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The high-flux backscattering spectrometer located at the NIST Center
for Neutron Research incorporates several state-of-the-art
neutron optics devices (see the details
webpage for more information) to achieve the highest possible flux
on sample while maintaining an excellent energy resolution.
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At the center of the beam, the neutron fluence rate is
3 x 105 n cm-2 s-1
as determined by gold foil activation measurements. A high-speed
Doppler-driven monochromator system allows energy transfers up to
±50 µeV. The instrumental energy resolution
is a Gaussian-like lineshape, with a full-width-half-maximum
(FWHM) about 1 µeV.
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Standard vanadium measured on the HFBS using a Doppler drive frequency
of 13.5 Hz (±30 µeV), and integrating
over
0.6 Å-1 < Q < 1.6 Å-1.
The solid line represents a fit to a Gaussian function plus a constant
background. The spectrum shows an almost Gaussian-like
energy resolution with a FWHM of 0.93 µeV.
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The resolution varies as the Doppler frequency changes. The
plot above shows the resolution measured at different
frequencies, i.e. different dynamic ranges.
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The distribution of neutron energies reflected from the PST
chopper is skewed to energies greater than Eo, and
this can be seen in the monitor spectrum above (more details on
this can be found in the instrument paper). The monitor
is located between the monochromator and the sample, and detects
the neutrons before they hit the sample.
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A measurement of the methyl tunnel splitting in 2,6 lutidine
(C7H9N) on the HFBS displays the large
dynamic range as well as the excellent energy resolution (about
1.01 µeV FWHM at this dynamic range) of the
instrument. The data has been summed over 10 detectors spanning
0.6 Å-1 and
1.6 Å-1.
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The backscattering spectrometer can also be operated with the Doppler
monochromator at rest, in which case it probes only the
energy resolution limited elastic scattering intensity. This fixed
window scan (FWS) method is a useful technique for determining under
what conditions the dynamics of the system being studied lie within
the dynamic range of the spectrometer. In addition, this technique
can be useful for probing phase transitions. As an example of this,
the heating and cooling curves for toluene are shown above. In this
figure, the data were summed over ten detectors and warming and
cooling rates of 0.1 K min-1 were used. The large
hysteresis on heating and cooling, indicative of undercooling, is
clear. The solid melts at 179 K and the liquid solidifies around
160 K.
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A topic of interest is the structural relaxation in polymer
systems. A measurement of the glass former polystyrene (PS)
in a solution of deuterated toluene (15% concentration PS by
molecular weight) reveals clear non-exponential structural
relaxation. A spectrum collected at
0.99 Å-1
and a temperature of 220 K is shown here. The
data was modeled with a Kohlrausch spectral function. Because
the data has particularly large wings, and is quite narrow
near ω = 0, this example illustrates the
need for a large dynamic range and excellent energy resolution
in order to extract reliable estimates on the stretching factor,
b, and the relaxation time, t.
(M. P. Nieh, et al.)
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