If you have read the How It Works section under SEMs, you will understand that when an incident electron beam hits atoms of the sample, secondary and backscattered electrons are emitted from the sample surface. However, these are not the only signals emitted from the sample.

When the incident beam bounces through the sample creating secondary electrons, it leaves thousands of the sample atoms with holes in the electron shells where the secondary electrons used to be.

If these "holes" are in inner shells, the atoms are not in a stable state. To stabilize the atoms, electrons from outer shells will drop into the inner shells, however, because the outer shells are at a higher energy state, to do this the atom must lose some energy. It does this in the form of X-rays.

The X-rays emitted from the sample atoms are characteristic in energy and wavelength to, not only the element of the parent atom, but which shells lost electrons and which shells replaced them.

For instance, if the innermost shell (the K shell) electron of an iron atom is replaced by an L shell electron, a 6400 eV K alpha X-ray is emitted from the sample

Or, if the innermost shell (the K shell) electron of an iron atom is replaced by an M shell electron, a 7057 eV K beta X-ray is emitted from the sample

Or, if the L shell electron of an iron atom is replaced by an M shell electron, a 704 eV L alpha X-ray is emitted from the sample

1                                                                                          NEXT