Page 85 - ILL Annual Report 2019

MODERNISATION PROGRAMMES AND TECHNICAL DEVELOPMENTS
82-83
Péter Falus. Hungarian
The ILL
‘I am an instrument scientist at the ILL.
My main area of interest is improving spectroscopy instruments for soft matter studies. I have worked at the X-ray Photon Correlation Spectroscopy beamline at the
Advanced Photon Source and I participated in the upgrade of the IN15 neutron Spin-Echo Spectrometer. I have been the scientific project leader of the WASP instrument since 2013.’
WASP takes off
The first neutron spin-echo instrument, IN11, was built at the ILL and was in operation for 40 years. After all those years of exciting science, the moment has come to turn the page and welcome the newest spin-echo instrument: WASP. The new instrument has just begun commissioning but already surpasses IN11 in both intensity and the highest available resolution. In 2020, WASP will take over from IN11 and will complement the high-resolution spin-echo instrument
IN15 for wide-angle measurements.
Neutron Spin-Echo (NSE) reveals atomic motions by encoding the neutron energy into the number of rotations of the neutron spin. Since the number of spin rotations depends on the strength of the magnetic field, the secret to a good NSE instrument is creating symmetric magnetic fields (so-called ‘precession fields’) before and after the sample.
All functioning, non-resonant NSE spectrometers use the basic IN11A design, where the precession field is generated by long solenoids along the neutron beam. This construction—while it gives high resolution—limits the angular coverage and count rate of the instruments.
About 20 years ago, the IN11 set-up was upgraded to make a wide-angle coverage, neutron spin-echo instrument, IN11C [1], equipped with a flattened solenoid. It has a 30 degree-wide angular coverage but limited range in spin-echo time. This instrument was practically trading intensity for resolution, but was a very successful compromise at the time. The SPAN instrument [2] at HZB extended the angular coverage even further. A pair of coils in the anti-Helmholtz configuration created an azimuthally symmetric magnetic field, which in theory could allow almost 360-degree detector coverage.
The newly commissioned WASP uses an improved SPAN construction [3]. It aims to have a detected intensity 500 times higher than that of IN11A while the resolution remains the same. Compared with IN11C, WASP aims to have 25 times the intensity and six times the resolution.
AUTHORS
O. Czakkel, P. Falus, B. Faragó and P. Fouquet (ILL) REFERENCES
[1] B. Farago, Physica B 241–243 (1998) 113
[2] C. Pappas et al., Physica B 267–268 (1999) 285 [3] P. Fouquet et al., J. Neutron Res. 15 (2007) 39
1.0 0.8 0.6 0.4 0.2
Resolution at 6 Å wavelength and Q=1.6 Å -1 wavenumber
WASP Oct 2019
Best IN11C 1998 WASP Oct 2018
0.00 1 2 3 4 5 NSE time [nsec]
Figure 1
Resolution functions versus spin-echo time. The longer the function time, the higher the instrument resolution. The red WASP curve clearly surpasses the yellow best resolution of IN11C.
Construction of the instrument was completed in October 2018, and during 2019 the commissioning and alignment continued to achieve echo signals in all three 30-degree-wide detector banks. The three banks are motorised, covering the -60 to +135 degree scattering angle range. At a 6 Å wavelength on the same sample a single WASP detector bank has eight times the IN11C count rate, while 25 times higher intensity can be achieved using all three banks. In the future, more banks will cover the whole angular range without having to move the detectors. In the medium term, a chopper system will run the instrument with pulsed neutrons simulating capabilities at ESS or SNS. The paramagnetic echo option has just been installed and the range of wavelengths and sample environments available will gradually be expanded.
Before any NSE measurement, a resolution function is measured on a non-moving reference. Ideally, this resolution has a constant value of unity as a function of time. Imperfections in the instrument will make this resolution function drop for higher spin-echo times. The higher the spin-echo times that we can reach without the resolution dropping
to zero, the better the instrument’s energy resolution. Figure 1 plots the resolutions of IN11C and WASP before and after tuning at a 6Å wavelength, respectively, showing WASP to surpass IN11C in both intensity and resolution.
In conclusion, the new instrument WASP bridges the gap between high-resolution spin-echo and backscattering instruments. It offers a similar wave number range to that achieved through backscattering, and slightly higher resolution. It also complements backscattering by measuring S(Q,t) in the time space, not S(Q,w) in the energy space. At the other end of the spectrum, WASP and IN15 will complement each other too. The lowest WASP Q-values will slightly overlap with the highest IN15 Q-values, and the WASP NSE times at the longest wavelengths will be just about the same as the IN15 NSE times at the shortest wavelengths.
WASP is ready to welcome all users who need better resolution than backscattering can achieve, or wider q-range than classical spin-echo can achieve.
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Resolution