X-ray absorption spectroscopy requires a precisely tunable X-ray source. One possibility are laboratory based installations as for example a laser-produced plasma source.83 A synchrotron radiation facility is able to provide radiation of a very broad spectral range with very high intensity. High-energy electrons are deflected in magnetic fields, losing energy in a form of synchrotron radiation. A scheme of a typical synchrotron is shown in Figure 16. All synchrotron facilities differ in their setup, electron energy, mode of operation, etc. Typically they consist of:71
1. An electron gun (1)
2. A linear electron accelerator (2)
3. A booster ring which accelerates the electrons to their final energy (3)
4. A storage ring in which the electron beam is maintained on a specific orbit (4) a. Magnetic fields focus the beam and keep the electrons in that orbit
b. Wiggler or undulator devices which produce the synchrotron radiation by deflecting the electrons in magnetic fields
5. Beamlines tangential to the storage ring at the wiggler/undulator positions to receive the generated light beam
Figure 16 Schematic illustration of a synchrotron facility. It consists of an electron gun (1), a linear accelerator (2), a booster ring (3), a storage ring (4) and several beamlines (5). (© SOLEIL synchrotron. Adapted with permission from Ref. 84)
As the electron beam is deflected by the magnetic fields of a bending magnet/wiggler/undulator, it emits a spectrum of radiation along its path which is received by the beamline, schematically illustrated in Figure 17. One of the first components of a beamline
31 is a cooled slit serving as an aperture. This is necessary to reduce the heat load which is otherwise deposited on the following optical components. In the optical hutch the beam is collimated by a first mirror. In the DCM the beam is diffracted according to the Bragg law
UV = 2W XYU Z (13)
on both crystals (n – positive integer giving the order of diffraction, λ – wavelength of the incident beam, d – distance between the lattice plane, θ – diffraction angle). The monochromatized beam is then focused by the second mirror onto the sample in the experimental hutch. The first ionization chamber measures (cf. Figure 17), the intensity of the incident beam. There are different experimental modes for measurement of an X-ray absorption spectrum.
In transmission mode the absorption of the sample is measured before and after the sample with the help of the first and the second ionization chambers (Equation 4). Between the second and third ionization chambers a reference compound, commonly a thin metal foil, is positioned. This reference which has its absorption edge close to the edge of the investigated element is used for calibration of the DCM beamline and alignment of the spectra. In a fluorescence mode measurement, the emitted fluorescence of the sample is detected using a solid-state fluorescence detector. This detector records a complete emission spectrum. The energy range corresponding to the emission line of the element of interest is selected. Both modes can be considered as conventional XAS setup.71, 85 For advanced HR-XANES investigations, the fluorescence detector is exchanged with a MAC-spectrometer.54
Figure 17 Schematic illustration of a typical beamline (angles and distances are not to scale). The synchrotron ring with its radiation producing devices are named as “source”. The ionization chambers are abbreviated as ICn. (Reproduced with permission from Ref. 54)
In the following section, the MAC-spectrometer build and installed by the Helmholtz Young Investigator Group (HYIG) with principle investigator Tonya Vitova is described.86 The
spectrometer (cf. Figure 18) has been commissioned at the INE-Beamline for Actinide Research5 at Angströmquelle Karlsruhe (ANKA)87 synchrotron radiation facility as part of the doctoral project of Prüßmann.54 In 2016 it was moved to the recently built and commissioned ACT-Beamline at ANKA (cf. Figure 19).88
Figure 18 3D CAD model (left) and pictures of the MAC-spectrometer setup including the He environment chamber.
Figure 19 Schematic illustration of the ACT-Beamline and photographic pictures of the installed spectrometer including He environment chamber.
The spectrometer is an adapted design of the spectrometer used at the ID26-Beamline at the European Synchrotron Radiation Facility (ESRF).67 It is built in Johann geometry89 for up to five spherically bent analyzer crystals with 1 m bending radius. Sample, crystal and detector are
33 positioned on a Rowland circle, whose diameter equals the bending radius of the crystal (cf. Figure 20).
Figure 20 Schematic illustration of the Johann geometry applied for the MAC-spectrometer (left). (Reproduced with permission from Ref. 90) Picture of the installed spectrometer at the INE-Beamline at ANKA. The paths of the incident and fluorescence X-ray beams are indicated in red.
The selection of an emission energy can be achieved via the same diffraction principle as in the DCM. The position of the individual crystal is calculated according to the type of crystal (Si/Ge) used and the required Bragg angle.90 Each of the five crystals possesses four degrees of freedom as shown in Figure 21.
Figure 21 Crystal positioning stages with four degrees of freedom for each crystal to fulfil the Bragg condition for the desired energy. Only four of the five crystals are mounted. (Figure taken from Prüßmann, doctoral thesis)54
For An M edge HR-XANES investigations a He atmosphere is essential as photons with an energy of 3–4 keV are absorbed in air within several centimeters.91 Therefore, a He environment setup has been designed and assembled comprising the HYIG MAC-spectrometer and maintaining a 99% He-atmosphere (cf. Figure 22).
Figure 22 He environment for the MAC- spectrometer for low energy HR-XANES measurements with energies of 3-4 keV.
35