X-ray Analysis in the TEM

If the sampling volume is now treated as a truncated cone, and if the specimen thickness is t nm, then the lateral beam spread B (in nm) for thin specimens is given by  

Where Z = atomic number, A — atomic weight, E the electron energy (keV), and p the density (g/cm3). 

With a thermionic electron source, and a foil thickness of 100 nm, the volume of specimen excited is of the order 10-5 pm3. With a FEG source in a dedicated STEM, however, with a foil of 1 nm thickness this specimen-beam interaction volume can be as small as 10 8pm3. Very small signal levels are thus to be expected in AEM, hence the importance of employing higher brightness sources and the need to modify the specimen-detector configuration to maximize the collection angle. 

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