Glossary of Radiological Terms
(continued from previous page)
August 2004 Page 9 of 16
Penetrating radiation: radiation that can penetrate the skin and reach internal organs and tissues.
Photons (
gamma rays and x-rays), neutrons, and protons are penetrating radiations. However, alpha
particles and all but extremely high-energy beta particles are not considered penetrating radiation.
Photon: discrete "packet" of pure electromagnetic energy. Photons have no mass and travel at the speed
of light. The term "photon" was developed to describe energy when it acts like a particle (causing
interactions at the molecular or atomic level), rather than a wave.
Gamma rays and x-rays are photons.
Pitchblende: a brown to black mineral that has a distinctive luster. It consists mainly of urananite (UO
2
),
but also contains
radium (Ra). It is the main source of uranium (U) ore.
Plume: the material spreading from a particular source and traveling through environmental media, such
as air or ground water. For example, a plume could describe the dispersal of particles, gases, vapors, and
aerosols in the atmosphere, or the movement of contamination through an aquifer (For example, dilution,
mixing, or adsorption onto soil).
Plutonium (Pu): a heavy, man-made, radioactive metallic
element. The most important isotope is Pu-
239, which has a half-life of 24,000 years. Pu-239 can be used in reactor fuel and is the primary isotope in
weapons. One kilogram is equivalent to about 22 million kilowatt-hours of heat energy. The complete
detonation of a kilogram of plutonium produces an explosion equal to about 20,000 tons of chemical
explosive. All isotopes of plutonium are readily absorbed by the bones and can be lethal depending on the
dose and exposure time.
Polonium (Po): a radioactive chemical
element and a product of radium (Ra) decay. Polonium is found in
uranium (U) ores.
Prenatal radiation exposure: radiation exposure to an embryo or fetus while it is still in its mother’s
womb. At certain stages of the pregnancy, the fetus is particularly sensitive to radiation and the health
consequences could be severe above 5
rads, especially to brain function. For more information, see CDC’s
fact sheet, “Possible Health Effects of Radiation Exposure on Unborn Babies,” at
http://www.bt.cdc.gov/radiation/prenatal.asp.
Protective Action Guide (PAG): a guide that tells state and local authorities at what projected dose
they should take action to protect people from exposure to unplanned releases of
radioactive material into
the environment.
Proton: a small atomic particle, typically found within an atom's
nucleus, that possesses a positive
electrical charge. Even though protons and
neutrons are about 2,000 times heavier than electrons, they
are tiny. The number of protons is unique for each chemical element. See also
nucleon.
Quality factor (Q): the factor by which the
absorbed dose (rad or gray) is multiplied to obtain a quantity
that expresses, on a common scale for all
ionizing radiation, the biological damage (rem) to an exposed
person. It is used because some types of radiation, such as
alpha particles, are more biologically
damaging internally than other types. For more information, see “
Primer on Radiation Measurement” at
the end of this document.
Rad (radiation absorbed dose): a basic unit of
absorbed radiation dose. It is a measure of the amount
of energy absorbed by the body. The rad is the traditional unit of absorbed dose. It is being replaced by
the unit
gray (Gy), which is equivalent to 100 rad. One rad equals the dose delivered to an object of 100
ergs of energy per gram of material. For more information, see “
Primer on Radiation Measurement” at the
end of this document.