Transmission imaging with simultaneous acquisition of absorption, differential phase contrast and dark field imaging:
A set of different integrating (e.g. photo diiodes, photo multiplier) and spatially resolving detectors (CCD) are integrated in the I08 end station. Although integrating detectors are generally preferred due to acquisition speed and dynamics, a wealth of important additional scattering information from the specimen is not acquired by this kind of detectors. I08-SXM with its specific applications on the analysis of organic-inorganic matter focuses therefore on CCD technologies for transmission detectors. Such allow simultaneous acquisition and display of absorption and phase-sensitive contrast images providing sensitive additional information. These detector system is also used for XANES/ NEXAFS mapping, where a set of images at different energies is acquired and the spectral contributions (oxidation states) are analysed by post-processing the data.
SXM transmission micrographs of a Siemens test pattern in absorption (top) and differential phase contrast (bottom)
Near Edge X-Ray Absorption Fine Structure, NEXAFS, spectroscopy refers to the absorption fine structure close to an absorption edge, about the first 30eV above the actual edge. This region usually shows the largest variations in the x-ray absorption coefficient and is often dominated by intense, narrow resonances. NEXAFS is also called X-Ray Absorption Near Edge Structure, XANES. Today, the term NEXAFS is typically used for soft x-ray absorption spectra and XANES for hard x-ray spectra. At I08-SXM, typically a stack of images at different photon energies is acquired, and the NEXAFS spectra depicted from the stack and further analysed by techniques like principal component or cluster analysis.
X-ray absorption techniques as XANES/ NEXAFS are preferred for analysing different chemical oxidation states of elements. This approach is limited to a small number of elements per experiment as moving to a different element requires a partial resetup of the instrument. A different approach is multi-elemental X-ray fluorescence analysis where atomic states are excited and secondary X-ray photons are emitted. The photon energy of such photons give an exact fingerprint of the element and can be collected by an energy-dispersive detector. Another advantage is the increased sensitivity for trace element analysis compared to XANES/ NEXAFS. I08-SXM implements a large-area, single-element SDD detector optimised for X-ray fluorescence detection down to carbon.