4.4 Interaction with Electromagnetic Radiation
49
energy to an orbital electron of the interacting material and this electron is ejected out
of the orbital. The scattered photons leave the material with the remaining energy.
These scattered photons may either undergo a series of similar Compton scattering
(until their energy falls below 0.51 MeV) and eventually end up in a photoelectrictype interaction, or get scattered away into the space without having the chance
to interact with interacting material. Thus, the probability of Compton scattering
decreases with increasing energy of γ -rays (Fig. 4.1). The ejected electrons from the
orbital of interacting material, on the other hand, transfer their energies in a fashion
as described for a β-particle (refer Sect. 4.3.1).
If the spectrum of the energy of photons is measured, the Compton scattering
would give a continuous “background” with one broad spectrum due to ejected
electrons (of a type similar to β-spectrum) followed by one specific sharp intensity
due to photoelectric effect (refer Fig. 2.3 in Chap. 2).
4.4.4 Pair Production
Photons of energy greater than 2 × 0.51 MeV (0.51 MeV is the energy equivalent to
one electron) may be absorbed by pair production, where γ -photon of 1.02 MeV is
converted in the vicinity of a nucleus, into a pair of electrons, one positive (known
as positron) and one negative (like a β-particle). This process is known as pair
production. If a γ -ray possesses energy greater than 1.02 MeV, the excess energy
is shared equally between the two electrons as kinetic energy. This negative electron
like a β-particle can now interact with the target, producing secondary ion-pairs.
The process of the formation of secondary ion-pairs continues until the electron has
lost all its kinetic energy. Likewise, the positron also produces ionization with the
interacting material (i.e., it can eject the orbital electron of the interacting material),
until its kinetic energy has become zero.
However, after the positron loses all its kinetic energy and slows down to almost
zero, it encounters a free electron of the interacting material. During the process
of this interaction, the positron and electron of the target material get annihilated,
converting their masses into energy. This energy appears in the form of two γ -rays,
each possessing 0.51 MeV energy. These γ -rays may interact with the target by the
process of Photoelectric effect. However, if by chance the newly formed two γ -rays
possess energy higher than 0.51 MeV, they may interact with the target material
by the process of Compton scattering as well. The probability of pair production
increases with the energy of γ -rays (Fig. 4.1).
Hence, if the energy spectrum of γ -rays with energy greater than 1.02 MeV is
measured, in addition to a peak corresponding to its actual energy, we would also
observe low energy photons (appearing as background noise) due to the Compton
scattering process, a pair production photopeak of energy 1.02 MeV (due to the anni-
49
energy to an orbital electron of the interacting material and this electron is ejected out
of the orbital. The scattered photons leave the material with the remaining energy.
These scattered photons may either undergo a series of similar Compton scattering
(until their energy falls below 0.51 MeV) and eventually end up in a photoelectrictype interaction, or get scattered away into the space without having the chance
to interact with interacting material. Thus, the probability of Compton scattering
decreases with increasing energy of γ -rays (Fig. 4.1). The ejected electrons from the
orbital of interacting material, on the other hand, transfer their energies in a fashion
as described for a β-particle (refer Sect. 4.3.1).
If the spectrum of the energy of photons is measured, the Compton scattering
would give a continuous “background” with one broad spectrum due to ejected
electrons (of a type similar to β-spectrum) followed by one specific sharp intensity
due to photoelectric effect (refer Fig. 2.3 in Chap. 2).
4.4.4 Pair Production
Photons of energy greater than 2 × 0.51 MeV (0.51 MeV is the energy equivalent to
one electron) may be absorbed by pair production, where γ -photon of 1.02 MeV is
converted in the vicinity of a nucleus, into a pair of electrons, one positive (known
as positron) and one negative (like a β-particle). This process is known as pair
production. If a γ -ray possesses energy greater than 1.02 MeV, the excess energy
is shared equally between the two electrons as kinetic energy. This negative electron
like a β-particle can now interact with the target, producing secondary ion-pairs.
The process of the formation of secondary ion-pairs continues until the electron has
lost all its kinetic energy. Likewise, the positron also produces ionization with the
interacting material (i.e., it can eject the orbital electron of the interacting material),
until its kinetic energy has become zero.
However, after the positron loses all its kinetic energy and slows down to almost
zero, it encounters a free electron of the interacting material. During the process
of this interaction, the positron and electron of the target material get annihilated,
converting their masses into energy. This energy appears in the form of two γ -rays,
each possessing 0.51 MeV energy. These γ -rays may interact with the target by the
process of Photoelectric effect. However, if by chance the newly formed two γ -rays
possess energy higher than 0.51 MeV, they may interact with the target material
by the process of Compton scattering as well. The probability of pair production
increases with the energy of γ -rays (Fig. 4.1).
Hence, if the energy spectrum of γ -rays with energy greater than 1.02 MeV is
measured, in addition to a peak corresponding to its actual energy, we would also
observe low energy photons (appearing as background noise) due to the Compton
scattering process, a pair production photopeak of energy 1.02 MeV (due to the anni-