As penetrating radiation moves from point to point in matter, it
loses its energy through various interactions with the atoms it
encounters. The rate at which this energy loss occurs depends
upon the type and energy of the radiation and the density and
atomic composition of the matter through which it is passing.
The various types of penetrating radiation impart their energy to
matter primarily through excitation and ionization of orbital
electrons. The term "excitation" is used to describe
an interaction where electrons acquire energy from a passing charged
particle but are not removed completely from their atom. Excited
electrons may subsequently emit energy in the form of x-rays during
the process of returning to a lower energy state. The term "ionization"
refers to the complete removal of an electron from an atom following
the transfer of energy from a passing charged particle. In describing
the intensity of ionization, the term "specific ionization"
is often used. This is defined as the number of ion pairs formed
per unit path length for a given type of radiation.
Because of their double charge and relatively slow velocity,
alpha particles have a high specific ionization and a relatively
short range in matter (a few centimeters in air and only fractions
of a millimeter in tissue). Beta particles have a much lower specific
ionization than alpha particles and, generally, a greater range.
For example, the relatively energetic beta particles from P32
have a maximum range of seven meters in air and eight millimeters in tissue.
The low energy betas from H3, on the other hand, are stopped by
only six millimeters of air or six micrometers of tissue.
Gamma-rays, x-rays, and neutrons are referred to as indirectly
ionizing radiation since, having no charge, they do not directly
apply impulses to orbital electrons as do alpha and beta particles.
Electromagnetic radiation proceeds through matter until there is
a chance of interaction with a particle. If the particle is an
electron, it may receive enough energy to be ionized, whereupon
it causes further ionization by direct interactions with other
electrons. As a result, indirectly ionizing radiation (e.g. gamma,
x-rays, and neutrons) can cause the liberation of directly ionizing
particles (electrons) deep inside a medium. Because these neutral
radiations undergo only chance encounters with matter, they do
not have finite ranges, but rather are attenuated in an exponential
manner. In other words, a given gamma ray has a definite probability
of passing through any medium of any depth.
Neutrons lose energy in matter by collisions which transfer kinetic
energy. This process is called moderation and is most effective
if the matter the neutrons collide with has about the same mass
as the neutron. Once slowed down to the same average energy as the matter being
interacted with (thermal energies), the neutrons have a much greater
chance of interacting with a nucleus. Such interactions can result
in material becoming radioactive or can cause radiation to be