If the energy of the incoming electrons
exceeds the binding energy of the electrons on a certain shell of
the target atoms, an additional process can happen:
In a collision, the incoming electron (1) can push the target
electron (2) out of its place on the shell. This event will leave an
unstable atom behind. The gap on the shell will be filled
immediately by an electron (3) from an outer shell or even from the
conduction band of the target material. This replacement electron
will thereby change its energy by a well defined amount depending on
the binding energy levels of the electrons in the target material,
which are characteristic for that material. The resulting X-rays (4)
with very distinct photon energy values are therefore called
characteristic radiation. As the binding energy values for the outer
shells are not high enough for most elements to generate photons of
noticeable energy, usually only characteristic radiation generated
by electrons jumping into the K-shell is considered.
See x-ray specrta
Ref: Siemens.com |
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