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 2.4 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.
Users may find this Powerpoint slides useful.
Specifications:
- Source: neutron guide NG-5, 50 mm x 120 mm
- Monochromator: PG(002) vert. focusing, 30' FWHM, incident energies from 14 to
2 meV, wavelengths from 2.4 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 Å 1 0.28
55-40-40-40 4 Å 0.24 0.17
69-80-80-80 5 Å 0.26 0.13
84-80-80-80 6.1 Å 0.03 0.05
*values for integrated intensities, standard 3-axis mode
|
