Page 80 - ILLS Annual Report 2018

MODERNISATION PROGRAMME AND INSTRUMENT UPGRADES
First results with BATS on IN16B – inverted neutron time-of-flight spectroscopy
While neutron backscattering spectrometers at reactors attain unequalled energy resolutions below 1 μeV, they are rather limited in their accessible energy-transfer range. With the BATS option commissioned in 2018, the IN16B backscattering spectrometer at the ILL now offers a flexible set-up with an extension in energy transfer exceeding one order of magnitude at the cost of a slightly broadened resolution. Ongoing enhancements of the neutron optics will also make the count rates of
the new BATS option highly competitive.
AUTHORS
M. Appel (ILL and FAU Erlangen-Nürnberg, Germany) B. Frick (ILL)
A. Magerl (FAU Erlangen-Nürnberg, Germany)
REFERENCES
[1] B. Spiegelhalder, U. Keller and E. Käppeler, German Patent No. DE 10 2014 004 994 B3 (2015)
[2] M. Appel, B. Frick and A. Magerl, Sci. Rep. 8 (2018) 13580
[3] M. Appel, B. Frick and A. Magerl, Physica B 562, 6-8 (2019),
DOI: 10.1016/j.physb.2018.11.062
Many neutron spectroscopy experiments aim to study dynamics on the pico- to nanosecond time scale over distances of Ångstrom to several nanometres, such as the rotational or translational diffusion of molecules, the diffusion of protons or lithium in energy materials or the multi-scale dynamics in biological or glassy materials. Frequently, separate measurements on neutron time-of-flight (ToF) and backscattering machines need to be combined to cover the necessary, broad dynamic range. The recently commissioned Backscattering and ToF Spectrometer (BATS) option on IN16B at the ILL combines the advantages of both techniques in
one set-up. In BATS mode, the incident neutron beam is
not conditioned using a crystal monochromator; instead, a novel, flexible chopper system installed 34 m upstream of the sample position creates short neutron pulses such that the incident neutron energies in the sample can be reconstructed from a ToF measurement. Neutrons scattered from the sample are analysed by the same, large backscattering crystal analysers (typically Si 111) as in IN16B’s conventional mode of operation. The chopper system is the first to use a new generation of oriented carbon fibre-reinforced polymer composite disks [1] of 750 mm diameter, rotating at up to 19 000 rpm. It has been designed for maximum flexibility [2] and features multiple slits of seven different widths on two counter-rotating chopper pairs. While the BATS option was anticipated in the ILL’s design of IN16B from the beginning, the design and realisation of the chopper system over
the last few years was made possible by grants from the German Ministry of Education and Research (BMBF, grants 05K13WE1 and 05K16WEA).
Figure 1
Neutron scattering spectrum recorded on γ-picoline-N-oxide
at T = 1.8 K with BATS on IN16B summed over all detectors. Three consecutive measurements (6 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 is 3.8 μeV (from Ref. [3]).
100 10−1 10−2 10−3 10−4 10−5
Intensity (a.u.)
ANNUAL REPORT 2018
−400 −300
−200 −100
E (μeV)
0 100 200 300