50
4 Interaction of Radiation with Matter
hilation of positrons with electrons), and a peak corresponding to the photoelectric
effect (due to the interaction of photons of energy equal to 0.51 MeV). The resolution
of all these peaks would depend upon the sensitivity of the measuring instrument.
4.5 Consequences of Interactions
From these discussions, it is obvious that when any radiation either electromagnetic
(X -rays or γ -rays) or particulate (α-particles or β-particles) interacts with matter it
can either ionize the material (due to the removal of electrons from the interacting
material) or cause either a photoelectric effect, pair production or excite the material
(due to insufficient energy transfer to the atom) to a higher energy. So, two processes
can take place when such radiations interact with matter: ionization causing the
formation of ion-pairs or the excitation of atoms of the interacting material.
4.5.1 Process of Excitation
When the interaction follows the process of excitation, the excited atoms of the
material soon fall to its original ground state by emitting photons of equivalent energy
(i.e., excess energy gained by the interacting radiation). These photons interact with
matter by three processes discussed earlier. The creation of excited species by these
radiations can easily be done with solid or liquid materials.
4.5.2 Process of Ionization
When the interaction follows a process of ionization, then depending upon the energy
of the radiation, primary and/or secondary ion-pairs are formed. The kinetic energy
associated with ion-pairs then becomes equivalent to the energy of the electromagnetic radiation. Since it would be difficult to measure the kinetic energy of ion-pairs
formed in a solid, one normally uses gases as a constituent of target material for the
interaction. This immediately imposes a limitation to detect γ -rays by this process,
because the probability of their interaction with the gaseous particles is very low.
4.6 Types of Counters
For measuring the radioactivity of any isotope, we shall have to develop a counting
instrument which could detect either the presence of ion-pairs or photon emitted by
the excited species. The instruments which operate to measure either ion-pairs or
photons emitted due to the interaction of radiation with matter are of two types:
4 Interaction of Radiation with Matter
hilation of positrons with electrons), and a peak corresponding to the photoelectric
effect (due to the interaction of photons of energy equal to 0.51 MeV). The resolution
of all these peaks would depend upon the sensitivity of the measuring instrument.
4.5 Consequences of Interactions
From these discussions, it is obvious that when any radiation either electromagnetic
(X -rays or γ -rays) or particulate (α-particles or β-particles) interacts with matter it
can either ionize the material (due to the removal of electrons from the interacting
material) or cause either a photoelectric effect, pair production or excite the material
(due to insufficient energy transfer to the atom) to a higher energy. So, two processes
can take place when such radiations interact with matter: ionization causing the
formation of ion-pairs or the excitation of atoms of the interacting material.
4.5.1 Process of Excitation
When the interaction follows the process of excitation, the excited atoms of the
material soon fall to its original ground state by emitting photons of equivalent energy
(i.e., excess energy gained by the interacting radiation). These photons interact with
matter by three processes discussed earlier. The creation of excited species by these
radiations can easily be done with solid or liquid materials.
4.5.2 Process of Ionization
When the interaction follows a process of ionization, then depending upon the energy
of the radiation, primary and/or secondary ion-pairs are formed. The kinetic energy
associated with ion-pairs then becomes equivalent to the energy of the electromagnetic radiation. Since it would be difficult to measure the kinetic energy of ion-pairs
formed in a solid, one normally uses gases as a constituent of target material for the
interaction. This immediately imposes a limitation to detect γ -rays by this process,
because the probability of their interaction with the gaseous particles is very low.
4.6 Types of Counters
For measuring the radioactivity of any isotope, we shall have to develop a counting
instrument which could detect either the presence of ion-pairs or photon emitted by
the excited species. The instruments which operate to measure either ion-pairs or
photons emitted due to the interaction of radiation with matter are of two types: