In a typical scanning X-ray microscope (SXM), a virtual or secondary source is demagnified by a zone plate to form a microprobe across which the specimen is raster-scanned. Since a zone plate has an infinite number of diffraction orders, a so-called order-selecting aperture (OSA) is placed between the zone plate and the specimen in order to block all undesired diffraction orders.
Unlike a full-field imaging microscope, a scanning X-ray microscope requires coherent illumination, in laser-terms a single-mode illumination, to reach diffraction-limited lateral resolution. According to the van Cittert - Zernike theorem, the degree of spatial coherence is controlled by modifying the size of the effective source.
Since the effective lateral resolution in a SXM results form the convolution of the diffraction-limited point-spread function with the demagnified image of the source, a good rule of thumb to maximise resolution while keeping a high photon flux is to set the geometrical demagnification of the source roughly equal to the diffraction-limited lateral resolution.
Like other scanning probe techniques, SXM allows simultaneous monitoring of different signals, using appropriate X-rays or electron detectors. Thus simultaneous images of the transmitted X-rays and emitted photons and electrons can be recorded. Scanning instruments are therefore well suited for a combination of imaging and spectromicroscopy. The versatility of the detector setup allows also simultaneous acquisition of different contrast modes including absorption, differential phase and interference contrast, and dark-field imaging. The price to pay for this flexibility is an increased demand on the experimental setup, where the positioning accuracy of the optical element typically requires the use of sophisticated mechanical setups and optical feed-back systems.
Key figures of the I08-SXM end station are:
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