IN16B: commissioning of the two projects BATS and GaAs


The two IN16B instrument projects BATS and GaAs have just been successfully commissioned. They were developed in collaboration between the Friedrich-Alexander University Erlangen-Nürnberg and the ILL over the last years, and both ventures were largely financed by the German Federal Ministry of Education and Research through its ‘Verbundforschung’ programme.

The BATS (Backscattering And Time-of-flight Spectrometer) option is an inverted ToF extension with the Si 111 and Si 311 analysers on IN16B with μeV energy resolution. It extends the energy transfer by a factor six for energy windows placed symmetrically around the elastic line and much further with inelastic offsets (see Fig. 1 (png - 111 Ki)).
To achieve the high energy resolution an advanced chopper system with a new technique to produce discs from oriented carbon fibres was specifically developed for BATS by Airbus DS. For the first time, such a neutron backscattering spectrometer is conceived at reactors where the independence from a given pulsed source frequency allows for high flexibility.

The GaAs project is a prototyping of a higher-resolution backscattering spectrometer using the GaAs 200 reflection, which has a 10 times smaller Darwin width than the usually used Si 111. Several challenges have been met along the way to reach a significant improvement like the single crystal quality (collaboration with Freiberger Compound Materials GmbH), precision of alignment and compensation for gravity. During the commissioning of a GaAs prototype, a world record in energy resolution of 77 neV FWHM has been achieved (see Fig. 2 (png - 220 Ki)), which presents the first major improvement in energy resolution since the invention of neutron backscattering 50 years ago.
While the GaAs project demonstrated the possibilities for future spectrometers with excellent energy resolution, the highly flexible BATS option with its extended energy transfer range is available to the user community and open for proposals.

(png - 111 Ki)
(png - 220 Ki)
Figure 1: Tunneling spectrum obtained with BATS on IN16B using three different offsets for the energy transfer window.
Figure 2: Hyperfine splitting in cobalt measured with the GaAs prototype on IN16B, compared to data from two Si configurations.
Numbers left of the curves indicate the instrumental resolution.



Dr Bernhard Frick, ILL

Dr Markus Appel, ILL