
  • Flux at sample: 3.9x10**6 n/cm2/s at 4 A, open/40'coll.
  • Resolution: from 0.02 to 1.0 meV 
Collimation(')   lambda       rel. signal*     FWHM(meV)

55-80-80-80            4           1                0.28

55-40-40-40            4           0.24             0.17

69-80-80-80            5           0.26             0.13

84-80-80-80            6.1         0.03             0.05
*values for integrated intensities, standard 3-axis mode Last modified 30-August-2001