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SPHERES (SPectrometer for High Energy RESolution) is a third-generation backscattering instrument with focussing optics and a phase-space-transform chopper. It is a versatile spectrometer for investigating atomic and molecular dynamics on a GHz scale.
The necessary filtering of neutron energies is achieved by Bragg reflection from perfect monochromator and analyzer crystals under angles close to 180°. The backscattering geometry makes it unavoidable to use a primary beam deflector and a duty-cycle chopper. In SPHERES, these two functions are realized jointly by a chopper that bears deflector crystals on its circumference. This leads to a particularly compact spectrometer layout so that full use can be made of the focussing neutron guide. As an additional advantage, the fast motion of the deflector crystals achieves a phase-space transform of the primary spectrum, thereby enhancing the flux at the sample.
The principal figures of merit (spectral flux, resolution, dynamic range, signal-to-noise ratio Fig. 1)qualify SPHERES as one of the best of its class . Count rates and signal-to-noise ratio have been improved by filling the entire instrument with argon, thereby avoiding air scattering in the secondary spectrometer. Another gain in flux will be achieved by a more efficient phase-space transform chopper, which is currently under development.
As a multi-detector instrument with relaxed angular resolution, SPHERES is particularly suited for studying tagged-particle motion by incoherent scattering. A hot topic is the dynamics of water in confined geometry. The unprecedented sensitivity of SPHERES helps us to detect the onset of quasielastic scattering deep in the supercooled state . Other important applications are hyperfine splitting in magnetic materials  and rotational tunneling . The high count rates allow inelastic temperature scans (Fig. 2) and real-time kinetic experiments .
Raw histograms are accumulated on an equidistant ω grid. A script driven program, SLAW , is provided to normalize the raw counts, to perform optional binning, and to deliver S(q,ω) in a variety of output formats so that users are not bound to any specific data analysis programm. In data fitting, it is critcially important to convolute theoretical models with the measured resolution function in an efficient and numerically stable way. We strive to support best practice through our FRIDA package .
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 J. Wuttke: SLAW – a neutron histogram to scattering law converter, http://apps.jcns.fz-juelich.de/slaw
 J. Wuttke: FRIDA – fast reliable interactive data analysis, http://apps.jcns.fz-juelich.de/doku/frida/start