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4 Interaction of Radiation with Matter
4.2.1 Ionization
This process results in the removal of an electron from an atom or molecule, thereby
leaving the atom or molecule with a net positive charge. This causes the respective
atom or molecule to become ionized.
4.2.2 Excitation
The process of excitation is the addition of insufficient energy (but sufficient enough
to transfer an electron from the ground state to an excited state) to the target material to
produce the ionization of material. The excited atom or molecule may lose its excess
energy when the electron from its higher energy shell returns to its original ground
state. When this occurs, the excess energy is liberated as photons which can escape
or get absorbed in different parts of the target material. The nuclear excitation of the
target material is of significance only for neutrons having relatively high energies
and may not be of our interest.
4.3 Interactions with Particulate Radiation
A discussion on the types of nuclear radiations and their interaction can be conveniently divided into three main categories.
4.3.1 α-Particles
α-particles are ejected from atomic nuclei with a velocity in the order of 5% of light
velocity. High mass number, charge, and velocity serve to make the α-particle an
efficient projectile when it encounters atoms of an absorbing material and it has a high
probability of interacting with the orbital electrons. Thus, the number of ion- pairs
produced in unit length of track (i.e., the specific ionization) is high, and energy is
rapidly transferred to the medium; its penetrating power is, therefore, comparatively
poor.
α-particles may undergo either elastic (no transfer of energy) or inelastic (transfer
of energy from the α-radiation to the target particle) collisions. Inelastic collisions
result in ionization and/or excitation. After collision the kinetic energy of the αparticle is gradually dissipated by such interactions until eventually it captures two
electrons and becomes a helium atom. Briefly, an α-particle is a highly ionizing and
weakly penetrating radiation. α-particles from a given radioactive nuclei are emitted
with the same energy; hence the range of α-particles, apart from straggling, will
4 Interaction of Radiation with Matter
4.2.1 Ionization
This process results in the removal of an electron from an atom or molecule, thereby
leaving the atom or molecule with a net positive charge. This causes the respective
atom or molecule to become ionized.
4.2.2 Excitation
The process of excitation is the addition of insufficient energy (but sufficient enough
to transfer an electron from the ground state to an excited state) to the target material to
produce the ionization of material. The excited atom or molecule may lose its excess
energy when the electron from its higher energy shell returns to its original ground
state. When this occurs, the excess energy is liberated as photons which can escape
or get absorbed in different parts of the target material. The nuclear excitation of the
target material is of significance only for neutrons having relatively high energies
and may not be of our interest.
4.3 Interactions with Particulate Radiation
A discussion on the types of nuclear radiations and their interaction can be conveniently divided into three main categories.
4.3.1 α-Particles
α-particles are ejected from atomic nuclei with a velocity in the order of 5% of light
velocity. High mass number, charge, and velocity serve to make the α-particle an
efficient projectile when it encounters atoms of an absorbing material and it has a high
probability of interacting with the orbital electrons. Thus, the number of ion- pairs
produced in unit length of track (i.e., the specific ionization) is high, and energy is
rapidly transferred to the medium; its penetrating power is, therefore, comparatively
poor.
α-particles may undergo either elastic (no transfer of energy) or inelastic (transfer
of energy from the α-radiation to the target particle) collisions. Inelastic collisions
result in ionization and/or excitation. After collision the kinetic energy of the αparticle is gradually dissipated by such interactions until eventually it captures two
electrons and becomes a helium atom. Briefly, an α-particle is a highly ionizing and
weakly penetrating radiation. α-particles from a given radioactive nuclei are emitted
with the same energy; hence the range of α-particles, apart from straggling, will