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85748 Garching

SPHERES

Backscattering spectrometer

SPHERES scheme SPHERES scheme

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 [1]. 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 [2]. Other important applications are hyperfine splitting in magnetic materials [3] and rotational tunneling [4]. The high count rates allow inelastic temperature scans (Fig. 2) and real-time kinetic experiments [6].

Raw histograms are accumulated on an equidistant ω grid. A script driven program, SLAW [7], 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 [8].

[1] J. Wuttke et al., Rev. Sci. Instrum. 83, 075109 (2012).
[2] W. Doster et al., Phys. Rev. Lett. 104, 098101 (2010).
[3] T. Chatterji, G. J. Schneider, and R. M. Galera, Phys. Rev. B 78, 012411 (2008).
[4] M. Prager, A. Pawlukojc, A. Wischnewski, J. Wuttke, J. Chem. Phys. 127, 214509 (2007).
[5] W. Häußler et al., Neutron News 22, 24 (2011).
[6] A. Leon, J. Wuttke, J. Phys.: Condens Matter 23, 254214 (2011).
[7] J. Wuttke: SLAW – a neutron histogram to scattering law converter, http://apps.jcns.fz-juelich.de/slaw
[8] J. Wuttke: FRIDA – fast reliable interactive data analysis, http://apps.jcns.fz-juelich.de/doku/frida/start

Typical Applications
  • Hyperfine splitting
  • Molecular reorientations and rotational tunneling
  • Dynamic signature of phase transitions
  • Hydrogen diffusion
  • Liquid dynamics
  • Polymer relaxation
  • Protein aggregation
Sample Environment
  • Cryofurnace 2..700 K
  • Dilution inset 20 mK
  • Furnace
Technical Data

Primary beam

  • Neutron guide: NL6-S
  • Neutron wavelength: 6.27 Å
  • Neutron energy: 2.08 meV

Main parameters

  • Resolution FWHM: 0.60 – 0.65 µeV
  • Dynamic range: ± 31 µeV
  • Q range: 0.2 – 1.8 Å-1
  • Flux after selector: 1010 s-1
  • Flux at sample: 8 ·105 s-1
  • Illuminated area: 30 × 30 mm2

Instrument Scientists

Dr. Michaela Zamponi
Phone: +49.(0)89.289.10793
E-Mail:

Dr. Marina Khaneft
Phone: +49.(0)89.289.11676
E-Mail:

Dr. Gerald J. Schneider
Phone: +49.(0)89.289.10718
E-Mail:

SPHERES
Phone: +49.(0)89.289.14875

Operated by

JCNS

Gallery

Resolution at SPHERES
Resolution at SPHERES

Figure 1: A resolution of 0.65 ueV, a dynamic range of ±31 µeV, and a signal-to-noise ratio of 1000 : 1 or better are routinely achieved in user experiments [5].

Inelastic temperature scan
Inelastic temperature scan

Figure 2: This inelastic temperature scan, measured during 23 h,
revealed a hitherto unknown phase transition in Mg(NH3)6Cl2 [5].

SPHERES
SPHERES
| © © Andreas Heddergott/ TU Muenchen

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