Physics and Engineering of Radiation Detection

2.5. Interaction of Electrons with Matter 123

Primary

Ionization

Secondary

Ionization

Incident

Electron

Tertiary

Ionization

Figure 2.5.2: Depiction of electron-induced ionization processes. At

each stage of ionization, if the ejected electron has an energy greater

than the binding energy of the atom, it can cause another ionization.

on its energy and the type of target atom. It has been seen that, at each step of
this ionization, only about one third of the electrons cause subsequent ionizations.
The ionization of atoms or molecules is a highly researched area due to its utility
in material and physics research. Electrons have be ability to penetrate deep into
the materials and can therefore be used to extract information about the structure
of the material.
As can be seen from Fig.2.5.1, the ionization with electrons dominates at low to
moderate energies.Electron impact ionizationis a term that is extensively used in
literature to characterize the process of ionization with electrons at relatively high
energies. This useful process is routinely employed in spectroscopy of materials in
gaseous state.
Symbolically, for an atom or a moleculeXq, with total positive chargeq,the
electron ionization process can be written as

e+Xq→Xq+1+2e. (2.5.1)

A.2 MoellerScattering

This refers to the elastic scattering of an electron from another electron (or a positron
from another positron). The interaction can be symbolically described by

e+e→e+e. (2.5.2)

In quantum electrodynamics, Moeller scattering is said to occur due to exchange
of virtual photons between the electrons. In classical electrodynamic terms one can
simply call it a consequence of Coulomb repulsion between the two electrons.

A.3 BhabhaScattering

It is the scattering of an electron from a positron. The reaction can be written as

e+e+→e+e+. (2.5.3)