X-ray generation volume

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

In the late twenties, research started using powerful X-ray generators. With X-rays, it was then possible to use photons that would penetrate vessels and evenly irradiate a reasonable physical volume. With this capability and the development of small ionization chambers to measure X-ray dose, it now became possible to carry out quantitative radiolysis experiments in liquids. 

The volume emitting X-rays (emission volume) is that in which primary electron energy is still greater than the energy required to excite the specific line of characteristic X-rays (Eex). The emission volume is determined by the acceleration voltage of electrons (E0) and the characteristics of the specimen s component elements, such as mass density (p) and Eex. The emission volume has been estimated with the depth of characteristic X-ray generation (R). 

Figure 12.5 Low-power X-ray generator. Reserved principally for portable instruments because of their smaller volume (diam. 15mm). The production of Cu X-rays on the diagram corresponds to the cooling time. The heating/cooling cycle is about 3 min. When the crystal temperature reaches its low point, the heating phase starts again. The above are characteristics of the model Amptek COOL-X.

Figure 21 Comparison of volume map for HRV-14 generated from x-ray data (left) and small-molecule energy-minimized structures (right). 

Fig. 4. A schematic two-dimensional illustration of the idea for an information theory model of hydrophobic hydration. Direct insertion of a solute of substantial size (the larger circle) will be impractical. For smaller solutes (the smaller circles) the situation is tractable a successful insertion is found, for example, in the upper panel on the right. For either the small or the large solute, statistical information can be collected that leads to reasonable but approximate models of the hydration free energy, Eq. (7). An important issue is that the solvent configurations (here, the point sets) are supplied by simulation or X-ray or neutron scattering experiments. Therefore, solvent structural assumptions can be avoided to some degree. The point set for the upper panel is obtained by pseudo-random-number generation so the correct inference would be of a Poisson distribution of points and = kTpv where v is the van der Waals volume of the solute. Quasi-random series were used for the bottom panel so those inferences should be different. See Pratt et al. (1999).

Stoichiometric variations in compositions of a material and of surface layers can be revealed by AEM. Because a relatively small amount of scattering occurs through a thin HRTEM specimen, X-rays are generated from a volume that is considerably less than in the case of electron microprobe analysis (EPMA). For quantitative microanalysis, a ratio method for thin crystals (57) is used, given by the equation ... 

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