Three-axis spectrometer FLEXX
FLEXX is a work-horse instrument for inelastic neutron scattering matching the 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 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 the 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 [5-7]. The neutrons are subsequently imaged onto a double focusing monochromator, maximizing the neutron flux at the sample - experiments confirmed an order of magnitude gain in intensity [3,4]. 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.
In 2016, the construction of the new multi-analyzer MultiFLEXX was completed. It consists of 31 angular segments that each contain 5 vertically scattering analyzers . 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 .
Fig.1: The upgraded cold neutron triple-axis spectrometer FLEXX at its new position in neutron guide hall I at the BER II facility.