High resolution X-ray Emission Spectroscopy (XES)

The von-Hamos-principle employs as dispersive element a cylindrically bent crystal.

© AG Kanngießer

The energies of X-ray emission lines are sensitive to the chemical environment of the probed atom. With a suitable analytical method this chemical shift can be used to probe the chemical state of specific atoms. High resolution spectrometers have to be used for such investigations as the change in emission energy is in the order of a few eV.

Up until now XES is used at synchrotron radiation facilities because an intense source for excitation is mandatory due to the small throughput of wavelength dispersive spectrometers. Together with the MBI and the PTB we are working on a new type of spectrometer which will render XES with commercially available X-ray tubes in the laboratory feasible.

Spectrometer setup for the testing and developing of the optimal concept.

© AG Kanngießer

We are developing a new von Hamos spectrometer which will facilitate micro-XES with a spatial resolution of a few 10µm using X-ray tube excitation. Our research activities comprise the development and optimisation of the instrument as well as application to various fields like material research or archaeometry.

Principle of a von Hamos spectrometer

The setup for wavelength dispersed detection of X-rays proposed by von Hamos in 1932 consists of a cylindrically bent crystal and a position sensitive detector. Along the cylinder axis the energy of the Bragg-reflected X-rays changes. The sagittal bending focuses the reflected rays and increases the solid angle of detection.

Use of HOPG

Highly Oriented Pyrolytic Graphite (HOPG) is a polycrystalline material. The orientation of the crystallites spreads slightly around the surface normal. This mosaicity considerably increases the solid angle of detection because the Bragg condition is fulfilled within an extended region along the meridional direction. For this reason the reflectivity of HOPG crystals exceeds the reflectivity of perfect crystals by one order of magnitude in the first order of reflection. Of course, on the other hand, the spectrograph’s resolution is reduced. Nevertheless, energy resolutions of E/deltaE~1000 to E/delta~4000 can be reached depending on the crystal’s thickness, mosaicity, the distance between the spectrograph’s components and the used order of reflection.

HOPG crystals have due to the high mosaicity of the material an increased reflectivity compared to other crystals.

© AG Kanngießer

Contact: Lars Anklamm, Wolfgang Malzer

© Optigraph

© MBI

© PTB