&EPA
United States Air and -
Environmental Protection Radiation
Agency (6601J)
EPA 402-F-98-009
May 1998
Ionizing Radiation Series No. 1
General Description
Ionizing radiation is radiation that has
sufficient energy to remove electrons from
atoms. In this document, it will be referred
to simply as radiation. One source of
radiation is the nuclei of unstable atoms.
For these radioactive atoms (also referred to
as radionuclides or radioisotopes) to become
more stable, the nuclei eject or emit
subatomic particles and high-energy photons
(gamma rays). This process is called
radioactive decay. Unstable isotopes of
radium, radon, uranium, and thorium, for
example, exist naturally. Others are
continually being made naturally or by
human activities such as the splitting of
atoms in a nuclear reactor. Either way, they
release ionizing radiation. The major types
of radiation emitted as a result of
spontaneous decay are alpha and beta
particles, and gamma rays. X rays, another
major type of radiation, arise from processes
outside of the nucleus.
Alpha Particles
Alpha particles are energetic, positively
charged particles (helium nuclei) that
rapidly lose energy when passing through
matter. They are commonly emitted in .the
radioactive decay of the heaviest radioactive
elements such as uranium and radium as
well as by some manmade elements. Alpha
particles lose energy rapidly in matter and
do not penetrate very far; however, they can
cause damage over their short path through
tissue. These particles are usually
completely absorbed by the outer dead layer
of the human skin and, so, alpha emitting'
radioisotopes are not a hazard outside the
body. However, they can be very harmful if
they are ingested or inhaled. Alpha
particles can be stopped completely by a
sheet of paper.
Beta Particles
Beta particles are fast moving, positively or
negatively charged electrons emitted from
the nucleus during radioactive decay.
Humans are exposed to beta particles from
manmade and natural sources such as
tritium, carbon-14, and strontium-90. Beta
particles are more penetrating than alpha
particles, but are less damaging over equally
traveled distances. Some beta particles are
capable of penetrating the skin and causing
radiation damage; however, as with alpha
emitters, beta emitters are generally more
hazardous when they are inhaled or
ingested. Beta panicles travel appreciable
distances in air, but can be reduced or
stopped by a layer of clothing or by a few
millimeters of a substance such as
aluminum.
Gamma Rays
Like visible light and x rays, gamma rays are
weightless packets of energy called photons.
Gamma rays often accompany the emission
of alpha or beta particles from a nucleus.
They have neither a charge nor a mass and
are very penetrating. One source of gamma
rays in the ' environment is naturally
occurring potassium-40. Manmade sources
include plutonium-239 and cesium-137.
Gamma rays can easily pass completely
through the human body or be absorbed by
tissue, thus constituting a radiation hazard
for the entire body. Several feet of concrete
or a few inches of lead may be required to
stop the more energetic gamma rays.
X Rays
X rays are high-energy photons produced by
the interaction of charged particles with
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matter. X rays and gamma rays have essentially
the same properties, but differ in origin; i.e., x
rays are emitted from processes outside the
nucleus, while gamma rays originate inside the
nucleus. They are generally lower in energy and
therefore less penetrating than gamma rays.
Literally thousands of x-ray machines are used
daily in medicine and industry for examinations,
inspections, and process controls. X rays are also
used for cancer therapy to destroy malignant cells.
Because of their many uses, x rays are the single
largest source of manmade radiation exposure. A
few millimeters of lead can stop medical x rays.
SOURCES OF RADIATION
Natural Radiation
Humans are primarily exposed to natural
radiation from the sun, cosmic rays, and naturally
occurring radioactive elements found in the
earth's crust. Radon, which emanates from the
ground, is another important source of natural
radiation. Cosmic rays from space include
energetic protons, electrons, gamma rays, and x
rays. The primary radioactive elements found in
the earth's crust are uranium, thorium, and
potassium, and their radioactive derivatives. These
elements emit alpha and beta particles, or gamma
rays.
Manmade Radiation
Radiation is used on an ever increasing scale in
medicine, dentistry, and industry. Main users of
manmade radiation include: medical facilities such
as hospitals and pharmaceutical facilities; research
and teaching institutions; nuclear reactors and
their supporting facilities such as uranium
mills and fuel preparation plants; and Federal
facilities involved in nuclear weapons production
as part of their normal operation.
Many of these facilities generate some radioactive
waste; and some release a controlled amount of
radiation into the environment. Radioactive
materials are also used in common consumer
products such as digital and luminous-dial
wristwatches, ceramic glazes, artificial teeth, and
smoke detectors.
Health Effects of Radiation Exposure
Depending on the level of exposure, radiation can
pose a health risk. It can adversely affect
individuals directly exposed as well as their
descendants. Radiation can affect cells of the
body, increasing the risk of cancer or harmful
genetic mutations that can be passed on to future
generations; or, if the dosage is large enough to
cause massive tissue damage, it may lead to death
within a few weeks of exposure.
Suggested Reading
To learn more about radiation, we suggest you
read the following books:
Cember, H. Introduction to Health Physics. New
York: Pergamon Press, 1983.
Martin, A. and Harbison, S.A; An Introduction
to Radiation Protection. 3rd ed. London:
Chapman and Hall, 1986.
Shapiro, J. Radiation Protection. Cambridge:
Harvard University Press, 1972.
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Radiation Source
Alpha Particles
Stopped by a
sheet of paper
Beta Particles
Gamma Rays
Stopped by several
feet of concrete
or a few inches of lead
Stopped by a layer of clothing
or by a few millimeters of a
substance such as aluminum
THE PENETRATING POWER OF ALPHA AND BETA PARTICLES AND GAMMA RAYS.
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