Three-axis spectrometer FLEXX

Fig.1: The upgraded cold neutron triple-axis spectrometer FLEXX.

FLEXX is a high-output instrument for inelastic neutron scattering using the outstanding sample environment capabilities at HZB (Fig. 1). Low-energy excitations such as phonons and magnons can be measured in a large parameter space, at temperatures as low as 40 mK and magnetic fields as high as 17 T. The instrument is in user operation for external researchers for a range of scientific topics including quantum magnetism and unconventional superconductivity. FLEXX also provides important insights for in-house research on energy materials, with focus on the transport properties in thermoelectric materials.

Since the neutron guide upgrade accomplished in 2012, FLEXX is located at a guide-end position in an area of the guide hall with intrinsically low background. During the upgrade, the primary spectrometer of FLEXX was completely rebuilt with m=3 guides including a converging elliptical section to focus neutrons onto a virtual source [6-8]. From the virtual source, the neutrons are imaged onto a double focusing monochromator, which maximizes the neutron flux at the sample. Experiments have confirmed an order of magnitude gain in intensity [4,5]. In addition, a velocity selector is used to remove higher-order scattering which eliminates the need for filters. A polarizing S-bender may be translated into the beam before the elliptical guide section where the beam is collimated, allowing the gains from focusing neutrons onto the sample to be realized for polarized measurements as well (Fig. 2). A focusing Heusler analyzer is available to further enhance polarized neutron capabilities at FLEXX.

Fig. 2: Schematic drawing of the key components of FLEXX.

In 2016, the construction of the new multi-analyzer MultiFLEXX was completed [1]. It consists of 31 angular segments that each contain 5 vertically scattering analyzers [2]. Each analyzer is set to a fixed wavelength, corresponding to final energies between 2.5 meV and 4.5 meV. Thus, MultiFLEXX allows a large region of reciprocal space to be measured simultaneously by simply rotating the sample. Switching between FLEXX and MultiFLEXX takes only half an hour, and all usual sample environment set-ups are compatible with MultiFLEXX. It is thus a convenient new tool for parametric overview studies of excitations as a function of magnetic field or temperature.

Larmor labeling is seen as one of the key ingredients in the development of novel neutron instrumentation. FLEXX puts special emphasis on exploiting the neutron resonance spin echo (NRSE) technique for high-resolution spectroscopy on dispersive quasi-particle excitations. This enables unique measurements in single crystal spectroscopy over large portions of the Brillouin zone. To keep the experimental opportunities at BER II at the forefront the NRSE option available at FLEXX was upgraded along with the change of the primary spectrometer [3].

References

[1] F. Groitl, R. Toft-Petersen, D.L. Quintero-Castro, S. Meng, Z. Lu, Z. Huesges, M.D. Le, S. Alimov, T. Wilpert, K. Kiefer, S. Gerischer, A. Bertin, K. Habicht, MultiFLEXX - The new multianalyzer at the cold triple-axis spectrometer FLEXX, Sci. Rep. 7 (2017) 13637.

[2] R. Toft-Petersen, F. Groitl, M. Kure, J. Lim, P. Čermák, S. Alimov, T. Wilpert, M.D. Le, D. Quintero-Castro, C. Niedermayer, A. Schneidewind, K. Habicht, Experimental characterization of a prototype secondary spectrometer for vertically scattering multiple energy analysis at cold-neutron triple axis spectrometers, Nucl. Instrum. Methods Phys. Res. A 830, 338-344 (2016).

[3] F. Groitl, T. Keller, D. Quintero-Castro, K. Habicht, Neutron resonance spin-echo upgrade at the three-axis spectrometer FLEXX, Rev. Sci. Instrum. 86 (2015) 025110.

[4] K. Habicht, D.L. Quintero-Castro, R. Toft-Petersen, M. Kure, L. Mäde, F. Groitl, M.D. Le, The upgraded cold neutron triple-axis spectrometer FLEXX - enhanced capabilities by new instrumental options, EPJ Web Conf. 83 (2015) 03007.

[5] M.D. Le, D.L. Quintero-Castro, R. Toft-Petersen, F. Groitl, M. Skoulatos, K.C. Rule, K. Habicht, Gains from the upgrade of the cold neutron triple axis spectrometer FLEXX at the BER-II reactor, Nucl. Instrum. Methods Phys. Res. A 729 (2013) 220-226.

[6] K. Habicht, M. Skoulatos, Optimization of virtual source parameters in neutron scattering instrumentation, J. Phys. Conf. Ser. 340/1 (2012) 012029.

[7] M. Skoulatos, K. Habicht, K. Lieutenant, Improving energy resolution on neutron monochromator arrays, J. Phys. Conf. Ser. 340/1 (2012) 012019.

[8] M. Skoulatos, K. Habicht, Upgrade of the primary spectrometer of the cold triple-axis spectrometer FLEX at the BER II reactor, Nucl. Instrum. Methods Phys. Res. A 647/1 (2011) 100–106.