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Von Hamos geometry

The von Hamos geometry was first developed by von Hamos in 1934 [1]. 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-rays 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. The obvious disadvantage is that only one energy will be sagittaly focussed in the CCD plane. 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. Hence the solid angle of detection was enhanced to the maximum by maximizing the figure of revolution to a full cylinder. Of course in this geometry the CCD cannot be placed on the cylinder axis as well. It is situated 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: Christopher Schlesiger, Wolfgang Malzer

Relevant publications:

L. Anklamm, C. Schlesiger, W. Malzer, D. Grötzsch, M. Neitzel and B. Kanngießer, Rev. Sci. Instrum. 85, 053110 (2014)

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