IONIZING RADIATION 335 9. GLOSSARY Absorbed Dose—The energy imparted by ionizing radiation per unit mass of irradiated material. The units of absorbed dose are the rad and the gray (Gy). (See also Rad, Gray, and Units, Radiological.) Absorbed dose is defined per unit mass of absorbing material. Absorbed Fraction—A term used in internal dosimetry. It is that fraction of energy radiated by the source organ that is absorbed by the target organ. For example, for 131I in the thyroid (source organ), the absorbed fraction could be the fraction of gamma radiation absorbed in the liver (one of the target organs). Absorber—Any material that absorbs or lessens the intensity of ionizing radiation. Neutron absorbers (boron, hafnium, and cadmium) are used as material in control rods for reactors. Concrete, steel, and lead are typical absorbers for x rays and gamma rays. A thin sheet of paper or metal will absorb alpha particles and all except the most energetic beta particles. Absorption—The process by which radiation imparts some or all of its energy to any material through which it passes. Absorption Ratio, Differential—The ratio of concentration of a nuclide in a given organ or tissue to the concentration that would be obtained if the same administered quantity of this nuclide were uniformly distributed throughout the body. Activation—The process of inducing radioactivity by neutron irradiation of a target material. Activity—The number of nuclear transformations occurring in a given quantity of material per unit time. (See Curie, Becquerel, and Units, Radiological, for more information on activity.) Activity Median Aerodynamic Diameter (AMAD)—The diameter of a unit density sphere with the same terminal settling velocity in air as that of the aerosol particle whose activity is the median for the entire aerosol. Acute Exposure—An exposure to ionizing radiation for a duration of less than 15 days. Regarding acute radiation syndrome, high radiation levels involve an exposure period up to 2 days. Acute Radiation Syndrome—The signs and symptoms which, taken together, characterize a person suffering from the effects of intense radiation. The effects occur within hours of exposure. ALARA—The acronym for “As Low As is Reasonably Achievable.” This term refers to the practice of making every reasonable effort to keep exposure to radiation as far below the dose limit as possible while still achieving the purpose for which radiation is licensed to be used. The benefits of reducing dose must be weighed against economic, engineering, and social costs of doing so. Alpha Particle (symbolized by Greek letter α)—A charged particle emitted from the nucleus of certain radioactive atoms. An alpha particle has a mass of 4 atomic mass units (amu) and is equal to a helium nucleus (i.e., two protons and two neutrons, and a charge of +2). IONIZING RADIATION 336 9. GLOSSARY Annihilation Radiation—The photons produced when an electron and a positron unite and cease to exist. The annihilation of a positron-electron pair results in the production of two photons, each of 0.51 MeV in energy (see pair production). Annual Limit on Intake (ALI)—The derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year. For a given radionuclide, ALI is defined as the smaller of the intakes that would result in a committed effective dose equivalent of 5 rem or a committed dose equivalent of 50 rem to any individual organ or tissue (see also Committed Effective Dose). Antineutrino— A neutral particle of rest mass near zero that is emitted during beta transformation (nucleus with a neutron excess) which occurs via the pathway by converting a neutron into a proton: n---> p + e- + anti-nu(e), where n means neutron, p means proton, e- means electron, and anti-nu(e) means an antineutrino of the electron type. Artificial Radioactivity—The radioactivity produced by particle bombardment or electromagnetic irradiation in an accelerator or reactor and not existing in nature. Atomic Mass—The mass of a neutral atom of a nuclide, usually expressed in terms of "atomic mass units." The "atomic mass unit" is one-twelfth the mass of one neutral atom of carbon-12; equivalent to 1.6604x10-24 gm. (Symbol: u) Atomic Mass Number—The total number of nucleons (neutron plus protons) in the nucleus of an atom. Atomic Number—The number of protons in the nucleus of an atom. The "effective atomic number" is calculated from the composition and atomic numbers of a compound or mixture. An element of this atomic number would interact with photons in the same way as the compound or mixture. (Symbol: Z). Atomic Weight—The weighted mean of the masses of the neutral atoms of an element expressed in atomic mass units. Background Radiation—Radiation resulting from cosmic rays and naturally occurring radioactive material. Background radiation is always present and its level can change with altitude and the amount of radioactive material present in soil and building materials. Becquerel (Bq)—A unit of measure for the quantity of radioactive material; one becquerel is that quantity of radioactive material in which one atom decays in one second. (See also Units, Radiological.) Beta Particle (symbolized by Greek letter β) —A charged particle emitted from the nucleus of some radioactive atoms. A beta particle has a mass and charge equal in magnitude to that of the electron. The charge may be either +1 or -1, and may be shown with the respective symbol, β+ or β-. Bioassay—A determination of the kind, quantity, concentration, or location of radioactive material in the body by either direct measurement or the analysis and evaluation of materials excreted or removed from the body. Bone Seeker—Any compound or ion in the body that preferentially migrates into actively forming bone to become part of the hydroxyapatite mixture. IONIZING RADIATION 337 9. GLOSSARY Branching—The occurrence of two modes by which a radionuclide can undergo radioactive transformation. For example, 214Bi can undergo α or β- transformation, 64Cu can undergo β-, β+, or electron capture transformation. An individual atom of a nuclide exhibiting branching disintegrates by one mode only. The fraction disintegrating by a particular mode is the "branching fraction" for that mode. The "branching ratio" is the ratio of two specified branching fractions (also called multiple transformation or disintegration). Bremsstrahlung—Electromagnetic radiation (photons) produced by the acceleration that a fast charged particle (usually an electron) undergoes from the effect of an electric or magnetic field, for instance, from the field of another charged particle (usually a nucleus). Bremsstrahlung is emitted when beta particles or electrons are stopped by a shield. Cancer Effect Level (CEL)—The lowest dose of chemical in a study, or group of studies, that produces significant increases in the incidence of cancer (or tumors) between the exposed population and its appropriate control. Carcinogen—A chemical capable of inducing cancer. Carcinoma—A malignant neoplasm composed of epithelial cells, regardless of their derivation. Cataract—A clouding of the crystalline lens of the eye that obstructs the passage of light. Chronic Exposure—An exposure to ionizing radiation for 365 days or more, as specified in the ATSDR toxicological profiles. Collective Dose—The sum of the individual doses received in a given period of time by a specified population from exposure to a specified source of radiation, in units such as person @ rem or person @ Sv. Committed Dose Equivalent (HT50)—The dose equivalent to organs or tissues of reference (T) that will be received from an intake of radioactive material by an individual during the 50-year period following the intake. Committed Effective Dose—The International Commission on Radiological Protection (ICRP) term for committed effective dose equivalent. (See Committed Effective Dose Equivalent.) Committed Effective Dose Equivalent (HE50)—The sum of the products of the weighting factors applicable to each of the body organs or tissues that are irradiated and the committed dose equivalent to the organs or tissues (HE50 = 3WTHT50). The committed effective dose equivalent is used in radiation safety because it implicitly includes the relative carcinogenic sensitivity of the various tissues. Compton Scattering—An attenuation process observed for x or gamma radiation in which an incident photon interacts with an orbital electron of an atom to produce a recoil electron and a scattered photon of energy less than the incident photon. Contamination, Radioactive—The deposition of radioactive material in any place where it is not desired. Cosmic Rays—High-energy particulate and electromagnetic radiations that originate outside the earth's atmosphere. IONIZING RADIATION 338 9. GLOSSARY Count (Radiation Measurements)—The external indication of a radiation-measuring device designed to enumerate ionizing events. It may refer to a single detected event or to the total number registered in a given period of time. This term can be used with equipment and geometry efficiencies to quantify the rate of transformation of ionizing events. Counter—A general description applied to radiation detection instruments or survey meters that detect and measure radiation. The signal that announces the detection of an ionization event is called a count. (See also Counter, Geiger-Mueller and Counter, Scintillation.) Counter, Geiger-Mueller—A sensitive, gas-filled radiation-measuring device that responds to individual ionizing particles. Counter, Scintillation—The combination of phosphor, a photomultiplier tube, and associated circuits for counting light emissions produced in the phosphors by ionizing radiation. Cumulative Dose—The total dose resulting from continuous or intermittent exposures of radiation to the same region of the body, or to the whole body, from internally deposited radioisotopes over a period of time. (See Also Weighting Factor.) Curie (Ci)— The quantity of radioactive material in which 37 billion radioactive atoms transform per second, which is approximately the activity of 1 gram of radium.