XES: A Johann-type spectrometer with 5 spherically bent crystals on 0.5 and 1.0 m Rowland circles 

The spherically-bent crystal analyzers, the sample and the detector are mounted in vertical Rowland geometry. Each of the five crystals monochromatizes the fluorescence according to Bragg’s law and focusses the reflected monochromatic photons into the detector. The resolution of the instrument depends on the selected Bragg angle of the analyzer and is thus linked to the selected energy. The best resolution is obtained in the backscattering geometry at Bragg angles close to 90° due to the geometrical effect of the Johann geometry. The resolution is further improved by a motorized slit (typically 1 mm) in front of the detector. ROBL’s 5-crystal spectrometer covers the Bragg angles 65-89°. For concentrated samples an avalanche photodiode (APD) with an area of 10 x 10 mm2, and for dilute samples a Ketek detector with 10 mm diameter and 450 µm Si thickness are available.

The 5-crystal spectrometer can be operated with crystal analyzers with 0.5 m and 1 m bending radius. Available spherically-bent striped crystal analyzers (~100 mm diameter) cover most elements of the periodic table in the energy range 3 – 20 keV. Note that the whole spectrometer setup (sample, analyzer crystals and detector) can be placed into a He-filled bag to obtain sufficient fluorescence yield also at the lowest energies.