United States                 Air and
                             Environmental Protection        Radiation
                             Agency                     (ANR-459)
Ionizing  Radiation
                                                        September 1990
                             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
                                            travehid 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 particles travel  appreciable
                                            distances  in  air,  but can be  reduced or
                                            stoppexi by a layer of clothing or by a few
                                            millimeters   of   a  substance  such  as
                                            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 veiry 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

 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.


 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

 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.