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HFBS: Performance and Data

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.

Vanadium ± 36 µeVVanadium resolution: Standard vanadium measured on the HFBS using a Doppler drive frequencyof 13.5 Hz (±36 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.
Resolution as function of dynamic rangeHFBS resolution as a function of dynamic range: The resolution varies as the Doppler frequency changes. The plot above shows the resolution measured at different frequencies, i.e. different dynamic ranges.

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 (though this energy range is not advisable). The instrumental energy resolution is a Gaussian-like lineshape, with a full-width-half-maximum (FWHM) about 1 µeV.

Monitor performance

The use of a PST chopper introduces a change in the distrubution of energies not uniform over the energy range. Below is shown the spectra of the monitor detector as a function of energy for largerst energy range that we use, 50 Hz or ±36 µeV

Monitor Flux DistributionMonitor Performance: The energy distribution of energies based on the speed of the PST.

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.


Examples

Below are three examples of HFBS experiments that benifit from the high resolution and increase flux.

Lutidine methyl tunneling

Tunneling spectrum of 2,6 lutidine

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.

Fixed window scan for toluene, from 100 K to 200 K.

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.

Dynamic measurement of polystyrene in toluene.

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.)

A description of the performance of the instrument is found here on this page. The resolution is calculated using a gaussian fit to a vanadium mesurement. The current resolution for backscattering is: (10-2008)


Energy Range Detectors: 1-4 (µeV) Detectors: 5-16 (µeV)
11 µeV 1.07 0.79
17 µeV 1.13 0.83
36 µeV 1.60 1.04

Detectors 1-4 are slightly out of backscattering geometry and have a degraded resolution. Detectors 5-16 are in backscattering geometry and have almost linear resolution averaged in the above table.