Different applications proposed for the I08-SXM instrument require different imaging and spectromicroscopy modes. In absorption contrast, lateral variations of contrast levels are determined by local density and attenuation cross sections of the constituent atoms for a given photon energy. This is the most wide-spread contrast technique used and applicable when the specimen provides sufficient attenuation. Contrast techniques like different phase or interference contrast or dark-field imaging are preferable when the scattering properties dominate the attenuation, especially for low-absorbing organic matter. Phase-sensitive techniques are in many cases superior to absorption contrast because the X-ray dose required to image a specimen with the required signal-to-noise ration can dramatically be reduced.
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.
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