Physics and Engineering of Radiation Detection

122 Chapter 2. Interaction of Radiation with Matter

2.5 InteractionofElectronswithMatter

The way an electron beam would behave when passing through matter depends, to
a large extent, on its energy. At low to moderate energies, the primary modes of
interaction are

ionization,

Moeller scattering,

Bhabha scattering, and

electron-positron annihilation.

At higher energies the emission of Bremsstrahlung dominates as shown in Fig.2.5.1.

Bremsstrahlung

Positrons Lead (Z = 82)

Electrons

Ionization

Møller (e−)

Bhabha (e+)

Positron

annihilation

1.0

0.5

0.20

0.15

0.10

0.05

(cm

2 g

−^1

)

E (MeV)

(^01101001000)
(^1) E
−
dEdx
(X
− 0
1 )
Figure 2.5.1: Fractional energy loss of electrons and positrons
per radiation length as a function of energy (19)

2.5.A InteractionModes

A.1 Ionization............................

If an incident electron departs enough energy to the atom, it may eject one of its
loosely bound electrons, resulting in the ionization of the atom. The energy of the
ejected electron depends on the incident electron energy as well as its binding energy.
If the energy carried away by the ejected electron is enough energy it can produce
secondary ionization in the same manner as the primary ionization. The process can
continue until the energy of the ejected electron is less than the ionization potential
of the atom. This process is graphically depicted in Fig.2.5.2. It should, however,
be noted that not all electrons that have energy higher than the ionization potential
of the atom produce subsequent ionization. The probability with which an electron
can cause ionization depends on its cross section, which to a large extent depends