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HFBS: instrument details
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A brief description of the backscattering spectrometer is given on this
webpage. For detailed information, please see the following (downloadable)
paper:
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The High Flux Backscattering Spectrometer at the NIST Center for
Neutron Research
A. Meyer, R. M. Dimeo, P. M. Gehring, and D. A. Neumann,
Rev. Sci. Instrum., 74, 2759 (2003).
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The backscattering technique
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Backscattering spectroscopy is based on the fact that the
wavelength spread, δλ, of a Bragg-diffracted neutron beam
decreases as the scattering angle, 2Θ, approaches 180°. The
equation below, derived by differentiating Bragg's law and then dividing
the result by λ, shows the dependence of the relative wavelength
spread on the crystal planes and the scattering angle:
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As Θ approaches 90°, the angular term vanishes. There is
thus a minimum in the attainable wavelength spread that depends on the
average value and spread of the spacing, d, between the Bragg planes in the
crystal. The δd/d for the silicon (111) plane, the most commonly
used configuration in backscattering instruments, is 1.86 x 10-5.
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In the case of the HFBS, we have the following:
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The NCNR backscattering spectrometer
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A backscattering spectrometer is an inverse-geometry instrument in
which only those neutrons with a particular final energy (2.08 meV in this
case) are detected in the counting process. The motions in the sample are
probed by varying the energy of the incident neutrons (by Doppler motion
of the monochromator) and measuring the gain or loss in energy that they
undergo in their interaction with the sample.
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The backscattering spectrometer at the NCNR is located on neutron
guide NG2. The photograph below shows the instrument as seen from the
north-east corner of the Guide Hall.
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Plan view of the HFBS:
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| Converging neutron guide
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The HFBS is located at the end of a dedicated neutron guide, NG2.
Neutrons from the source reach the instrument by travelling along an evacuated
glass guide 41.1 m in length, with a cross section of 15 cm by 6 cm. The
top and bottom interior surfaces are coated with NiCTi supermirrors with
a critical angle of reflection given by 4πΘc =
Qcλ = (0.044 Å-1) λ, while
the interior side surfaces are coated with 58Ni equivalent
supermirrors for which 4πΘc
= (0.026 Å-1) λ.
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| About 26 m downstream of the source, a cut in the guide accomodates
a neutron velocity selector, and bismuth and beryllium filters. These serve
to reduce the background in the instrument by limiting the number of γ-
rays and those neutrons with energies outside the usable range of the HFBS.
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As the neutrons enter the scattering chamber, a converging guide is
used to focus the large beam from the guide down to a size that is
commensurate with typical sample dimensions: 2.8 cm wide by 2.8 cm high.
The guide entrance is located 41.3 m downstream from the cold source, just
after the local beam shutter and before the PST chopper. All four of its
interior surfaces are coated with 2&ThetacNi
equivalent supermirrors. The overall gain in intensity due to the
converging guide is a factor of 3.4 over the beam from the main guide.
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The phase space transformation chopper
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The phase space transform (PST) chopper exploits the physics of
Bragg diffraction from moving crystals to shift the neutron energies
toward the desired value of 2.08 meV. Neutrons that are diffracted from
the rotating PST chopper are Doppler-shifted up or down in energy
depending on whether they are less than or greater than 2.08 meV,
respectively. Essentially, the PST transforms the shape of the incoming
neutron beam in phase space to enhance the flux at the backscattering
energy of 2.08 meV at a cost of an increased horizontal divergence (the
divergence increases from 3° to 17°).
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The idea, developed by Schelten and Alefeld [1], was proposed
to address the significant mismatch in divergence between the primary
and secondary spectrometer. The HFBS is the first instrument in the
world to incorporate such a device.
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Measured flux gain as a function of crystal speed. The gain peaks
at 4.2 at an operating speed of 250 m/s.
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The PST on the HFBS is a 1 m diameter disk whose periphery is divided
into 6 sectors (see photograph reproduced below). Alternate sectors
are covered with crystals of highly oriented pyrolytic graphite (HOPG)
that are 34.5 mm high, 28.0 mm wide, and 1.5 mm thick. The mosaic
spread of an individual crystal is 2.5° and three crystals are
stacked together to yield an overall mosaic spread of 7.5°.
As seen from the graph above, a rotation rate of 4732 RPM results
in the maximum gain of 4.2 times the flux at standstill.
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[1]. J. Schelten, B. Alefeld, in: R. Scherm, H. H. Stiller (Eds.),
Proc. Workshop on Neutron Scattering Instrumentation for SNQ, Report
Jul-1954, 1984.
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Monochromator and Doppler drive
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The neutrons diffracted from the PST are then monochromated by silicon
crystals that are mounted on a support structure that is curved to focus
the neutrons onto the sample position. The strain arising from the bending
serves to enhance the backscattered flux at the expense of energy resolution.
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The HFBS employs a mechanical Doppler drive to produce an oscillatory
motion of the monochromator in order to vary the incident energy of the
neutrons. The motion is oriented along the average silicon wafer [111]
direction so as to maintain the backscattering condition to achieve the
sub µeV energy resolution. In contrast to other backscattering
spectrometers, the HFBS drive system is a cam-based one which allows
the velocity profile of the motion to be determined by the shape of
the cam. As implemented on the HFBS, the velocity profile of the
monochromator is a rounded triangle. This shape avoids the large
accelerations resulting from a pure triangle profile but still allows
almost equal weighting to all energy transfers in the dynamic range.
This is not possible with a conventional sinusoidal velocity profile
(found in locomotive-type drives) which applies more weight to large
energy transfers.
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The Doppler drive is designed to achieve a top frequency of 25 Hz which
corresponds to an acceleration in excess of 100 g's. The support
structure is constructed from a graphite composite with a foam core to
minimize the total mass of the assembly. In addition, the maximum
deflection of the structure at the highest operating speed is designed
to be less than 0.25 mm to keep the energy resolution as narrow as
possible.
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The drawing above shows the monochromator support installed on the
Doppler drive and cam system. Specifications on the support are given
below:
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Dan Neumann inspects the Debye-Scherrer rings inside the HFBS
scattering chamber. At the center of the rings is the transmitted
beam monitor (TBM). During normal operation, the chamber is
evacuated to reduce air scattering of the neutrons.
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Neutron detectors
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The detectors used in the HFBS are pressured 3He detectors,
with the exception of a fission chamber that is used to monitor the incident
beam.
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The HFBS sample area viewed from a location downstream of the neutron guide.
The shiny rectangle directly in front is the cover for the primary beam stop
for the source neutrons coming down NG2. To the right of that is the detector
assembly.
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The HFBS sample area viewed from a location downstream of the monochromator,
roughly 45° to the view in the previous photograph. The low angle
detectors, mounted on the PST, are visible in this shot.
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The following is a list of all the detectors:
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Low angle (1 through 3): Three rectangular detectors with
38 mm x 38 mm active area, and 3He fill pressure of 2.5 bar.
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High angle (4 through 16): One 25 mm-diameter detector (#4),
and twelve 12.5 mm-diameter detectors, with 3He
fill pressure of 6 bars.
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Monitors: Pencil-type 3He detector to monitor the white
beam (white beam monitor, WBM), fission chamber placed to intercept the
neutrons just before they hit the sample (incident beam monitor, IBM), and
a 3He rectangular detector for the transmitted beam
(transmitted beam monitor, TBM), located at the center of the Debye-Scherrer
rings (visible in the photograph in the preceeding section on the analyzers).
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