Page 81 - ILLS Annual Report 2018

Using Si 111 analysers, the energy resolution for spectroscopy experiments can now be varied between 1.5 μeV and 7.5 μeV in seven steps, with an energy-transfer window with a full width of 360 μeV and the additional possibility of inelastic offsets. In comparison, the standard, conventional backscattering mode of IN16B provides 0.75 μeV resolution within a window of ±31 μeV. The maximum elastic momentum transfer is identical in both cases, namely 1.8 Å-1.
First results obtained during the commissioning are now available [3]. Figure 1 shows the tunnelling spectrum of methyl groups in γ-picoline-N-oxide measured at
T = 1.8 K with an energy resolution of 3.8 μeV at the elastic line. Consecutive measurements with shifted energy observation windows have been conducted. The result demonstrates a clean, straightforward stitching of spectra with different offsets, as well as an excellent signal-to-noise ratio above 104. Figure 2 shows a measurement of the broad quasi-elastic scattering on
a glycerol sample at T = 400 K. For this experiment,
the resolution was relaxed to 6.2 μeV in order to
benefit from a higher beam intensity. As in the previous example, several measurements with different offsets were performed, demonstrating the feasibility of reaching energy transfers beyond -600 μeV.
In the near future, enhancements of the neutron optics system are to be completed in order to unlock the full potential of BATS. First, beam focusing on the sample will be much improved by an optional exchange of the last neutron guide piece—the expected flux density on small samples necessary for best BATS resolution
(4 × 10 mm2) is expected to increase by one order of magnitude. Secondly, the challenging and innovative project of realising an adaptive, variable, horizontal focusing and defocusing line around the BATS chopper system will eliminate the intensity loss caused by using slits in the chopper disks that are narrower than the existing neutron guide width. The latter system will increase the neutron flux for the best resolution setting by another factor of five. With these future enhancements, the BATS option will be highly competitive and enable excellent experiments from various scientific fields— beamtime proposals are welcome!
100 Q =1.6A˚−1 Q = 0.8 A˚ −1
10−1
10−2
Figure 2
MODERNISATION PROGRAMMES AND TECHNICAL DEVELOPMENTS
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Intensity (a.u.)
− 600
− 400
− 200 0
E (μeV)
Neutron scattering spectra recorded on glycerol at T = 400 K with BATS on IN16B for two different momentum transfers Q. Three consecutive measurements (3 h counting time each) with different offsets of the observed energy-transfer window are shown in different colours. The energy resolution FWHM at the elastic peak obtained with a Vanadium standard (black line) is 6.2 μeV (from [3]).
The new BATS chopper system: each housing contains a counter-rotating pair of disks spinning at up to 19 000 rpm.
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