Physics and Engineering of Radiation Detection

86 Chapter 2. Interaction of Radiation with Matter

σThis called Thompson scattering cross section.
An interesting aspect of photoelectric effect is that if the incident photon has
sufficient energy to overcome the binding energy of an inner shell electron, that
electron might get ejected leaving a vacancy behind. This vacancy can then be
filled by an outer shell electron to stabilize the atom. Such a transition would
emit a photon with energy equal to the difference of the two energy levels. These
photons are generally in the x-ray region of electromagnetic spectrum and are called
fluorescence photons. In experiments with photons where elements with high atomic
number (such as lead) are used for shielding, the emission of characteristic x-rays
from the shield is not uncommon. This adds to the overall background and care
must be taken to eliminate it from the data.
An x-ray photon emitted as a consequence of photoelectric effect can also knock
off another orbital electron provided its energy is equal to the binding energy of
that electron. This electron is called Auger electron. The process is essentially
radiationless because the excess energy of the atom is used and taken away by the
Auger electron. Auger process is shown graphically in Fig.2.3.3.
Although here we have shown that the photoelectric effect can lead to the emission
of Auger electrons, however this process is not in any way restricted to photoelectric
effect. In fact in Auger electron spectroscopy, an electron beam is used to knock
off inner shell electrons from target atoms, that eventually leads to the emission of
Auger electrons.

Photon

Incident

Photoelectron

M L K

Auger Electron

Orbital Electron

Figure 2.3.3: Depiction of photoelectric effect leading to the emission of

an Auger electron. A K-shell electron is seen to have been knocked off by

the incident photon, creating a vacancy that must be filled in order for

the atom to be stable again. Another electron from the M-shell fills this

gap but releases some energy in the process (shown as a photon) equal

to the difference between the two energy level. This photon is shown to

have knocked off another electron from the M-shell. The end result is a

radiationless electron emission. This electron is called Auger electron.