Absolute risk: Expression of excess risk due to exposure as
the arithmetic difference between the risk among those exposed and
that obtained in the absence of exposure.
Actinide: Element with an atomic number from 90 through 103.
Action levels: The inner-most level around the operating
level on a control chart. Any corrective action is intended
to bring the measured values back toward the operating level and
within the action levels.
Active detector: A radiation detector which processes and
displays a signal at the time of detection.
Activity: The rate of decay of a radioactive material.
The traditional unit is the curie (Ci). The SI unit is the
Acute exposure: Radiation exposure in a short period of time
Aerosol: Any material (liquid or solid) of fine enough
particle size that when dispersed in air will remain suspended.
Agreement State: Any state with which the U.S. Nuclear
Regulatory Commission (NRC) has entered into an effective
licensing agreement under Section 274(b) of the Atomic Energy Act
of 1954, as amended, to enable the state to regulate source,
special nuclear, and byproduct materials.
ALARA (As Low As Reasonably Achievable): A principle of
radiation protection philosophy that requires that exposures to
ionizing radiation be kept as low as reasonably achievable,
economic and societal factors being taken into account. The ALARA
principle is satisfied when the expenditure of further resources
would be unwarranted by the reduction in exposure that would be
Alpha (α) decay: A process of radioactive decay wherein the
mass number of the original atom decreases by 4 and the atomic
number decreases by two with the difference made up via the
emission of an alpha particle.
Alpha (α) particle: A completely-ionized nucleus of a
helium-4 (He-4) atom which results from the radioactive decay of
heavy elements. Alpha particles have a mass of 4 amu, a
charge of +2 ec, but relatively low penetrating powers.
Alpha particles can be completely stopped by a few inches of air,
a sheet of paper, or the dead outer layers of skin.
Aquifer: A saturated geologic feature that is permeable and
capable of transmitting usable quantities of water under ordinary
Atomic Energy Act (AEA): Law passed originally in 1946 and
extensively revised in 1954 that governs production and use of
radioactive materials (i.e., byproduct material, source material,
and special nuclear material) for defense and peaceful purposes
and regulation of such radioactive materials to protect public
health and safety. The Act provides authority for licensing of
nuclear activities by the U.S. Nuclear Regulatory Commission or
Agreement States and regulation by the U.S. Department of Energy
of its atomic energy defense, research and development activities.
Atomic number (Z): The number of protons in a nucleus.
An element is defined by the number of protons.
Background: A value associated with a null exposure.
Background level: The ambient concentration of radon.
Background radiation: The ambient, natural radiation from
terrestrial sources (e.g. radon) or extraterrestrial sources (e.g.
solar radiation, cosmic rays). Also includes artificial
sources, such as from fallout from the atmospheric testing of
nuclear weapons or nuclear reactor accidents.
Becquerel (Bq): The SI unit for radioactivity with the
dimensions of disintegrations per second (s-1).
Beta (β) decay /
beta-plus/minus (β±) decay:
A process of radioactive decay wherein the atomic number changes
by one with no change in mass number with the difference in charge
made up by the emission of a beta
Beta (β) particle: An
energetic electron emitted by a nucleus during radioactive decay.
Usually refers to a beta-minus (β-)
particle, which is an electron, but may be generally used to
indicate either beta-minus or beta-plus emission.
particle: An energetic electron emitted during radioactive
particle: An energetic positron (anti-electron) during