I
m z2
K.E.
DOE-HDBK-1019/1-93
INTERACTION OF RADIATION WITH MATTER
Atomic and Nuclear Physics
NP-01
Page 64
Rev. 0
Because ionizing radiation creates ions in pairs, the intensity of ionization or the specific
ionization is defined as the number of ion-pairs formed per centimeter of travel in a given
material. The amount of ionization produced by a charged particle per unit path length, which
is a measure of its ionizing power, is roughly proportional to the particle's mass and the square
of its charge as illustrated in the equation below.
where:
I is the ionizing power
m is the mass of the particle
z is the number of unit charges it carries
K.E. is its kinetic energy
Since m for an alpha particle is about 7300 times as large as m for a beta particle, and z is twice
as great, an alpha will produce much more ionization per unit path length than a beta particle
of the same energy. This phenomenon occurs because the larger alpha particle moves slower
for a given energy and thus acts on a given electron for a longer time.
Alpha Radiation
Alpha radiation is normally produced from the radioactive decay of heavy nuclides and from
certain nuclear reactions. The alpha particle consists of 2 neutrons and 2 protons, so it is
essentially the same as the nucleus of a helium atom. Because it has no electrons, the alpha
particle has a charge of +2. This positive charge causes the alpha particle to strip electrons
from the orbits of atoms in its vicinity. As the alpha particle passes through material, it removes
electrons from the orbits of atoms it passes near. Energy is required to remove electrons and
the energy of the alpha particle is reduced by each reaction. Eventually the particle will expend
its kinetic energy, gain 2 electrons in orbit, and become a helium atom. Because of its strong
positive charge and large mass, the alpha particle deposits a large amount of energy in a short
distance of travel. This rapid, large deposition of energy limits the penetration of alpha
particles. The most energetic alpha particles are stopped by a few centimeters of air or a sheet
of paper.
Beta-Minus Radiation
A beta-minus particle is an electron that has been ejected at a high velocity from an unstable
nucleus. An electron has a small mass and an electrical charge of -1. Beta particles cause
ionization by displacing electrons from atom orbits. The ionization occurs from collisions with
orbiting electrons. Each collision removes kinetic energy from the beta particle, causing it to
slow down. Eventually the beta particle will be slowed enough to allow it to be captured as an
