Absorption contrast

Figure 3.17 Imaging artifact for a specimen consisting of a thin uniform layer except for a 12 pm diameter hole (aperture), using a nonconfocal 32 x Schwarzschild objective (NA = 0.65) and X = 6 pm. Solid line calculated absorption profile through the diameter of the hole. Solid circles measured absorption profile for an actual specimen. Note the poor absorption contrast (—50%) and bump at the holes center (15 pm location).

Figure 4.3.10 Full-field X-ray microscopy on a 5 wt% Rh/Al2C>3 catalyst during catalytic partial oxidation of methane (A) amount of oxidized Rh species (corresponds to XANES species 1 in [D]), (B) reduced Rh species (reduced species 2 in [D]), (C) the distribution of other elements that show a featureless absorption spectrum in the given energy range, and (D) spectra used for X-ray absorption contrast (original image taken by X-ray camera was 3.0 mm x 1.5 mm the reaction gas mixture 6% CH4/3% ()2/I Ie enters from the left) (reproduced with permission from ref. [69], Copyright ACS, 2006).

Spiller, R.C. Stool water content and colonic drug absorption contrasting effects of lactulose and codiene. Pharm. 

Hebden, J.M., Gilchrist, P.J., Perkins, A.C., Wilson, C.G. and Spiller, R.C. (1999) Stool water content and colonic drug absorption contrasting effects of lactulose and codeine. Pharmaceutical Research, 16, 1254-1259. 

In this context, the Rietveld error represents the uncertainty in the mathematical fit between the observed and calculated patterns and is the value most often quoted as the error in the phase abundance. Contrasting with this is the standard deviation of the mean abundances, which represents the expected precision in the analysis and is 3 to 4 times greater than the Rietveld derived errors. The good level of fit achieved in conducting these analyses (evidenced in the low i -factors) could lead the analyst to conclude that the mean value the standard deviation of the mean is an adequate measure of the phase abundances and their errors. However, the Rietveld errors and the replication errors are at least an order of magnitude smaller than the bias (measured - weighed). The bias, due to the presence of severe microabsorption, represents the true accuracy that can be achieved in this system if the analyst takes no further steps to identify the cause and minimize the effect of absorption contrast or other aberrations which may affect accuracy. 

J. C. Taylor and C. E. Matulis, Absorption contrast effects in the quantitative XRD analysis of powders by full multi-phase profile refinement, J. Appl. Crystallogr., 1991, 24, 14-17. 

Quite recently, a new type of radiology was developed, under names such as phase contrast radiology, refractive index radiology, or coherent radiology [1,21—41]. The foundation is the use of image contrast mechanisms different from absorption contrast, the basis of conventional radiology. 

For X-rays, 6 is very small (10 -10 ) and positive, i.e., the refractive index is slightly less than unity. This property of X-rays causes a near absence of reflection at interfaces and results in very clear images, even of thick specimens. X-ray microscopic samples are amplitude objects as well phase objects and X-ray microscopy can be performed in amplitude (absorption) contrast and phase contrast. Hence, at high photon energies for low-density materials it is much more favorable to image an object in phase contrast rather than in absorption contrast. In addition to natural contrast mechanisms. X-rays can provide chemical bond mapping or trace elements mapping of many elements. 

Corrosion under paint films and atmospheric corrosion. The loss of metal due to corrosion can be observed with absorption contrast, and any changes in the paint film (e.g., cracks, swelling with water) could be observed with phase contrast imaging. If suitable ions are introduced, their distribution could be tracked with energy difference imaging, described above. For atmospheric corrosion, salt droplets and crystallization of salts and corrosion products could be observed and correlated with pit development. 

The images produced in transmission electron microscopy are essentially due to local diffraction phenomena absorption contrast plays only a minor role. Not only is electron diffraction responsi-... 

In transmission electron microscopy (TEM), a beam of electrons is passed through a thin sample, such that an image is formed as a result of absorption or diffraction contrast. In the case of polymers, a combination of disorder and radiation sensitivity means that, of these, absorption contrast is most important, in which case, high resolution images can be generated where image contrast is based on the spatial variation in electron density. In the case of materials such as nanocomposites, the distribution of the nanoparticles can therefore easily be imaged, as a result of the difference in the atomic number between the nanoparticles and the matrix polymer, as shown in Eig. 2.18. 

The three most common X-ray contrast methods used for spatially resolved measurements are absorption, fluorescence and diffiaction/scattering. X-ray absorption spectroscopy measurements are a special case of utilizing the absorption contrast, due to the strong contrast variation in the vicinity of an absorption edge. The selection of a suitable instrument and contrast method strongly depends on the relevant scientific question. In the following we will briefly introduce a few fundamental characteristics of X-rays and highlight the three most common methods to obtain contrast. 

Absorption contrast images in general show physical distribution of matter such as morphology e.g. particle assembly in an anode, similar to optical and electron microscope images. Special cases are... 

X-ray absorption contrast the most commonly applied contrast method is frequently used in daily live for e.g. taking radiographs for medical reasons. Absorption imaging visualizes matter according to its density and absorption cross sections. 

Fig. 3 Principles of data processing for three-dimensional XANES microscopy. (1) One image is acquired in absorption contrast at each energy in the XANES scan. (2) XANES are constnicted from each pixel plotting normalized absorption versus enta-gy. (3) XANES from each pixel is fit to create a chemical phase map. (4) The sample is rotated to collect a 3D dataset and a phasemap is determined for each angle and (5) tomographic reconstruction and rendering for 3D phase distribution. Reprinted with permission from [20]. Copyright 2011...

Most X-ray microscopes and spatially resolved imaging techniques especially with the option of tomography and thus 3D imaging utilize the absorption contrast of a material. In the following section we illustrate the results obtained for Li-ion batteries by a few selected examples from this vastly growing research area. 

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