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Von Hamos geometry

Figure 1: von Hamos geometry for the XAFS and XES spectrometer, respectively. Both are based on the standard von Hamos geometry but adapted to the specific needs. This is done by, on the one hand, tilting the detector perpendicular to the incoming beams and on the other hand by enlarging the optic to a full figure of revolution with a detector oriented perpendicular to the cylinder axis.

The von Hamos geometry was first developed by von Hamos in 1934. He proposed to use cylindrically bent crystals in contrast to flat ones to enlarge the solid angle of detection.

The principle of the von Hamos geometry is rather simple. The X-ray source is placed on the cylinder axis of the dispersive element (for hard X-rays usually crystals). The divergent emitted X-rays will be diffracted at the dispersive element according to Bragg’s law along a circle segment of the cylinder. All these diffracted X-ray s will be focused onto one point on the cylinder axis again.

Different changes to this standard von Hamos geometry were made to optimize the setups for our needs.

For the XAFS spectrometer the CCD is placed perpendicular to the reflected beam, see Figure 1 top. The obvious disadvantage is that only one energy will be sagittaly focused in the CCD plane (here E3). Nevertheless, this results in the significant advantage of a much larger observable energy range, which is in the normal setup just limited by the length of the dispersive element and not the CCD size anymore. Furthermore, multi pixel events, induced due to the shallow angle for higher energies, which lead to a degradation in spectral resolving power, are minimized.

The XES spectrometer is optimized especially for high efficiency of the setup. The goal is to increase the collected fluorescence radiation that is emitted from a sample isotropically into a half sphere. Hence the solid angle of detection was optimized by maximizing the figure of revolution to a full cylinder, see figure 1 bottom. Of course, in this geometry the CCD cannot be placed on the cylinder axis as well. It is oriented perpendicular to the cylinder axis of the optic. In this configuration we can use the symmetry of the optic and therefore of the expected image for simple alignment of the whole spectrometer.

Point of contact: Wolfgang Malzer, Christopher Schlesiger

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