Physics and Engineering of Radiation Detection

1.3. Radioactivity and Radioactive Decay 7

The process of internal conversion can occur to electrons in any electronic orbit.
If an electron from one of the inner shells is expelled, it leaves behind a vacancy
that could be filled up by an electron in one of the higher shells. If that happens,
the excess energy is emitted in the form of an x-ray photon. This photon can either
escape the atom or can knock off another electron from the atom. The knocked-off
electron is known asAuger electron. The process of internal conversion followed by
emission of an Auger electron is graphically depicted in Fig.1.3.1. Auger electron
emission is not specific to the decay process. It can happen whenever an electron
from one of the inner electronic orbitals leaves the atom. An example of such a
process is thephotoelectric effect, which we will study in some detail in the next
Chapter.

M L K

Auger Electron

Internal Conversion

Electron

Orbital Electron

Figure 1.3.1: Depiction of internal conversion leading to the emission of an

Auger electron. The internal conversion electron from the K-shell creates

a vacancy that must be filled in order for the atom to regain stability.

Another electron from the M-shell fills this gap but releases some energy in

the process (shown as a photon). This photon is shown to have knocked

off another electron from the M-shell. The end result is the emission of an

internal conversion electron and an Auger electron.

At this point the reader might be wondering why the radioisotopes emitting
neutrons and protons have not been mentioned so far. These decays are in fact
possible. However, such isotopes are not found in nature except for the ones that
undergo spontaneous fission. On the other hand, one can produce such a radionuclide
by bombarding a suitable material with high energy particles, something that can
be done at a particle accelerator. The basic idea behind such a process is to depart
enough energy to a nucleus abundant in protons and neutrons such that it becomes
unstable. This instability forces it to eventually decay by emitting a proton or a
neutron. There are also someβ-decays, which are followed by proton emission. Such
a process is generally known asbeta-delayed proton emission.Anexampleofthe