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Suggested Citation:"Appendix B: Glossary." National Research Council. 1972. Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18994.
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Suggested Citation:"Appendix B: Glossary." National Research Council. 1972. Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18994.
Page 214
Suggested Citation:"Appendix B: Glossary." National Research Council. 1972. Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18994.
Page 215
Suggested Citation:"Appendix B: Glossary." National Research Council. 1972. Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18994.
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Appendix B. GLOSSARY ABCC: Atomic Bomb Casualty Commission Absolute risk: Product of assumed relative risk times the total population at risk. The number of cases that will result from exposure of a given population. Absorption coefficient: Fractional decrease in the intensity of a beam of x or gamma radiation per unit thickness (linear absorption coefficient), per unit mass (mass ab- sorption coefficient), or per atom (atomic absorption coefficient) of absorber, due to deposition of energy in the absorber. The total absorption coefficient is the sum of individual energy absorption processes (Compton effect, photoelectric effect, and pair production). Accelerator (particle accelerator): A device for imparting large kinetic energy to electrically charged particles such as electrons, protons, deuterons and helium ions. Common types of particle accelerators are direct voltage accelerators, cyclotrons, etatrons, and linear accelera- tors. Alpha particle: A charged particle emitted from the nucle- us of an atom having a mass and charge equal in magni- tude to a "helium nucleus: i.e., two protons and two neu- trons. Angstrom unit: One angstrom unit equals 10-* cm (Symbol: A). Anion: Negatively charged ion. 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 neu- tral atom of carbon-12; equivalent to 1 6604 X 10-24 g Symbol: u). Attenuation: The process by which a beam of radiation is reduced in intensity when passing through some materi- al. It is the combination of absorption and scattering processes and leads to a decrease in flux density of the beam when projected through matter. Average life (mean life): The average of the individual lives of all the atoms of a particular radioactive sub- stance. It is 1.443 times the radioactive half-life. BEAR Committee: Advisory Committee on the Biological Effects of Atomic Radiation (Precursor of the BEIR Committee). BEIR Committee: Advisory Committee on the Biological Effects of Ionizing Radiation. Beta particle: Charged particle emitted from the nucleus of an atom, with a mass and charge equal in magnitude to that of the electron. Bone seeker: Any compound or ion which migrates in the body preferentially into bone. Bremsstrahlung: Secondary photon radiation produced by deceleration of charged particles passing through mat- ter. Carrier: A quantity of non-radioactive or non-labeled ma- terial of the same chemical composition as its corre- sponding radioactive or labeled counterpart. When mixed with the corresponding radioactive labeled materi- al, so as to form a chemically inseparable mixture, the carrier permits chemical (and some physical) manipula- tion of the mixture with less label or radioactivity loss than would be true for the undiluted label or radioactivi- ty. Cation: Positively charged ion. Chamber, ionization: An instrument designed to measure a quantity of ionizing radiation in terms of the charge of electricity associated with ions produced within a denned volume. Curie: The special unit of activity. One curie equals 3,700 x 10i0 nuclear transformations per second. (Abbr. Ci.) Common fractions are: Megacurie: One million curies (Abbr. MCi) Microcurie: One millionth ofa curie (3.7 x 104 disintegrations per second. Abbr.fiCi) Millicurie: One-thousandth of a curie (3.7 x 10? disintegrations per second. Abbr. mCi.) Nanocurie: One-billionth of a curie (Abbr. nCi) Picocurie: One-millionth of a microcurie(3.7 x 10-2) disintegrations per second. (Abbr. pCi) Daughter: Synonym for decay product. Decay product: A nuclide resulting from the radioactive disintegration of a radionuclide, formed either directly or as the result of successive transformations in a radio- active series. A decay product may be either radioactive or stable. Decay, radioactive: Disintegration of the nucleus of an unstable nuclide by spontaneous emission of charged particles and/or photons. Dose: A general form denoting the quantity of radiation or energy absorbed. For special purposes it must be appro- priately qualified. If unqualified, it refers to absorbed dose. Absorbed dose: The energy imparted to matter by ioniz- ing radiation per unit mass of irradiated material at the place of interest. The unit of absorbed dose is the rad. One rad = 100 ergs per gram, or 0.01 J/pg. Cumulative dose: Total dose resulting from repeated exposure to radiation. Dose equivalent (DE): Quantity that expresses all radia- tions on a common scale for calculating the effective absorbed dose. It is denned as the product of the ab- sorbed dose in rads and certain modifying factors. The unit of DE is the rem. Genetically significant dose (GSD): The gonad dose from medical exposure which, if received by every member of the population, would be expected to produce the same 213

total genetic effect on the population as the sum of the individual doses actually received. The GSD can be ex- pressed algebraically as: GSD = _NiPi DJ = Average gonad dose to persons age i who re- ceived x-ray examinations NJ = Number of persons in population of age i who receive x-ray examinations Pi = Expected future number of children for person of age i Ni = number of persons in population of age i. In 1964 the GSD was computed to be 55 millirads per person per year, for the United States. An estimated 55% of the population were receiv- ing x-rays at that time. Thus, the average dose to those receiving medical radiation could be computed to be approximately 80 mil- lirads. Maximum Permissible Dose Equivalent (MPD): The greatest dose equivalent that a person or specified part thereof shall be allowed to re- ceive in a given period of time. Median Lethal Dose (MLD): Dose of radiation required to kill, within a specified period, 50% of the individuals in a large group of animals or organisms. Also called LDso. Permissible Dose: The dose of radiation which may be received by an individual within a specified period with expectation of no signifi- cantly harmful result. Threshold Dose: The minimum absorbed dose that will produce a detectable degree of any given effect. Doubling Dose: The amount of radiation needed to double the natural incidence of a genetic or somatic anomaly. Dose, Fractionation: A method of administering radiation, in which relatively small doses are given daily or at longer intervals. Dose, Protraction: A method of administering radiation by delivering it continuously over a relatively long period at a low dose rate. Dose rate: Absorbed dose delivered per unit time. Electron Volt: A unit of energy equivalent to the energy gained by an electron in passing through a potential difference of one volt. Larger multiple units of the elec- tron volt are frequently used: KeV for thousand or kilo electron volts; MeV for million or meg-a electron volts. (Abbr. eV, 1 eV = 1.6 X 10-i2 erg.) EPA: Environmental Protection Agency Exposure: A measure of the ionization produced in air by x or gamma radiation. It is the sum of the electrical charges on all ions of one sign produced in air when all electrons liberated by photons in a volume element of air are completely stopped in air, divided by the mass of the air in the volume element. The special unit of exposure is the roentgen. Acute exposure: Radiation exposure of short duration. Chronic exposure: Radiation exposure of long duration by fractionation or protraction. Fission, nuclear: A nuclear transformation characterized by the splitting of a nucleus into at least two other nu- clei and the release of a relatively large amount of ener- gy- Fission products: Elements or compounds resulting from fission. Fission yield: The percentage of fissions leading to a par- ticular nuclide. FRC: Federal Radiation Council Fuel cycle: The sequence of steps, such as utilization, re- processing, and refabrication, through which nuclear fuel passes. Fusion, nuclear: Act of coalescing two or more atomic nu- clei Gamma ray: Short wavelength electromagnetic radiation of nuclear origin (range of energy from lOKeV to 9MeV) emitted from the nucleus. Gram atomic weight: A mass in grams numerically equal to the atomic weight of an element. Gram molecular weight (gram-mole): Mass in grams numer- cally equal to the molecular weight of a substance. Gram-Rad: Unit of integral dose equal to 100 ergs. Half-life, biological: The time required for the body to eliminate one-half of an administered dosage of any substance by regular processes of elimination. Approxi- mately the same for both stable and radioactive isotopes of a particular element. Half-life, effective: Time required for a radioactive ele- ment in an animal body to be diminished 50% as a result of the combined action of radioactive decay and biologi- cal elimination. Effecti vehalf-life = Biological half-life x radioactive Half-life Biological half-life+ Radioactive half-life Half-life, radioactive: Time required for a radioactive substance to lose 50% of its activity by decay. Each ra- dionuclide has a unique half-life. ICRP: International Commission on Radiological Protec- tion ICRU: International Commission on Radiation Units and Measurements Incidence: The rate of occurrence of a disease within a specified period of time; usually expressed in number of cases per million (106) per year. Ion: Atomic particle, atom, or chemical radical bearing an electrical charge, either negative or positive. Ion exchange: A chemical process involving reversible interchange of ions between a solution and a particular solid material such as an ion exchange resin consisting of a matrix of insoluble material interspersed with fixed ions of opposite charge. Ionization: The process by which a neutral atom or mole- cule acquires a positive or negative charge. Primary ionization: In collision theory; the ionization produced by the primary particles as contrasted to the "total ionization" which includes the "secondary ioniza- tion" produced by delta rays. Secondary ionization: Ionization produced by delta rays, lonization density: Number of ion pairs per unit volume. Ionization path (track): The trail of ion pairs produced by an ionizing radiation in its passage through matter. Isotopes: Nuclides having the same number of protons in their nuclei, and hence the same atomic number, but dif- fering in the number of neutrons, and therefore in the mass number. Almost identical chemical properties exist between isotopes of a particular element. The term should not be used as a synonym for nuclide. Labeled compound: A compound consisting, in part, of la- beled molecules. By observations of radioactivity or iso- topic composition, this compound or its fragments may be followed through physical, chemical, or biological processes. 214

Latent period: The period or state of seeming inactivity between the time of exposure of tissue to an injurious agent and response. LD50 (radiation dose) (See: Dose, median lethal. Linear energy transfer (LET): The average amount of energy lost per unit of particle spur-track length. Low-LET: Radiation characteristic of electrons, x rays, and gamma rays High-LET: Radiation characteristic of protons or fast neutrons Average LET is specified to even out the effect of a parti- cle that is slowing down near the end of its path and to allow for the fact that secondary particles from photon or fast-neutron beams are not all of the same energy. AVERAGE LET VALUES Particle Electron Proton Deuteron Alpha Charge -i +1 +1 +2 Energy (KeV) 1 10 100 1000 100 2000 5000 10000 10000 200000 100 Average LET (KeV/micron) 12.3 2.3 0.42 0.25 90 16 8 4 1.0 260 95 5 Tissue Penetration (microns) .01 1 180 5000 3 80 350 1400 700 190000 1 35 20000 5000 200000 Linear hypothesis: The assumption that a dose-effect curve derived from data in the high dose and high dose- rate ranges may be extrapolated through the low dose and low dose range to zero, implying that, theoretically, any amount of radiation will cause some damage. Nam-rems: See person-rems. Maximum credible accident: The worst accident in a reac- tor or nuclear energy installation that, by agreement, need be taken into account in deriving protective mea- sures. Medical exposure: Exposure to ionizing radition in the course of diagnostic or therapeutic procedures. As used in this report, the term includes: 1. Diagnostic radiology (e.g., x rays) 2. Exposure to radioisotopes in nuclear medicine (e.g., iodine-131 in thyroid treatment) 3. Therapeutic radiation (e.g., cobalt treatment for cancer) 4. Dental exposure Micron: Unite of length equal to 10-6 meters, (symboljx) Morbidity: 1. The condition of being diseased. 2. The ratio of sick to well persons in a com- munity. NAS-NRC: National Academy of Sciences - National Re- search Council NCRP: National Council on Radiation Protection and Measurements Neoplasm: Any new and abnormal growth, such as a tumor. The term "neoplastic disease" refers to any disease which forms tumors, malignant or benign. Nuclide: A species of atom characterized by the constitu- tion of its nucleus. The nuclear constitution is specified by the number of protons (Z), number of neutrons (N), and energy content; or, alternatively, by the atomic number (Z),mass number A=(N+Z), and atomic mass. To be regarded as a distinct nuclide, the atom must be capa- ble of existing for a measurable time. Thus, nuclear iso- mers are separate nuclides, whereas promptly decaying excited nuclear states and unstable intermediates in nu- clear reactions are not so considered. Person-rems: The product of the average individual dose in a population times the number of individuals in the popu- lation. Syn: man-rems. Plateau: A period of above-normal, relative uniform, incid- ence of morbidity or mortality in response to a given biological insult. Prevalence: The number of cases of disease in existence at a certain time in a designated area. Quality Factor (QF): The linear-energy-transfer-dependent factor by which absorbed doses are multiplied to obtain (for radiation protection purposes) a quantity that ex- presses — on a common scale for all ionizing radiations — the effectiveness of the absorbed dose. Rad. The unit of absorbed dose equal to 0.01 J/kg in any medium. Radiation: 1) The emission and propagation of energy through space or through a material medium in the form of waves; e.g., the emission and propagation of electro- magnetic waves, or of sound and elastic waves. 2) The energy propagated through space or through a material medium as waves. The term radiation or radiant energy, when unqualified, usually refers to electromagnetic ra- diation. Such radiation is commonly classified by fre- quency: Hertzian, infrared, visible, ultraviolet, x ray, and gamma ray. 3) Corpuscular emissions, such as alpha and beta radiation, or rays of mixed or unknown type, as cosmic radiation. Background radiation: Radiation arising from radioac- tive material other than the one directly under consider- ation. Background radiation due to cosmic rays and nat- ural radioactivity is always present. There may also be background radiation due to the presence of radioactive substances in other parts of the building, in the building material itself, etc. External radiation: Radiation from a source outside the body. 215

Internal radiation: Radiation from a source within the body (as a result of deposition of radionuclides in body tissue). Ionizing radiation: Any electromagnetic or particulate radiation capable of producing ions, directly or indirect- ly, in its passage through matter. Secondary radiation: Radiation resulting from absorp- tion or other radiation in matter. It may be either elec- tromagnetic or particulate. Radioactivity: The property of certain nuclides of sponta- neously emitting particles or gamma radiation or of emitting X radiation following orbital electron capture or of undergoing spontaneous fission. Artificial radioactivity: Manmade radioactivity pro- duced by particle bombardment or electromagnetic irra- diation. Natural radioactivity: The property of radioactivity exhibited by naturally occurring radionuclides. Radiosensitivity: Relative susceptibility of cells, tissues, organs, organisms, or any living substance to the inju- rious action of radiation. Radiosensitivity and its anto- nym radioresistance, are currently used in a compara- tive sense, rather than in an absolute one. Rate, recovery: The rate at which recovery takes place after radiation injury. It may proceed at different rates for different tissues. "Differential recovery rate": Among tissues recovering at different rates, those hav- ing slower rates will ultimately suffer greater damage from a series of successive irradiations. This differential effect is considered in fractionated radiation therapy if the neoplastic tissues have a slower recovery rate than surrounding normal structures. Rays: Alpha: Beams of helium nuclei (2 protons and 2 neutrons) Beta: Beams of electrons or positrons. Gamma: Beams of high-energy photons from radioac- tively decaying elements. X: Beams of mixed lower energy photons. Neutron: Beams of neutrons. Proton: Beams of protons. Reactor, breeder: A reactor which produces more fissile material than it consumes; i.e., has a conversion ratio greater than unity. Reactor converter: A reactor which produces fissile atoms from fertile atoms, but has a conversion ratio less than one. Reactor, nuclear: An apparatus in which nuclear fission may be sustained in a self-supporting chain reaction. Relative Biological Effectiveness (RBE): The RBE is a fac- tor used to compare the biological effectiveness of ab- sorbed radiation doses (i.e., rads) due to different types of ionizing radiation; more specifically, it is the experi- mentally determined ratio of an absorbed dose of a ra- diation in question to the absorbed dose of a reference radiation required to produce an idential biological ef- fect in a particular experimental organism or tissues. The RBE is the ratio of rem to rad. (If 1 rad of fast neu- trons equalled in lethality 3.2 rads of 250 KVP x rays, the RBE of the fast neutrons would be 3.2). Relative risk: The ratio of the risk in those exposed to the risk to those not exposed (incidence in exposed popula- tion to incidence in control population). Rem: A special unit of dose equivalent. The dose equivalent in rems is numerically equal to the absorbed dose in rads multiplied by the quality factor, the distribution factor, and any other necessary modifying factors. The rem rep- resents that quantity of radiation that is equivalent—in biological damage of a specified sort—to 1 rad of 250 KVP x rays. See note p. 86. Roentgen (R): The special unit of exposure. One roentgen equals 2.58 X 10-4coulomb per kilogram of air. Sickness, radiation: A self-limited syndrome characterized by nausea, vomiting, diarrhea, and psychic depression, following exposure to appreciable doses of ionizing ra- diation, particularly to the abdominal region. Its mecha- nism is unknown and there is no satisfactory remedy. It usually appears a few hours after irradiation and may subside within a day. It may be sufficiently severe to ne- cessitate interrupting the treatment series or to incapa- citate the patients. Sigmoid curve: S-shaped curve, often characteristic, e.g., of a dose-effect curve in radiobiological studies. Softness: A relative specification of the quality or pene- trating power of x rays. In general, the longer the wave length the softer the radiation. Specific activity: Total activity of a given nuclide per gram of a compound, element, or radioactive nuclide. Target theory (Hit Theory): A theory explaining some bio- logical effects of radiation on the basis that ionization, occurring in a discrete volume (the target) within the cell, directly causes a lesion which subsequently results in a physiological response to the damage at that loca- tion. One, two, or more "hits" (ionizing events within the target) may be necessary to elicit the response. Threshold hypothesis: the assumption that no radiation injury occurs below a specified dose level. UNSCEAR: United Nations Scientific Committee on the Effects of Atomic Radiation Working Level (WL): Any combination of short-lived radon daughters in 1 liter of air that will result in the ultimate emission of 1.3 X 105 MeV of potential alpha energy. Working Level Month (WLM): Inhalation of air with a con- centration of 1 WL of radon daughters for 170 working hours results in an exposure of 1 WLM. X rays: Penetrating electromagnetic radiations whose wave lengths are shorter than those of visible light. They are usually produced by bombarding a metallic target with fast electrons in a high vacuum. In nuclear reactions, it is customary to refer to photons originating in the nucleus as gamma rays, and those originating in the extranuclear part of the atom as X rays. These rays are sometimes called roentgen rays, after their discover- er, W. C. Roentgen. 216

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In the summer of 1970, the Federal Radiation Council (whose activities have since been transferred to the Radiation Office of the EPA) asked the National Academy of Science for information relevant to an evaluation of present radiation protection guidelines. This report is a response to that request.

It presents a summary and analysis, by members of the Advisory Committee on the Biological Effects of Ionizing Radiations and its subcommittees, of current knowledge relating to risks from exposure to ionizing radiation. In many respects, the report is a sequel to the reports of the Committee on the Biological Effects of Atomic Radiation, published by the NRC-NAS from 1956-1961.

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