26 November 2014
I20-1 is now I20-scanning
I20-2 is now I20-EDE
Please reference the beamline and the branchlines with these new names in your publications.
I20 is equipped with two wigglers in the same straight section, one for the scanning branch (I20-scanning) and other for the dispersive branch (I20-EDE). The two beamlines can operate independently and simultaneously. The scanning branch offers monochromatic X-rays with high flux and high spectral purity in energy resolution and low harmonic content for transmission and fluorescence measurements. The dispersive branch offers the possibility of performing in-situ time-resolved X-ray spectroscopy studies.
The beamline aims to cover three very distinctive modes of operation:
1) X-ray Absorption Spectroscopy (XAS) on challenging samples
2) X-ray Emission Spectroscopy (XES)
3) Energy Dispersive EXAFS (EDE)
I20 supports a wide range of applications especially in biology, environmental science, chemistry and materials science.
The high flux and high spectral purity are the key design parameters of this beamline. The beamline is equipped with an in-house designed four-bounce monochromator which consists of two pairs of counter-rotating crystals. The advantages are: a) high stability and reproducibility, b) fixed exit by geometry, c) energy resolution of transmitted photons not determined by the incident beam divergence and, d) reduction of the tails in the reflectivity curves.
The determination of the local structure around a photoabsorbing atomic site present at low concentration (about 1.0 mM) and/or in X-ray unfavourable heavy matrix is one of the particular strengths of I20-scanning when operating in the conventional scanning XAS mode.
X-ray emission spectroscopy (XES) provides sensitivity to the valence state and the nature of the bound ligands.
The principle of the Energy Dispersive EXAFS (EDE) beamline is based on the diffraction of non monochromatic X-rays by a bent crystal (polychromator crystal). The polychromatic beam is focused onto the sample and then diverges towards a position sensitive detector where beam position is correlated to energy.
The dispersive configuration of XAS has two main advantages that make it scientifically attractive. First, the whole X-ray absorption spectrum is collected simultaneously which makes the technique especially useful for the study of fast processes. Second, the size of the focussed beam at the sample position is small and very stable due to the fact that no movement of optical elements is required to collect the spectral data.
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