SPHERES

This instrument is focussed on cold neutrons. All parameters given here are valid during the current operation of FRM II. Please get in touch with the instrument team well in advance for all further details (length of experiment etc.).

Insrumentscheme SPHERES

SPHERES (SPectrometer for High Energy RESolution) is a third-generation backscattering instrument with focussing optics and a phase-space-transform (PST) chopper [1]. It is a versatile spectrometer for the investigation of atomic and molecular dynamics on a GHz scale. The necessary filtering of neutron energies is achieved by Bragg reflection from perfect monochromator and analyser crystals under angles close to 180°. The backscattering geometry makes it unavoidable to use a primary beam deflector and a duty-cycle chopper.

At SPHERES, these two functions are both realised 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 redesign of PST chopper and focussing guide in recent years led to a more than doubled intensity. The newly installed background chopper also allows for a low background setup with significantly increased signal-to-noise ratio by eliminating every second pulse, albeit at the cost of intensity.

SPHERES enables investigations on a broad range of scientific topics. It is in particular sensitive to the incoherent scattering from hydrogen and allows to access dynamic processes up to a timescale of a few ns. Typical applications include for example relaxation processes in soft matter materials like polymers [2] and nanocomposites, dynamics in biological systems [3] or hydrogen diffusion in various systems like ionic conductors and fuel cells [4]. Furthermore, the high resolution of the spectrometer allows for investigation of dynamics of viscous liquids and water in confined geometry [5]. Other important applications are hyperfine splitting in magnetic materials [6] or reorientational motions and rotational tunneling [7].

Raw histograms are accumulated on an equidistant ω grid. A script driven program, SLAW [8], is provided to normalise 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 program. In data fitting, it is critically 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 [9].

[1] Wuttke et al., Rev. Sci. Instrum. 83, 075109 (2012).
[2] Zorn et al., Macromolecules 53, 6731 (2020).
[3] Schirò et al., Phys. Rev. Lett. 126, 088102 (2021).
[4] Appel et al., RSC Adv., 9, 37768 (2019).
[5] Soininen et al., J. Chem. Phys. 145, 234503 (2016).
[6] Chatterji et al., J. Phys.: Condens. Matter 31, 025801 (2019).
[7] Rok et al., CrystEngComm 22, 6811 (2020).
[8] Wuttke, J.: SLAW – a neutron histogram to scattering law converter, https://jugit.fz-juelich.de/mlz/spheres/slaw
[9] Wuttke, J.: FRIDA – fast reliable interactive data analysis, https://jugit.fz-juelich.de/mlz/frida