Le, M.D.; Skoulatos, M.; Quintero-Castro, D.L.; Toft-Petersen, R.; Groitl, F.; Rule, K.C.; Habicht, K.: The upgraded cold neutron three-axis spectrometer FLEXX at BER II at HZB. Neutron News 25 (2014), p. 19-22
10.1080/10448632.2014.902699
Open Access Version
Abstract:
The cold neutron three-axis spectrometer FLEXX is a work-horse instrument for inelastic neutron scattering matching the sample environment capabilities for high magnetic fields up to 17.5 T and low temperatures down to 30 mK at the BER II neutron source at HZB. During the upgrade of the BER II neutron source and its instruments the primary spectrometer of FLEX [1] was completely rebuilt leading to a substantial increase in the flux reaching the sample [2]. The major benefit from the exchange of the entire neutron guide system serving instruments in Neutron Guide Hall I is that it has allowed optimisations of individual instrument positions. As a consequence FLEXX was relocated to a guide-end position in an area of the guide hall with intrinsically low background (Fig. 1). The neutron guide system of FLEXX now hosts m=3 guides including a converging elliptical section to focus neutrons onto a virtual source [3, 4]. The neutrons are subsequently imaged onto a new double focussing monochromator, ensuring a substantial increase in neutrons reaching the sample [5]. In addition, a new velocity selector is used to remove higher order scattering which eliminates the need for filters. A polarising S-bender may be translated into the beam before the elliptical guide section where the beam is collimated allowing the gains from focussing neutrons onto the sample to be realised for polarised measurements also (Fig. 2). A Heulser analyser is now optionally available replacing the PG analyser when FLEXX is used for longitudinal polarisation analysis or operated with its neutron resonance spin echo option further enhancing polarised neutron capabilities at FLEXX.