Appl. Phys. A 74 [Suppl.], S133–S135 (2002) / Digital Object Identiﬁer (DOI) 10.1007/s003390201536 Applied Physics A Materials Science & Processing Status of the high-ﬂux backscattering spectrometer RSSM for the FRM-II reactor in Munich O. Kirstein ∗ , T. Kozielewski, M. Prager, D. Richter Institut für Festkörperforschung, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany Received: 16 July 2001/Accepted: 13 November 2001 –  Springer-Verlag 2002 Abstract. The new backscattering spectrometer RSSM for the FRM-II reactor uses all possibilities to optimize this type of spectrometer. In its high-intensity version it uses unpolished Si(111) crystals. The good energy resolution of δ E ≤ 0.9 μ eV is achieved by staying as closely as possible at a Bragg angle of 90 ◦ not only at the Doppler monochromator in the primary spectrometer but also at the analyzer system of the secondary spectrometer. The large solid angle of the ana- lyzer system compensates the low intensity which is a result of the good energy resolution. PACS: 28.20.Cz; 28.41.Rc; 61.12.Ex The backscattering spectrometer RSSM is based on the de- sign and performance of existing spectrometers like the IN16 at the ILL or the HFBS at NIST [1,2]. In order to maxi- mize the ﬂux at the sample position the spectrometer will be equipped with a focusing super mirror guide (realized e.g. at the IN16 or the BSJ at the Jülich DIDO reactor) and a phase-space-transformation chopper PST [3] (realized at ∗ Corresponding author. (Fax.: +49-2461/612-610, E-mail: o.kirstein@fz-juelich.de) Fig. 1. Schematic 3D view of the FRM-II backscattering spectrometer with the most important components the HFBS). This chopper changes the properties of neutrons reﬂected at a moving crystal in order to enhance the neu- tron intensity at the desired backscattering energy (RSSM: Si(111)-PG(002)-crystal system). 1 Backscattering Neutron backscattering spectroscopy, based upon an idea of Maier-Leibniz [4], uses the fact that the neutron wavelength Fig. 2. View of a large angle analyzer. The analyzer height is 2m. Shown is also CAD aided drawing Si crystal arrangement