Full-Field MXRF
Figure 2 : Elemental images collected from droplets while drying using an X-ray color camera in TXRF illumination geometry. The droplet consists of two 10 µL droplets and one 20 µL droplet: Mn (violet), Ni (red), and Cu+Sc (green). The X-ray source is located at the top of each picture (shaded regions are indicated by dotted lines). (a) Image taken after 7.5 min drying, (b) image taken after 11.75 min of drying, (c) image taken after 20.5 min, and (d) image of completely dried specimen after 27 min. Measurements were carried out at the Beamline/BESSYII instrument at Helmholtz-Zentrum Berlin.
Figure 4 : Schematic showing a sheet beam exciting a slice of specimen and an energy-dispersive detector detecting elemental signals. Reprinted from [ 5 ] with permission from the Royal Chemical Society.
channels. T e counts acquired for a specifi c element line can be extracted and displayed to show the spatial distribution of this element in an X-ray map. However, these array detectors do not achieve the high count rates of SDD detectors, for example, the SLCam ®
array camera count rates currently are
Figure 3 : Schematic showing multiple dimensions in the acquisition of an XRF spectrum at each pixel. Similar data cubes may be acquired in both scanning and full-fi eld microscopy modes. Reprinted with permission from Oliver Scharf.
detector, using arrays of capillary optics (>200 k capillaries) to guide more fl uorescence photons to the detector array, and improving event processing soſt ware.
Modern full-field array detectors like the SLcam® provide
spectral (energy) resolution of < 160 eV [ 4 ], comparable to single-chip silicon drift detectors (SDDs). At each pixel a full spectrum comprising an energy range of >10 keV is acquired. Figure 3 shows the typical data cube for a spectrum image with a 264×264 array and spectra collected over 1,024
40
only about 22 cps/pixel. The spatial resolution obtained with full-field setups is in the single-digit micrometer range and is comparable to spatial resolutions achieved in scanning mode at second-generation synchrotron facilities. Excitation and detection . In full-field X-ray microscopy, the sample is illuminated with a grazing incident beam, a total reflected beam, or a sheet-like beam [ 5 , 6 ]. A schematic of the setup using a sheet-like X-ray excitation beam from Radkte et al. [ 5 ] is shown in Figure 4 . A 3D image of an object can be obtained by translating the object through the sheet beam, where the in-depth image resolution is given by the thickness of the sheet beam. Elemental specific XRF data can be obtained from this excitation. Semiconductor array detectors such as charge coupled devices (CCDs) are used to achieve an elemental analysis as the object passes through the sheet beam. CCDs are in general energy-dispersive; however, to achieve energy (spectral) resolution comparable to SDDs, sophisticated devices like the silicon-based SLcam ® (developed by PNSensor GmbH, Munich, and IFG, Berlin, with other partners ) have to be used [ 4 ]. Such devices are often referred to as color X-ray cameras, or CXCs, because they provide energy resolution sufficient to discriminate
www.microscopy-today.com • 2015 May
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