Generation of X-rays in the Electron Microscope
The specimens examined in an electron microscope are made up of atoms. When electrons from the primary beam of the electron microscope are focussed on a specimen, they penetrate it and interact with the atoms that make up the sample material. In the scanning electron microscope (SEM), the electrons from the primary beam spread out in the sample to form the interaction volume. The size of the interaction volume depends on the accelerating voltage of the primary electron beam and the mean atomic number (or density) of the sample. The interaction volume will be larger for a larger accelerating voltage, but smaller for samples with a higher mean atomic number.
In the SEM, secondary electrons are produced from the surface of the sample or topmost part of the interaction volume; backscattered electrons come from the top half of the interaction volume; and X-rays are generated within the whole of the interaction volume. Therefore, the typical spatial resolution for X-ray microanalysis in the SEM is of the order of a few microns.
Figure: The typical spatial resolution of different signals, Secondary electrons, Backscattered electrons and X-rays, in the scanning electron microscope.
In the TEM, the sample is very thin (~10-200 nm) and there is minimal spreading of the electron beam as it passes through the sample. Therefore, the spatial resolution for X-ray microanalysis in the TEM is approximately of the same order as the sample thickness.