BT2 TRIPLE-AXIS/POLARIZED-BEAM SPECTROMETER

Current Schedule

The BT-2 spectrometer can be operated either as a standard triple-axis spectrometer, or as a polarized-beam spectrometer, depending on the choice of monochromator. A PG(002) monochromator is used for standard 3-axis operation, or a Heusler alloy monochromator for polarized beam experiments. Remotely positionable filters, either 6" of cooled (77K) polycrystalline Be, or 2" of pyrolytic graphite, may be inserted in the beam from the neutron source before the monochromator crystal. The collimator housings before and after the sample position have been designed to provide guide fields for polarized-beam operation, and the Söller collimators and blades are made from non-magnetic materials for the same reason. Spin-rotator devices can be mounted on x-rails before and after the sample position to flip the neutron spins. There is also a guide field that can selected by computer control to be either vertical to, or in, the scattering plane. The analyzer portion of the spectrometer uses the self-contained `NBS-type' shield that houses both the analyzer and detector in one unit.

An extensive variety of ancillary equipment to control the sample environment is available, as noted elsewhere.

Contact:    Jeff Lynn, (301)975-6246, Jeff.Lynn@nist.gov

                 Ying Chen, (301) 975-6442, Ying.Chen@nist.gov

Detailed Specifications   •   Operational Notes   •   Polarized Beam Operation

Recent Thermal Triple Axis Publications

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General Specifications for BT-2

• PG or cold Be filters, remotely insertable
• Vertically focused PG or Heusler alloy monochromator crystals
• Flat PG or Heusler alloy analyzer crystals, or polarizing supermirror
• Horizontally focussed PG analyzer, optimized for 14.7 meV
• Built-in guide fields, and insertable spin rotators
• Monochromator take-off angles 2 q _M from 21 to 77.5 degrees
• Variable incident energies from 4.8 to 54 meV
• Scattering angles from -9 to 120 degrees
• Söller slit collimation as tight as 21'-5'-5'-20'(FWHM)
• Vertical or horizontal guide field at sample position with 350º access

The spectrometer is controlled by the standard ICP program that operates all the neutron spectrometers in the confinement building and a number of the spectrometers in the guide hall. Sample mounting also uses the standard motorized goniometer, and will accept all the usual ancillary equipment including the dilution refrigerator, the ³He pumping system, and the vertical field superconducting magnets.

Fluxes

Some neutron fluxes measured at the sample position are given below:

PG(002) monochromator
40 meV, 60'-60' (no filter) 5.01´ 10⁷ n/cm²/s
14.7 meV, 60'-60' (PG filter) 1.63´ 10⁷n/cm²/s
14.7 meV, 20'-20' (PG filter) 2.51´ 10⁶ n/cm²/s
5.0 meV, 60'-60' (Be filter) 1.07´ 10⁶ n/cm²/s
5.0 meV, 20'-20' (Be filter) 1.72´ 10⁵ n/cm²/s

The Heusler Alloy polarizing monochromator is typically about 5 times lower in flux, including the obligatory factor of 2 for rejecting one spin state. Part of this reduction is due to the lower reflectivity of Heusler, and the rest is from the smaller (~18´ ) mosaic spread.

Last modified 29-August-2012 by website owner: NCNR (attn: )