Glossary

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A

α=β Ratio:

A measure of the curvature of the cell survival curve and a measure of the sensitivity of a tissue or tumor to dose fractionation. The dose at which the linear and quadratic components of cell killing are equal.

Abscopal Effect:

The radiation response in tissue at a distance from the irradiated site invoked by local irradiation.

Absorbed Dose Rate:

Absorbed dose divided by the time it takes to deliver that dose. High dose rates are usually more damaging to humans and animals than low-dose rates. This is because repair of damage is more efficient when the dose rate is low.

Absorbed Dose:

The amount of energy deposited in any substance by ionizing radiation per unit mass of the substance. It is expressed numerically in rads (traditional units) or grays (SI units).

Acceptable Daily Intake:

An estimate of the daily exposure dose that is likely to be without deleterious effect, even if continued exposure occurs over a lifetime.

Activity Median Aerodynamic Diameter (AMAD):

The value of aerodynamic diameter such that 50% of the airborne activity in a specified aerosol is associated with particles greater than the AMAD. Used when deposition depends principally on inertial impaction and sedimentation, typically when the AMAD is greater than about 0.5 lm.

Activity:

The expectation value of the number of nuclear transformations occurring in a given quantity of material per unit of time. The SI unit of activity is per second (s-1) and its special name is becquerel (Bq).

Adaptive Response:

A post-irradiation cellular response that typically serves to increase the resistance of the cell to a subsequent radiation exposure.

AEC:

The U.S. Atomic Energy Commission, 1947-1974. Broken up in 1974 into the U.S. Energy Research and Development Administration (ERDA) and the U.S. Nuclear Regulatory Commission (NRC). ERDA later became the U.S. Department of Energy (DOE).

ALARA:

Acronym for "As Low As Reasonably Achievable." It means making every reasonable effort to maintain exposures to ionizing radiation as far below the dose limits as practical. Be consistent with the purpose for which the licensed activity is undertaken, taking into account the state of technology, the economics of improvements in relation to state of technology, the economics of improvements in relation to benefits to the public health and safety, and other societal and socioeconomic considerations. These means are in relation to utilization of nuclear energy and licensed materials in the public interest.

Alpha Decay:

The emission of a nucleus of a helium atom from the nucleus of an element, generally of a heavy element, in the process of its radioactive decay.

Alpha Irradiation:

Radiation with alpha particles.

Alpha Particle:

A positively charged particle ejected spontaneously from the nuclei of some radioactive elements. It is identical to a helium nucleus that has a mass number of 4 and an electric charge of +2. It has low penetrating power and a short range (a few centimeters in air). The most energetic alpha particle will generally fail to penetrate the dead layers of cells covering the skin and can be easily stopped by a sheet of paper. Alpha particles represent much more of a health risk when emitted by radionuclides deposited inside the body.

Ambient Dose Equivalent:

The dose equivalent at a point in a radiation field that would be produced by the corresponding expanded and aligned field in the International Commission on Radiation Units and Measurements (ICRU) sphere at a depth of 10 mm on the radius vector opposing the direction of the aligned field. The unit of ambient dose equivalent is joule per kilogram (J kg^-1) and its special name is sievert (Sv).

Annual Intake (AI):

The derived limit for the permissible amount of radioactive material taken into the body of an adult radiation worker by inhalation or ingestion in a year. The ALI is the smaller value of intake of a given radionuclide in a year by the reference man that would result in either a committed effective dose equivalent of 5 rems (0.05 sievert) or a committed dose equivalent of 50 rems (0.5 sievert) to any individual organ or tissue.

Anthropogenic:

Of human origin.

Apoptosis:

An active biochemical process of programmed cell death following radiation or other insults.

Atomic Energy Commission:

A federal agency created in 1946 to manage the development, use, and control of nuclear energy for military and civilian applications. Abolished by the Energy Reorganization Act of 1974 and succeeded by the Energy Research and Development Administration (now part of the US Department of Energy) and the US Nuclear Regulatory Commission.

Atomic Energy:

Energy released in nuclear reactions. Of particular interest is the energy released when a neutron initiates the breaking up or fissioning of an atom's nucleus into smaller pieces (fission), or when two nuclei are joined together at millions of degrees of heat (fusion). It is more correctly called nuclear energy.

Attenuation:

Attenuation is the process by which the number of particles or photons entering a body of matter is reduced by absorption and scattering.

Averted Dose:

The dose prevented or avoided by the application of a protective measure or set of protective measures, i.e., the difference between the projected dose if the protective measure(s) had not been applied and the expected residual dose.

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B

Background Radiation:

Radiation from cosmic sources; naturally occurring radioactive materials, including radon (except as a decay product of source or special nuclear material), and global fallout as it exists in the environment from the testing of nuclear explosive devices. It does not include radiation from source, byproduct, or special nuclear materials regulated by the Nuclear Regulatory Commission. The typically quoted average individual exposure from background radiation is 360 millirems per year.

Baseline Rates:

The annual disease incidence observed in a population in the absence of exposure to the agent under study.

Becquerel (Bq):

The unit of radioactive decay equal to one disintegration per second. The Becquerel is the basic unit of radioactivity used in the international system of radiation units, referred to as the “SI” units. 37 billion (3.7x10¹⁰) becquerels = 1 curie (Ci).

BEIR:

Several committees of the National Academy of Sciences-National Research Council on Biological Effects of Ionizing Radiation and their reports. For example, BEIR VI defined the health effects of radon.

Benign:

Usually refers to nonmalignant tumors.

Beta Decay:

The emission of electrons or positrons (particles identical to electrons, but with a positive electrical charge) from the nucleus of an element in the process of radioactive decay of the element.

Beta Particle:

A charged particle emitted from a nucleus during radioactive decay, with a mass equal to 1/1837 that of a proton. A negatively charged beta particle is identical to an electron. A positively charged beta particle is called a positron. Exposure to large amounts of beta radiation from external sources may cause skin burns (erythema). Beta emitters can also be harmful if they enter the body. Thin sheets of metal or plastic may stop beta particles.

Beta Radiation:

Radiation consisting of beta particles.

Biodosimetry:

The use of biological changes to detect past radiation exposure. Chromosome aberrations have been used widely in biological dosimetry.

Biological Dosimetry:

Area of radiation dosimetry that uses biological damage produced by radiation to estimate radiation dose. Chromosomal damage in blood lymphocytes is often used in biological dosimetry for exposure of humans to gamma radiation.

Biological Half-Life:

The time required for a biological system, such as that of a human, to eliminate, by natural processes, half of the amount of a substance (such as a radioactive material) that has entered it.

Brachytherapy:

Radiation treatment of a patient using sealed or unsealed sources of radiation placed within the patient’s body.

Bystander Effects:

A response in unirradiated cells that is triggered by signals received from irradiated neighboring cells.

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C

Cancer:

A malignant tumor of potentially unlimited growth, capable of invading other tissue and of metastasis.

Categories of Exposure:

The International Commission on Radiological Protection distinguishes between three categories of radiation exposure: occupational, public, and medical exposures of patients.

Cesium, (¹³⁷Cs):

This environmentally important fission product is a beta-gamma emitter and has a long half-life (26.6 years). Cesium has metabolic properties similar to potassium. As a result, it is rather uniformly distributed in the body. It clears from the body quickly, with a half-life of days and weeks, and therefore has a rather low effectiveness in increasing cancer incidence.

Collective Dose:

The sum of the individual doses received in a given time period by a specified population from exposure to a specified source of radiation.

Committed Effective Dose, Ε(τ):

The committed dose equivalent for a given organ multiplied by a weighting factor (see the definition of Weighting Factor).

Committed Equivalent Dose Equivalent (CEDE), H_T(τ):

The dose to a specific organ or tissue that is received from an intake of radioactive material by an individual over a specified time after the intake. For radiation protection purposes, the specified time is to the age of 70, which is normally taken to be 50 years for a radiation worker and 70 years for a member of the public.

Confidence Limits:

An interval giving the lowest and highest estimate of a parameter that is statistically compatible with the data. For a 95% confidence interval, there is a 95% chance that the interval contains the parameter.

Contamination:

Undesired radioactive material that is deposited on the surface of or inside structures, areas, objects, or people.

Controlled Area:

A defined area in which specific protection measures and safety provisions are, or could be, required for controlling normal exposures or preventing the spread of contamination during normal working conditions, and preventing or limiting the extent of potential exposures. A controlled area is often within a supervised area, but need not be.

Cosmic Radiation:

Penetrating ionizing radiation, both particulate and electromagnetic, that originates in outer space. Secondary cosmic rays, formed by interactions in the earth's atmosphere, account for about 45 to 50 millirem of the 300 millirem of natural background radiation that an average member of the US public receives in a year.

Critical Organ:

That part of the body that is most susceptible to radiation damage resulting from the specific exposure conditions under consideration, taking into account the dose the various parts of the body receive under the exposure conditions.

Criticality:

A term used in reactor physics to describe the state when the number of neutrons released by fission is exactly balanced by the neutrons being absorbed (by fuel and poisons) and escaping the reactor core. A reactor is said to be "critical" when it achieves a self-sustaining nuclear chain reaction, as when the reactor is operating.

Cumulative Dose:

The total dose resulting from repeated exposures of ionizing radiation to the same portion of the body, or to the whole body, over a period of time.

Curie (Ci):

The original unit used to express the decay rate of a sample of radioactive material. The curie is equal to that quantity of radioactive material in which the number of atoms decaying per second is equal to 37 billion (3.7×10¹⁰). It was based on the rate of decay of atoms within one gram of radium. It is named for Marie and Pierre Curie who discovered radium in 1898. The curie is the basic unit of radioactivity used in the system of radiation units in the United States, referred to as "traditional" units.

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D

Daughter Products:

Decay products are also called "daughter products". They are radionuclides that are formed by the radioactive decay of parent radionuclides. In the case of radium-226, for example, nine successive different radioactive decay products are formed in what is called a "decay chain". The chain ends with the formation of lead-206, which is a stable nuclide.

Decay:

The decrease in the amount of any radioactive material with the passage of time due to the spontaneous emission from the atomic nuclei of either alpha or beta particles, often accompanied by gamma radiation.

Department of Energy (DOE), U.S.:

This federal agency's mission is to achieve efficiency in energy use, diversity in energy sources, a more productive and competitive economy, improved environmental quality, and a secure national defense. DOE was created on October 1, 1977, from the U.S. Energy and Research and Development Agency and incorporates various aspects of non-nuclear federal energy programs and policy. DOE is funding the Low-Dose Radiation Research Program described on this website.

Derived Air Concentration (DAC):

The concentration of radioactive material in air that will result in an annual limit of intake if an individual breathes that air for a year. For a radiation worker, it is assumed air is breathed for 2,000 hours in one year in the workplace.

Designated Area:

An area that is either controlled or supervised.

Deterministic Effect:

Health effects, the severity of which varies with the dose and for which a threshold is believed to exist. Deterministic effects generally result from the receipt of a relatively high dose over a short time period. Skin erythema (reddening) and radiation-induced cataract formation is an example of a deterministic effect (formerly called a nonstochastic effect).

Detriment:

The total harm to health experienced by an exposed group and its descendants as a result of the group’s exposure to a radiation source. Detriment is a multi-dimensional concept. Its principal components are the stochastic quantities: probability of attributable fatal cancer, weighted probability of attributable non-fatal cancer, weighted probability of severe heritable effects, and length of life lost if the harm occurs.

Detriment-Adjusted Risk:

The probability of the occurrence of a stochastic effect, modified to allow for the different components of the detriment to express the severity of the consequence(s).

Diagnostic Reference Level:

Used in medical imaging with ionizing radiation to indicate whether, in routine conditions, the patient dose or administered activity (amount of radioactive material) from a specified procedure is unusually high or low for that procedure.

Diploid:

A full set of genetic material consisting of paired chromosomes, one chromosome from each parental set.

Directional Dose Equivalent, H’(d, Ω):

The dose equivalent at a point in a radiation field that would be produced by the corresponding expanded field in the ICRU sphere at a depth, d, on a radius in a specified direction, Ω. The unit of directional dose equivalent is joule per kilogram (J kg-1) and its special name is sievert (Sv).

DNA Damage Signaling:

Interacting biochemical processes that recognize and respond to DNA damage in cells; for example, by causing the arrest of the reproductive cell cycle.

DNA Repair:

The cell's ability to repair DNA damage and restore the original base sequences. This process can restore DNA damage produced by normal physiological processes, ionizing radiation, or chemicals. There are many forms of DNA repair, and many genes responsible for and involved in DNA repair have been identified.

DNA Replication:

The use of existing DNA as a template for the synthesis of new DNA strands.

DNA Sequence:

The relative order of base pairs, whether in a fragment of DNA, a gene, a chromosome, or an entire genome.

DNA:

Deoxyribonucleic acid—the genetic material of life. Nuclear material that contains genes and is responsible for the genetic code.

Dose and Dose-Rate Effectiveness Factor (DDREF):

A judged factor that generalizes the usually lower biological effectiveness (per unit of dose) of radiation exposures at low doses and low dose rates as compared with exposures at high doses and high dose rates.

Dose Coefficient:

Used as a synonym for dose per unit intake of a radioactive substance, but sometimes also used to describe other coeffcients linking quantities or concentrations of activity to doses or dose rates, such as the external dose rate at a specified distance above a surface with a deposit of a specified activity per unit area of a specified radionuclide.

Dose Equivalent (H):

The product of absorbed dose in tissue multiplied by a quality factor, and then sometimes multiplied by other necessary modifying factors, to account for the potential for a biological effect resulting from the absorbed dose. (see Quality factor). It is expressed numerically in rems (traditional units) or sieverts (SI units).

Dose Limits:

The value of the effective dose or the equivalent dose to individuals from planned exposure situations that shall not be exceeded.

Dose of Record, H_p(10):

The effective dose of a worker assessed by the sum of the measured personal dose equivalent H_p(10) and the committed effective dose retrospectively determined for the Reference Person using results of individual monitoring of the worker and the International Commission on Radiological Protection (ICRP) reference biokinetic and dosimetric computational models. Dose of record may be assessed with site-specific parameters of exposure, such as the type of materials and AMAD, but the parameters of the Reference Person shall be fixed as defined by the ICRP. Dose of record is assigned to the worker for purposes of recording, reporting and retrospective demonstration of compliance with regulatory dose limits.

Dose Rate:

The radiation dose delivered per unit time.

Dose Response:

Correlation between a quantified exposure (dose) and the proportion of a population demonstrating a specific effect (response).

Dose:

A general term used to refer to the effect on a material that is exposed to radiation. It is used to refer either to the amount of energy absorbed by a material exposed to radiation (see Dose, absorbed) or to the potential biological effect in tissue exposed to radiation (see Dose, equivalent).

Dose-Response Assessment:

The process of characterizing the relationship between the dose of an agent administered or received and the incidence of an adverse health effect in exposed populations and estimating the incidence of the effect as a function of human exposure to the agent.

Dose-Threshold Hypothesis:

A given dose above background, below which it is hypothesized that the risk of excess cancer and/or heritable disease is zero. (See also Threshold Dose for Tissue Reactions).

Dosimetry:

The theory and application of the principles and techniques involved in the measurement and recording of ionizing radiation doses.

Doubling Dose (DD):

The dose of radiation (Gy) that is required to produce as many heritable mutations as those arising spontaneously in a generation.

DS02:

Dosimetry System 2002, a system for estimating gamma and neutron exposure under a large variety of situations and which allows the calculation of absorbed dose to specific organs for members of the Life Span Study. DS02 improved on the DS86 dose system.

DS86:

Dosimetry System 1986, a system for estimating gamma and neutron exposure under a large variety of situations and which then allowed the calculation of absorbed dose to specific organs for members of the Life Span Study.

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E

Effect:

A biological change caused by an exposure.

Effective Dose E:

The tissue-weighted sum of the equivalent doses in all specified tissues and organs of the body. The unit for the effective dose is the same as for absorbed dose, J kg-1, and its special name is sievert (Sv).

Effective Dose Equivalent (HE):

The sum over specified tissues of the products of the dose equivalent in a tissue and the weighting factor for that tissue.

Effective Half-Life:

The time required for the amount of a radionuclide deposited in a living organism to be diminished 50 percent as a result of the combined action of radioactive decay and biological elimination.

Electromagnetic Radiation:

A traveling wave motion resulting from changing electric or magnetic fields. Familiar types of electromagnetic radiation range from x rays (and gamma rays) of short wavelength, through the ultraviolet, visible, and infrared regions, to radar and radio waves of relatively long wavelength. Only the higher-energy (higher frequency/shorter wavelength) forms of electromagnetic radiation are ionizing. Radiation in the lower-energy ranges, such as visible, infrared, radar, and radio waves, are nonionizing.

Electron:

An elementary particle with a negative charge and a mass 1/1837 that of the proton. Electrons surround the positively charged nucleus of the atom.

Element:

One of the known chemical substances that cannot be broken down further without changing its chemical properties. Some examples include hydrogen, nitrogen, gold, lead, and uranium.

Environmental Protection Agency (EPA), U.S.:

Created in 1970, the EPA is responsible for working with state and local governments to set standards that help control and prevent pollution and minimize the potential health effects of solid and hazardous waste and toxic and radioactive substances.

Epidemiology:

The study of the distribution and dynamics of diseases and injuries in human populations. The two main types of epidemiological studies of chronic disease are cohort (follow-up) studies and case-control (retrospective) studies.

Equivalent Dose:

Absorbed dose averaged over an organ or tissue and weighted for the radiation quality for the type of radiation of concern.

Eukaryote:

Organisms with membrane-bound nucleus and chromosomes. Higher plants and animals are eukaryotes.

Excess Absolute Risk:

The rate of disease incidence or mortality in an exposed population minus the corresponding disease rate in an unexposed population. The excess absolute risk is often expressed as the additive excess rate per Gy or per Sv.

Excess Lifetime Cancer Risk (ELCR):

Potential carcinogenic effects characterized by estimating the probability of cancer incidence in a population of individuals for a specific lifetime. Projected from intakes (and exposures) and chemical-specific dose-response data (i.e., slope factors). By multiplying the intake by the slope factor, the ELCR result is a probability.

Excess Relative Risk:

The rate of disease in an exposed population divided by the rate of disease in an unexposed population, minus 1.0. This is often expressed as the excess relative risk per Gy or per Sv

Exclusion:

The deliberate exclusion of a particular category of exposure from the scope of an instrument of regulatory control.

Exemption:

The determination by a regulatory body that a source or practice activity involving radiation need not be subject to some or all aspects of regulatory control.

Exposed Individuals:

The International Commission on Radiological Protection distinguishes between three categories of exposed individuals: workers (informed individuals), the public (general individuals), and patients, including their comforters and caregivers.

Exposure Assessment:

The process of measuring or estimating the intensity, frequency, and duration of human exposures to an agent currently present in the environment or of estimating hypothetical exposures that might arise.

Exposure Level:

The amount or concentration of a chemical or field strength of a radiation field.

Exposure:

A general term used loosely to express what a person receives as a result of being exposed to ionizing radiation.

External Radiation Dose:

The situation in which the source of exposure is external to, that is, outside the body.

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F

Fallout:

Radioactive debris from a nuclear detonation or other source, usually deposited from airborne particulates.

Fissile Material:

Although sometimes used as a synonym for fissionable material, this term has acquired a more restricted meaning. Namely, any material that is fissionable by thermal (slow) neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239.

Fission Product:

The nuclei (fission fragments) formed by the fission of heavy elements, plus the nuclides formed by the subsequent decay products of the radioactive fission fragments.

Fission:

The splitting of the nucleus of an atom (generally of a heavy element) into at least two other nuclei and the release of a relatively large amount of energy. Two or three neutrons are usually released during this type of transformation.

Flow Cytometry:

The analysis of biological material by detection of properties of cells or sub-cellular fractions using a combination of fluorescence and a laser beam. This makes it possible to sort cells or sub cellular fractions for further analysis.

Frank-Effect Level (FEL):

Exposure level that produces unmistakable adverse effects, such as irreversible functional impairment or mortality, at a statistically or biologically significant increase in frequency or severity between an exposed population and its appropriate control.

FSU:

Functional subunits of tissues; for example, nephrons in kidney, alveoli in lung.

Fusion:

Combining of two nuclei to form a heavier one. Fusing the isotopes of light elements such as hydrogen or lithium results in a large release of energy.

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G

Gamete:

Mature male or female reproductive cells with a haploid set of chromosomes.

Gamma Multi-Hit Model:

A generalization of the one-hit, dose-response model that provides a better description of dose-response data.

Gamma Radiation:

High-energy, short wavelength, electromagnetic radiation emitted from the nucleus of an atom. Gamma radiation frequently accompanies the emission of alpha and beta particles and always accompanies fission.

Gamma Rays:

Very penetrating and are best stopped or shielded by dense materials, such as lead or uranium. Gamma rays are similar to x-rays.

Geiger-Mueller Counter:

A radiation detection and measuring instrument. It consists of a gas-filled tube containing electrodes, between which there is an electrical voltage, but no current flowing. When ionizing radiation passes through and ionizes the gas within the tube a short, intense pulse of current passes from the negative electrode to the positive electrode and is measured or counted. The number of pulses per second is an indication of the rate at which ionizing events are occurring within the tube. It was named for Hans Geiger and W. Mueller, who invented it in the 1920s. It is sometimes called simply a Geiger counter or a G-M counter, and is the most commonly used portable radiation instrument.

Genetic Effects of Radiation:

Effects arising from damage to genes in the germ cells of the mother or father, which are thus passed on to their children. The genetic effects of radiation will therefore not be seen in an irradiated individual but may occur in his or her offspring or in future generations.

Genetic Informatics:

Development of methods to search databases quickly, analyze DNA sequence information, and predict protein sequence and structure from DNA sequence data.

Genome:

All the genetic material in the chromosomes of a defined organism.

Genomic Instability:

Radiation-induced changes that occur several cell generations after the radiation exposure. Occurs at a higher frequency than would be predicted from radiation-induced mutations in single or groups of genes.

Genomics:

The study of an organism's entire genome. The field includes efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts.

Genotype:

The genetic constitution of an organism, as distinguished from its physical appearance (its phenotype).

Gray (Gy):

The international system (SI) unit of radiation dose expressed in terms of absorbed energy per unit mass of tissue. The gray is the unit of absorbed dose and has replaced the rad. 1 gray = 1 Joule/kilogram and also equals 100 rad.

Growth Factors:

Molecules that act to control cell reproduction and proliferation/differentiation of a population of cells.

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H

Half-Life (Biological):

The time required for the body to eliminate, by biological processes, one-half of the material originally taken in.

Half-Life (Effective):

The time required for a radionuclide contained in a biological system, such as a human or an animal, to reduce its activity by one-half as a combined result of radioactive decay and biological elimination.

Half-Life:

The time in which one-half of the activity of a particular radioactive substance is lost due to radioactive decay. Measured half-lives vary from millionths of a second to billions of years. Also called physical or radiological half-life.

Health Physics:

The science concerned with the recognition, evaluation, and control of health hazards to permit the safe use and application of ionizing radiation.

High-to-Low Dose Extrapolation:

The process of predicting human risks from low radiation exposures using either human or animal data on risks derived from high levels of exposure.

Homeostasis:

An ability of the body to maintain stability.

Hormesis:

The theory that small doses of radiation can induce beneficial biological processes and are healthful.

Human Equivalent Dose:

A dose that, when administered to humans, produces an effect equal to the same effect produced by a dose in animals.

Human Exposure Evaluation:

Describes the nature and size of the population exposed to a substance and the magnitude and duration of their exposure. The evaluation could concern past, current, or anticipated exposures.

Human Health Risk:

The likelihood that a given exposure or series of exposures may have damaged or will damage the health of individuals.

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I

ICRP:

International Commission on Radiological Protection: The international body charged with providing an overview of radiation standards and regulations and information to help standardize these regulations.

ICRU:

International Commission on Radiation Units and Measurements: The international organization charged with standardizing radiation units and measures.

In Vitro:

Studies carried out in cell or culture systems outside the whole organism.

In Vivo:

Studies carried out in cell or culture systems inside the whole organism.

Incidence:

The rate of occurrence of a disease in a population within a specified period of time, often expressed as the number of cases of a disease arising per 100,000 individuals per year (or per 100,000 person-years).

Induced Genomic Instability:

The induction of an altered cellular state characterized by a persistent increase over many generations in the spontaneous rate of mutation or other genome-related changes.

Induced Radioactivity:

Radioactivity produced in any material as a result of nuclear reactions, especially by absorption of neutrons.

Intake, I:

Activity that enters the body through the respiratory tract or the gastrointestinal tract or the skin.

• Acute intake: A single intake by inhalation or ingestion, taken to occur instantaneously.
• Chronic intake: An intake over a specified period of time.

Internal Radiation Dose:

The dose to organs of the body from radioactive materials deposited and retained inside the body. It may consist of any combination of alpha, beta, and gamma radiation.

Inverse Dose Rate Effect:

An effect in which, for a given exposure, the probability of effect increases as the dose rate is decreased.

Iodine, ¹³¹I:

This short-lived radionuclide (8.1 days) becomes concentrated in the thyroid gland and can produce large radiation doses to this gland. It is used in medicine to diagnose and treat thyroid disease. The deposition, retention and radiation dose from this isotope can be modified by taking potassium iodide pills. This radio-nuclide is released from nuclear weapons and radiation accidents involving nuclear power plants. It is the isotope that was responsible for an increase in the incidence of thyroid cancer in children exposed to fallout after the Chernobyl accident. However, there is no evidence that ¹³¹I has increased the incidence of thyroid cancer in adults exposed by this accident.

Ion:

(1) An atom that has too many or too few electrons, causing it to have an electrical charge, and therefore, to be chemically active. (2) An electron that is not associated (in orbit) with a nucleus.

Ionization:

The process of adding to or removing one or more electrons from atoms or molecules, thereby creating ions and free radicals. High temperatures, metabolic processes, electrical discharges, and radiation can cause ionization.

Ionize:

To split off one or more electrons from an atom, thus leaving it with a positive electric charge. The electrons usually attach to other atoms or molecules, giving them a negative charge.

Ionizing Radiation:

Any radiation capable of displacing electrons from atoms or molecules, thereby producing ions. Some examples are alpha, beta, gamma, x-rays, neutrons, and ultraviolet light.

Irradiate:

To expose or cause exposure to radiation.

Isotope:

One of two or more atoms with the same number of protons, but different numbers of neutrons in their nuclei. Thus, carbon-12, carbon-13, and carbon-14 are isotopes of the element, carbon, the numbers denoting the mass number of each isotope. Isotopes have very nearly the same chemical properties, but often have different physical properties. For example, carbon-12 and carbon-13 are stable; carbon-14 is unstable, that is, it is radioactive.

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J

Joule:

Measure of energy. Deposition of one Joule/Kg is equal to 1 Gy or 100 rads.

Justification:

The process of determining whether either (1) a planned activity involving radiation is, overall, beneficial, i.e. whether the benefits to individuals and to society from introducing or continuing the activity outweigh the harm (including radiation detriment) resulting from the activity; or (2) a proposed remedial action in an emergency or existing exposure situation is likely, overall, to be beneficial, i.e., whether the benefits to individuals and to society (including the reduction in radiation detriment) from introducing or continuing the remedial action outweigh its cost and any harm or damage it causes.

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K

Kerma:

The sum of the initial kinetic energies of all the charged ionizing particles liberated by uncharged particles per unit of mass of a specific material. SI unit of kerma is Joule/Kg and is the same as Gy.

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L

Lethal Dose Fifty:

A calculated dose of radiation or a chemical substance that is expected to kill 50% of the exposed individuals within 30 days. For single whole-body acute radiation exposure, the LD 50/30 is in the range from 400 to 500 rem (4 to 5 sieverts).

Life Span Study (LSS):

The long-term cohort study of health effects in the Japanese atomic bomb survivors in Hiroshima and Nagasaki.

Lifetime Exposure:

Total calculated exposure to radiation or a chemical that a human would receive in a lifetime (usually assumed to be 70 years).

Lifetime Risk Estimates:

Several types of lifetime risk estimates can be used to calculate the risk, over a lifetime, that an individual will develop, or die from, a specific disease caused by an exposure: (1) the excess lifetime risk (ELR), which is the difference between the proportion of people who develop or die from the disease in an exposed population and the corresponding proportion in a similar population without the exposure; (2) the risk of exposure-induced death (REID), which is defined as the difference in a cause-specific death rate for exposed and unexposed populations of a given sex and a given age at exposure, as an additional cause of death introduced into a population; (3) loss of life expectancy (LLE), which describes the decrease in life expectancy due to the exposure of interest; and (4) lifetime attributable risk (LAR), which is an approximation of the REID and describes excess deaths (or disease cases) over a follow-up period with population background rates determined by the experience of unexposed individuals. The LAR was used in this report to estimate lifetime risks.

Linear Dose Response:

A statistical model that expresses the risk of an effect (e.g., disease or abnormality) as being proportional to dose.

Linear Energy Transfer (LET):

The amount of energy deposited per unit of distance that the radiation travels in tissue. Alpha particles are examples of high LET radiation.

Linear-Quadratic Dose Response:

A statistical model that expresses the risk of an effect (e.g., disease, death, or abnormality) as the sum of two components, one proportional to dose (linear term) and the other one proportional to the square of dose (quadratic term).

LNTH:

The linear no-threshold model stating that any amount of radiation dose, no matter how small, results in increased radiation risk. For every unit of dose, there is an increase in risk.

Locus (Loci):

The position on a chromosome of a gene or other chromosome marker.

Lowest-Observed-Adverse-Effect-Level (LOAEL):

In an experiment, the lowest dose that produced an observable adverse effect.

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M

Malignant:

A cancer that tends to become progressively worse and to result in death if not treated; having the properties of anaplasia, invasiveness, and metastasis.

Marker:

An identifiable physical location on a chromosome (e.g., restriction enzyme cutting site, gene) whose inheritance can be monitored. Markers can be expressed DNA regions (genes) or DNA segments with no known coding function but with a determinable pattern of inheritance. See restriction fragment length polymorphism.

Medical Exposure:

Exposure incurred by patients as part of their own medical or dental diagnosis or treatment; by persons, other than those occupationally exposed, knowingly, while voluntarily helping in the support and comfort of patients; and by volunteers in a program of biomedical research involving their exposure.

Megabase (Mb):

Unit of length for DNA fragments, equal to 1 million nucleotides and roughly equal to 1 cM.

Megacurie:

One million (10⁶) curies.

Meiosis:

The process of two consecutive cell divisions in the diploid progenitors of sex cells. Meiosis results in four rather than two daughter cells, each with a haploid set of chromosomes.

Messenger RNA (mRNA):

RNA that serves as a template for protein synthesis.

Metaphase:

A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plate of the cell. The stage of the cell cycle that is used to evaluate chromosome aberrations and mark gene location.

Metastasis:

The spread of cancer from one organ or part to another not directly connected with it.

Microbeam:

A machine capable of delivering defined radiation doses or particle numbers to known cellular locations. Microbeams can deliver a known number of alpha particles to known cellular organelles. Used to measure bystander effects.

Microcurie:

One millionth (10^-6) of a curie.

Micronuclei:

Chromosome fragments or lagging chromosomes that are not incorporated into the nucleus at cell division. Used in biological dosimetry and detection of genomic instability.

Millirem:

One thousandth of a rem. (1 mrem = 10^-3 rem)

Minimum Detectable Level (MDL):

The threshold of detection for a biological response, substance, or device in question.

Mitosis:

The process of nuclear division in cells that produces daughter cells genetically identical to each other and to the parent cell.

Morbidity:

A departure from a state of physical or mental well-being, resulting from disease or injury. Frequently used only if the affected individual is aware of the condition.

Mortality Rate:

The number of deaths that occur in a given population during a given time interval, usually deaths per 103 or 105 people per year. Can be age, sex, race, and cause-specific.

Mortality:

Death; the death rate; ratio of number of deaths to a given population.

Multifactorial Diseases:

Diseases that are attributable to multiple genetic and environmental factors.

Multistage Model:

A carcinogenesis dose-response model in which cancer is assumed to originate as a "malignant" cell initiated by a series of somatic-like mutations occurring in a finite number of steps. It is also assumed that each mutational stage can be depicted as a Poisson process in which the transition rate is approximately linear in dose rate.

Multistage Tumorigenesis:

The stepwise acquisition of cellular properties that can lead to the development of a tumor from a single (target) cell.

Mutagen/Mutagenicity:

An agent that causes a permanent genetic change in a cell in addition to that occurring during normal genetic recombination. Mutagenicity is the capacity of a chemical or physical agent to cause such permanent genetic alterations.

Mutation Component (MC):

A quantity that provides a measure of the relative change in disease frequency per unit relative change in mutation rate, i.e., a measure of responsiveness; MC values differ for different classes of heritable disease.

Mutation:

Any heritable change in DNA sequence. Can be induced by changes at the chromosome, gene, or DNA level.

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N

Nanocurie:

One billionth (10^-9) of a curie.

National Academy of Sciences/National Research Council (NAS/NRC):

The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars involved in scientific and engineering research. As part of NAS, the National Research Council is designed to associate the broad community of science and technology with the needs of government. The NRC is the operating agency for NAS.

National Council on Radiation Protection and Measurements (NCRP):

A nonprofit corporation chartered by Congress to provide information that protects the public against radiation and provides recommendations on radiation measurements, quantities, and units.

Naturally Occurring Radioactive Material (NORM):

An acronym for Naturally Occurring Radioactive Material. Naturally occurring radioactive materials are common in virtually all rocks, minerals, and soils. They naturally contain small amounts of uranium, thorium, and a radioactive isotope of potassium. Plants and animals are also naturally radioactive; they contain small (but measurable) levels of radioactive potassium as well as radioactive carbon (C-14) and hydrogen (tritium, or H-3) that are formed by cosmic ray interactions in the atmosphere.

Neoplasm:

An aberrant new growth of abnormal cells or tissue in which the growth is uncontrollable and progressive.

Neutron:

An elementary particle slightly heavier than a proton, with no electric charge.

NIOSH:

National Institute for Occupational Safety and Health of the Public Health Service, U.S. Department of Health and Human Services (DHHS). A federal agency that, among other activities, tests and certifies respiratory protective devices and air-sampling detector tubes, recommends occupational exposure limits for various substances, and assists in occupational safety and health investigations and research.

No Observable Adverse Effect Level (NOAEL):

From long-term toxicological studies, the levels that indicate a safe, lifetime exposure level for a given chemical. Used to establish tolerance levels for human diets. Also written, NOEL.

Nominal Risk Coefficient:

Sex-averaged and age-at-exposure-averaged lifetime risk estimates for a representative population.

Non-Cancer Diseases:

Somatic diseases other than cancer, e.g., cardiovascular disease and cataracts.

Nonstochastic Effects:

Health effects, the severity of which varies with the dose and for which a threshold is believed to exist. Nonstochastic effects generally result from the receipt of a relatively high dose over a short time period. Skin erythema (reddening) and radiation-induced cataract formation is an example of a nonstochastic effect. This term has been replaced with Deterministic Effect.

Nuclear Energy:

The energy liberated by a nuclear reaction (fission or fusion) or by radioactive decay.

Nuclear Regulatory Commission (NRC), U.S.:

NRC is an independent agency created from the U.S. Atomic Energy Commission in 1975 to regulate civilian uses of nuclear material. Specifically, NRC is responsible for ensuring that activities associated with the operation of nuclear power and fuel-cycle plants and the use of radioactive materials in medical, industrial, and research applications are carried out with adequate protection of public health and safety, the environment, and national security.

Nucleic Acid:

A large molecule composed of nucleotide subunits.

Nucleon:

Common name for a constituent particle of the atomic nucleus. At present, applied to protons and neutrons, but may include any other particles found to exist in the nucleus.

Nucleotide:

A subunit of DNA or RNA consisting of a nitrogenous base (adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a phosphate molecule, and a sugar molecule (deoxyribose in DNA and ribose in RNA). Thousands of nucleotides are linked to form a DNA or RNA molecule. See DNA, base pair, RNA.

Nucleus:

The small, central, positively charged central core of an atom. Except for the nucleus of ordinary (light) hydrogen, which has a single proton, all atomic nuclei contain both protons and neutrons. The number of protons determines the total positive charge, or atomic number, which in turn determines the chemical element that a given atom represents. That is to say, all atoms of a given chemical element have the same number of protons in their nuclei. The total number of neutrons and protons is called the mass number.

Nuclide:

A general term that refers to any known isotope, either stable or unstable, of any element. Whereas a single element can have isotopes, when referring to the isotopes of more than one element, the proper term is nuclide. A radionuclide is an unstable nuclide.

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O

Occupational Exposure:

This refers to all exposure incurred by workers in the course of their work, with the exception of (1) excluded exposures and exposures from exempt activities involving radiation or exempt sources; (2) any medical exposure; and (3) the normal local natural background radiation.

Oncogene:

Genes that encode the potential for cancer and, when activated, can induce cancer.

Oncogenic:

A substance that causes tumors, whether benign or malignant.

One-Hit Model:

The basic dose-response model based on the concept that a tumor can be induced by a single receptor that has been exposed to a single quantum or effective dose unit of a chemical.

Operational Quantities:

Quantities used in practical applications for monitoring and investigating situations involving external exposure. They are defined for measurements and assessment of doses in the body. In internal dosimetry, no operational dose quantities have been defined that directly provide an assessment of equivalent or effective dose. Different methods are applied to assess the equivalent or effective dose due to radionuclides in the human body. They are mostly based on various activity measurements and the application of biokinetic models (computational models).

Optimization of Protection (and safety):

The process of determining what level of protection and safety makes exposures, and the probability and magnitude of potential exposures, as low as reasonably achievable, economic and societal factors being taken into account.

Organelle:

Any complex biological structure that forms a component of cells and performs a characteristic function. Examples of organelles are centrioles, the endoplasmic reticulum, kinetosomes, lysosomes, proteosomes, mitochondria, and ribosomes.

Organ-Weighting Factor (WT):

Factor indicating the ratio of stochastic-effects risk attributable to a given organ's or tissue's irradiation to the total risk when the whole body is uniformly irradiated.

OSHA:

Occupational Safety and Heath Administration of the U.S. Department of Labor. Federal agency with safety and health regulatory and enforcement authorities for most U.S. industry and business.

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P

Particle:

A tiny mass of material. Airborne particles that exist in the atmosphere as a solid or liquid can be natural, caused by stirring of soil dusts, or anthropogenic means. They vary in size from coarse (diameter >3 mm) to fine (<3 mm) . Sometimes "inhalable" or "respirable" is used to describe particles (<2 mm) that can be inhaled through the nose and enter the lungs.

Particulates:

Fine liquid or solid particles such as dust, smoke, mist, fumes, or smog, found in the air or emissions.

PCC:

Premature chromosome condensation, a method of studying chromosomes that are in the interphase stage of the cell cycle. PCC is used in many chromosome-repair studies.

PCR:

Polymerase Chain Reaction, a method used to amplify the amount of DNA from a given region of a gene or chromosome to rapidly produce a highly specific amplification of the desired sequence. PCR also can be used to detect the existence of the defined sequence in a DNA sample. This method has enabled major advances in molecular biology by providing adequate amounts of known DNA for study.

Personal Dose Equivalent, H_p(d):

An operational quantity: the dose equivalent in soft tissue (commonly interpreted as the "ICRU sphere") at an appropriate depth, d, below a specified point on the human body. The unit of personal dose equivalent is joule per kilogram (J kg^-1) and its special name is sievert (Sv). The specified point is usually given by the position where the individual’s dosimeter is worn.

Person-Gray (Gy):

The unit of population exposure obtained by summing dose equivalent values for all people in the exposed population. Thus, one person-Gy can result from one person being exposed to 1 Gy or to 100,000 people each exposed to 10 Gy.

Person-Year:

The sum of the number of years each person in a study population is at risk; a metric used to aggregate the total population at risk, assuming that 10 people at risk for one year is equivalent to 1 person at risk for 10 years.

Photon Radiation:

Forms of electromagnetic radiation such as x-rays, gamma-rays, and sunlight.

Photon:

A quantum (or packet) of energy emitted in the form of electromagnetic radiation. Gamma rays and x rays are examples of photons.

Picocurie:

One trillionth (10^-12) of a curie.

Planned Exposure Situations:

Everyday situations involving the planned operation of sources including decommissioning, disposal of radioactive waste and rehabilitation of the previously occupied land. Practices in operation are planned exposure situations.

Plutonium(²³⁹Pu):

A heavy, radioactive, man-made metallic element with atomic number 94. Its most important isotope is fissile plutonium-239 which is produced by neutron irradiation of uranium-238, followed by a two-step decay. It exists in only trace amounts in nature.

Polymerase, DNA or RNA:

Enzymes that catalyze the synthesis of nucleic acids on pre-existing nucleic acid templates, assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.

Pooled Analysis:

An analysis of epidemiological data from several studies based on original data from those studies that are analyzed in parallel.

Population Dose (population exposure):

The summation of individual radiation doses received by all those exposed to the source or event being considered.

Positron:

An elementary particle with a positive electric charge but in other respects identical to an electron.

Potential Exposure:

Exposure that is not expected to be delivered with certainty but that may result from an accident at a source or an event or sequence of events of a probabilistic nature, including equipment failures and operating errors.

Potential Recoverability Correction Factor (PRCF):

A set of factors that take account of knowledge that different classes of germ line mutation will show different degrees of recoverability in live-born offspring, i.e., through differing capacities to allow completion of embryonic/fetal development.

Primer:

Short preexisting polynucleotide chain to which DNA polymerase can add new deoxyribonucleotides.

Principles of Protection:

A set of principles that apply equally to all controllable exposure situations: the principle of justification, the principle of optimization of protection, and the principle of application of limits on maximum doses in planned situations.

Probe:

Single-stranded DNA or RNA molecules of specific base sequence, labeled either radioactively or immunologically, which are used to detect the complementary base sequence by hybridization.

Progenitor Cell:

Undifferentiated cell capable of limited proliferation.

Projected Dose:

The dose that would be expected to be incurred if no protective measure(s) were to be taken.

Prokaryote:

Cell or organism lacking a membrane-bound, structurally discrete nucleus and other subcellular compartments. Bacteria are prokaryotes. Compare to eukaryote.

Promoter:

(1) DNA site to which RNA polymerase will bind and initiate transcription. (2) Agent that is not carcinogenic by itself but is capable of amplifying a true carcinogen's effect by increasing the probability of late-stage cellular changes needed to complete the carcinogenic process. Promoters usually require protracted application to be effective in increasing cancer incidence.

Prospective Study:

An inquiry in which groups of individuals are selected, according to their exposure to certain factors and followed over time to determine differences in disease rates in relation to their factor exposure. Also called cohort study.

Protection Quantities:

Dose quantities that the ICRP has developed for radiological protection that allow quantification of the extent of exposure of the human body to ionizing radiation from both whole and partial body external irradiation and from intakes of radionuclides.

Proton:

An elementary nuclear particle located in the nucleus of an atom. The proton has a single positive electric charge.

Protraction:

The spreading out of a radiation dose over time by continuous or periodic delivery at a lower dose rate.

Public Exposure:

Exposure incurred by members of the public from radiation sources, excluding any occupational or medical exposure and the normal local natural background radiation.

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Q

Quadratic Dose Response Model:

A model that predicts that a biological effect continually increases out of proportion to an increase in dose. Effects at low doses would thus be relatively small.

Quality Factor (Q):

The factor by which the absorbed dose (rad or gray) must be multiplied to obtain a quantity that expresses, on a common scale for all ionizing radiation, the biological damage (rem or sievert) to the exposed tissue. It is used because some types of radiation, such as alpha particles, are more biologically damaging to live tissue than other types of radiation when the absorbed dose from both is equal. The term, quality factor, has now been replaced by "radiation weighting factor" in the latest system of recommendations for radiation protection.

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R

Rad:

The original unit developed for expressing absorbed dose, which is the amount of energy from any type of ionizing radiation (e.g., alpha, beta, gamma, neutrons, etc.) deposited in any medium (e.g., water, tissue, air). A dose of one rad is equivalent to the absorption of 100 ergs (a small but measurable amount of energy) per gram of absorbing tissue. The rad has been replaced by the gray in the SI system of units (1 gray = 100 rad).

Radiation (Ionizing):

Emission of particles (i.e., alpha, beta, or gamma) or rays (i.e., alpha, beta, gamma, or x-rays) by the nucleus of an atom.

Radiation Detriment:

A concept used to quantify the harmful health effects of radiation exposure in different parts of the body. It is defined by the International Commission on Radiological Protection as a function of several factors, including incidence of radiation-related cancer or heritable effects, lethality of these conditions, quality of life, and years of life lost owing to these conditions.

Radiation Shielding:

Reduction of radiation by interposing a shield of absorbing material between any radioactive source and a person, work area, or radiation-sensitive device.

Radiation Sickness (Syndrome):

The complex of symptoms characterizing the disease known as radiation injury, resulting from excessive exposure (greater than 200 rads or 2 gray) of the whole body (or large part) to ionizing radiation. The earliest of these symptoms are nausea, fatigue, vomiting, and diarrhea, which may be followed by loss of hair (epilation), hemorrhage, inflammation of the mouth and throat, and general loss of energy. In severe cases, where the radiation exposure has been approximately 1,000 rad (10 gray) or more, death may occur within two to four weeks.

Radiation Standards:

Dose and dose rate limits, permissible concentrations, rules for handling, regulations for transportation, regulations for industrial control of radiation, and control of radioactive material established by legislative or regulatory means for the safe use and application of ionizing radiation.

Radiation Units:

Units listed for easy conversion.
Units Conversion Factors
Becquerel (SI) 1 disintegration/s = 2.7 X 10^-11 Ci
Curie 3.7 X 10¹⁰ disintegrations/s = 3.7 X 10¹⁰ Bq
Gray (SI) 1 J/kg = 100 rad
Rad 100 ergs/g = 0.01 Gy
Rem 0.01 Sievert
Sievert (SI) 100 Rem

Radiation Weighting Factor, W_R:

The factor by which the absorbed dose (rad or gray) must be multiplied to obtain a quantity that expresses, on a common scale for all ionizing radiation, the biological damage (rem or sievert) to the exposed tissue. It is used because some types of radiation, such as alpha particles, are more biologically damaging to live tissue than other types of radiation when the absorbed dose from both is equal. This replaces the term quality factor in the latest system of recommendations for radiation protection.

Radioactive (Decay):

Property of undergoing spontaneous nuclear transformation in which nuclear particles or electromagnetic energy are emitted.

Radioactive Material:

Material designated in national law or by a regulatory body as being subject to regulatory control because of its radioactivity, often taking account of both activity and activity concentration.

Radioactivity:

The process of undergoing the transformation of an unstable nucleus by the spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nucleus of an unstable radionuclide. Often used also to express the rate at which radioactive material emits radiation. Measured in units of becquerels in the SI system of units or curies in the traditional system of units.

Radiogenic:

Caused by radiation.

Radioisotope:

An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation. Approximately 5,000 natural and artificial radioisotopes have been identified.

Radiological Attack:

The use of radioactive or nuclear materials for malicious purposes, such as blackmail, murder, sabotage, or terrorism.

Radionuclides:

A radioisotope.

Radiosensitivity:

The relative susceptibility of cells, tissues, organs, organisms, or other substances to the injurious action of radiation.

Radium (Ra):

A radioactive metallic element with atomic number 88. As found in nature, the most common isotope has a mass number of 226. It occurs in minute quantities associated with uranium in pitchblende, carnotite, and other minerals.

Radon (Rn):

A radioactive element that is one of the heaviest gases known. Its atomic number is 86. It is a daughter of radium and thorium.

Random Error:

Errors that vary in a non-reproducible way. These errors can be treated statistically by use of the laws of probability.

Reference Animals and Plants:

A hypothetical entity with the assumed basic characteristics of a specific type of animal or plant, as described to the generality of the taxonomic level of Family, with defined anatomical, physiological, and life-history properties, that can be used for the purposes of relating exposure to dose, and dose to effects, for that type of living organism.

Reference Dose:

Toxicity value for evaluating noncarcinogenic (systemic) effects of daily exposure to contaminant levels without appreciable deleterious effects during a lifetime.

Reference Level:

In emergency or existing controllable exposure situations, this represents the level of dose or risk, above which it is judged to be inappropriate to plan to allow exposures to occur, and below which optimization of protection should be implemented. The chosen value for a reference level will depend upon the prevailing circumstances of the exposure under consideration.

Reference Male and Reference Female (Reference Individual):

A person assumed to have the anatomical and physiological characteristics of an average individual. These assumed characteristics are used in calculations assessing internal dose (also may be called "Standard Man").

Reference Person:

Voxel phantoms for the human body (male and female voxel phantoms based on medical imaging data) with the anatomical and physiological characteristics defined in the report of the ICRP Task Group on Reference Man (Publication 89, ICRP 2002).

Reference Phantom:

Voxel phantoms for the human body (male and female voxel phantoms based on medical imaging data) with the anatomical and physiological characteristics defined in the report of the International Commission on Radiological Protection Task Group on Reference Man (Publication 89, ICRP 2002).

Reference Value:

The value of a parameter recommended by the International Commission on Radiological Protection for use in a biokinetic model in the absence of more specific information, i.e., the exact value used to calculate the dose coefficients presented in the report. Reference values may be specified to a greater degree of precision than that which would be chosen to reflect the uncertainty with which an experimental value is known, to avoid the accumulation of rounding errors in a calculation.

Relative Biological Effectiveness (RBE):

The ratio of a dose of a low-LET reference radiation to a dose of the radiation considered that gives an identical biological effect. RBE values vary with the dose, dose rate, and biological endpoint considered. In radiological protection, the RBE for stochastic effects at low doses (RBEM) is of particular interest.

Relative Life Lost:

The ratio of the proportion of observed years of life lost among people dying of a disease in an exposed population and the corresponding proportion in a similar population without the exposure.

Relative Survival:

The ratio of the proportion of cancer patients who survive for a specified number of years (e.g., 5 years) following diagnosis to the corresponding proportion in a comparable set of cancer-free individuals.

Rem:

A unit of equivalent absorbed dose of radiation, taking into account the relative biological effectiveness of the particular radiation. The dose in rems is the dose in rads multiplied by the quality factor Q derived from the RBE. Rem has been replaced in the SI system with sievert. One sievert equals 100 rem.

Repair Processes:

Metabolic processes within a cell that can repair radiation damage before it is expressed as a biological effect, such as cell killing.

Repository:

A permanent resting place for radioactive wastes that will finally decay to natural radioactivity levels.

Representative Person:

An individual receiving a dose that is representative of the more highly exposed individuals in the population (see Publication 101, ICRP 2006a). This term is the equivalent of, and replaces, "average member of the critical group" described in previous ICRP Recommendations.

Residual Dose:

The dose expected to be incurred after protective measure(s) have been fully implemented (or a decision has been taken not to implement any protective measures).

Ribonucleic Acid (RNA):

A chemical found in the nucleus and cytoplasm of cells; it plays an important role in protein synthesis and other chemical activities of the cell. The structure of RNA is similar to that of DNA. There are several classes of RNA molecules, including messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs, each serving a different purpose.

Ribosomes:

Small cellular components composed of specialized ribosomal RNA and protein; site of protein synthesis.

Risk Analysis:

A detailed examination including risk assessment, risk evaluation, and risk management alternatives; performed to understand the nature of unwanted negative consequences to human life, health, property, or the environment; an analytical process to provide information regarding undesirable events; the process of quantifying the probabilities and expected consequences of identified risks.

Risk Constraint:

A prospective and source-related restriction on the individual risk (in the sense of probability of detriment due to a potential exposure) from a source, which provides a basic level of protection for the individuals most at risk from a source and serves as an upper bound on the individual risk in optimization of protection for that source. This risk is a function of the probability of an unintended event causing a dose, and the probability of detriment due to that dose. Risk constraints correspond to dose constraints but refer to potential exposures.

Risk:

In many health fields, risk means the probability of incurring injury, disease, or death. Risk can be expressed as a value that ranges from zero (no injury or harm will occur) to one (harm or injury will definitely occur).

Risk-Specific Dose:

The dose associated with a specified risk level.

Roentgen:

A unit of gamma radiation measured by the amount of ionization per unit volume in air. In non-bony biological tissue 1 roentgen delivers a dose approximately equal to 1 rad.

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S

Sensitivity Analysis:

This aims to quantify how the results from a model depend upon the different variables included in it.

SI Units:

The international system of units as defined by the general conference on weights and measures in 1960. These units are generally based on meter/kilogram/second units, with special quantities for radiation including the becquerel, gray, sievert.

Sievert:

The international system (SI) unit for dose equivalent equal to 1 Joule/kilogram. The sievert has replaced the rem. One sievert is equivalent to 100 rem.

Somatic Cell:

Any cell in the body except gametes and their precursors.

Somatic Effects of Radiation:

Effects of radiation limited to the exposed individual, as distinguished from genetic effects which may affect subsequent unexposed generations.

Source Region, S_i:

An anatomical region within the reference phantom body that contains the radionuclide following its intake. The region may be an organ, a tissue, the contents of the gastrointestinal tract or urinary bladder, or the surfaces of tissues as in the skeleton, the alimentary tract, and the respiratory tract.

Source:

An entity for which radiological protection can be optimized as an integral whole, such as the x-ray equipment in a hospital, or the releases of radioactive materials from an installation. Sources of radiation, such as radiation generators and sealed radioactive materials, and, more generally, the cause of exposure to radiation or to radionuclides.

Specific Absorbed Fraction:

The fraction of energy of that emitted as a specified radiation type in a source region, S, that is absorbed in 1 kg of a target tissue, T.

Specific Activity:

A measure of the amount of radioactivity in a unit weight (generally one gram) of material.

Specific Energy:

The actual energy per unit mass deposited per unit volume in a given event. This is a stochastic quantity as opposed to the average value over a large number of instances (i.e., the absorbed dose).

Standard Deviation:

A measure of dispersion or variation, usually taken as the square root of the variance.

Standard Geometric Deviation:

Measure of value dispersion about a geometric mean; the frequency-distribution portion that is one standard geometric deviation to either side of the geometric mean; accounts for 68% of total samples.

Standard Mortality Ratio (SMR):

The ratio of the disease or mortality rate in a certain specific population compared to that in a standard population.

Standard Normal Deviation:

Measure of value dispersion about a mean; positive square root of the average of squares of individual deviations from the mean.

Statistical Power:

The probability that an epidemiological study will detect a given level of elevated risk with a specified degree of confidence.

Statistical Significance:

The statistical significance determined by using appropriate standard techniques of statistical analysis with results interpreted at the stated confidence level and based on data-relating species present in sufficient numbers at control areas to permit a valid statistical comparison with the areas being tested.

Stochastic Effects of Radiation:

Effects that occur by chance and which may occur without a threshold level of dose, whose probability is proportional to the dose and whose severity is independent of the dose. In the context of radiation protection, the main stochastic effect is cancer.

Strontium(⁹⁰Sr):

This is one of the most environmentally dangerous fission products produced by nuclear weapons and reactors. It has a long half-life (27.7 years), is readily taken into the body, and is deposited and retained in the bone matrix. It, as well as its daughter, Yttrium (⁹⁰Y), are beta emitters and result in radiation of the bone and bone marrow. An increase in bone cancer has been shown in experimental animals given large doses of ⁹⁰Sr.

Suppressor Gene:

A gene that can suppress the action of another gene, such as an oncogene.

Systematic Error:

Errors that are reproducible and tend to bias a result in one direction. Their causes can be assigned, at least in principle, and they can have constant and variable components. Generally these errors cannot be treated statistically.

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T

Target region, T_i:

Anatomical region within the body (reference phantom) in which radiation is absorbed. The region may be an organ or a specified tissue as in the gastrointestinal tract, urinary bladder, skeleton, and respiratory tract.

Target Theory (Hit Theory):

A theory explaining some biological effects of radiation on the basis that ionization, which occurs in a discrete volume (the target) within a cell, directly causes a lesion that later results in a physiological response to the damage at that location; one, two, or more hits (ionizing events within the target) may be necessary to elicit the response.

Terrestrial Radiation:

The portion of the natural background radiation that is emitted by naturally occurring radioactive materials, such as uranium, thorium, and radon in the earth.

Threshold Dose for Tissue Reactions:

Dose estimated to result in only 1% incidence of tissue reactions.

Threshold Dose:

The minimum application of a given substance required to produce an observable effect.

Threshold Hypothesis:

The assumption that no radiation injury occurs below a specified dose.

Threshold Limit Value (TLV):

Refers to airborne concentrations of substances. Represents conditions under which nearly all workers are believed to be protected while repeatedly exposed for an 8-hour day, 5 days a week [expressed as parts per million (ppm) for gases and vapors and as milligrams per cubic meter (mg/m³) for fumes, mists, and dusts].

Tissue Weighting Factor, W_T:

The factor by which the equivalent dose in a tissue or organ T is weighted to represent the relative contribution of that tissue or organ to the total health detriment resulting from uniform irradiation of the body (ICRP 1991b).

Total Effective Dose Equivalent (TEDE):

The sum of the deep-dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures).

Toxicity Assessment:

Characterization of the toxicological properties and effects of radiation or chemical, with special emphasis on establishment of dose-response characteristics.

Toxicity Profile:

An examination, summary, and interpretation of a hazardous substance to determine levels of exposure and associated health effects.

Toxicity:

Degree of danger a substance poses to animal or plant life.

Track Structure:

Spatial patterns of energy deposition in matter along the track from the passage of ionizing radiation.

Transport of Risk (also called Transfer of Risk):

Taking a risk coefficient estimated for one population and applying it to another population with different characteristics.

Transuranic Elements:

All elements beyond uranium on the periodic table. Transuranic elements are anthropogenic.

Transuranic Waste (TRU):

Waste that contains more than 100 nCi/g of alpha-emitting isotopes with atomic numbers greater than 92 and half-lives greater than 20 years. Such wastes result primarily from fuel reprocessing and from the fabrication of plutonium weapons and plutonium-bearing reactor fuel.

Tritium:

A radioactive isotope of hydrogen. Tritium contains one proton and two neutrons in its nucleus. Because it is chemically identical to the natural hydrogen atoms present in water, tritium can easily be taken into the body by ingestion. It decays by beta emission and has a radioactive half-life of about 12.5 years.

Tumor:

Any abnormal mass of cells resulting from excessive cellular multiplication or lack of differentiation.

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U

Uncertainty Analysis:

A detailed examination of the systematic and random errors of a measurement or estimate; an analytical process to provide information regarding the uncertainty.

Unit of Activity:

Units of Activity (Ci):	Units of Activity (Bq):
1 Centi Ci = 0.01 Ci	1 Kilo Bq (kBq) = 1,000 Bq
Milli 0.0001 Ci	Mega (MBq) 1,000,000 Bq
Micro 0.00001 Ci	Giga (GBq) 1,000,000,000 Bq
Nano 0.00000001 Ci	Tera (TBq) 1,000,000,000,000 Bq
Pico 0.000000000001 Ci	Peta (PBq) 1,000,000,000,000,000 Bq

Unit Risk:

The unit risk factors (URFs) provide estimates of risks due to a unit inventory of contaminant (i.e., risk/gram or risk/curie). URFs can be calculated for water, soil, air, and radiation.

United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR):

International body that publishes periodic reports on sources and effects of ionizing radiation.

Uranium:

A radioactive element with the atomic number 92 and, as found in natural ores, an atomic weight of approximately 238. The two principal natural isotopes are uranium-235 (0.7 percent of natural uranium), which is fissile, and uranium-238 (99.3 percent of natural uranium), which is fissionable by fast neutrons. Natural uranium also includes a minute amount of uranium-234.

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V

Virus:

A noncellular biological entity that can reproduce only within a host cell. Viruses consist of nucleic acid covered by protein; some animal viruses are also surrounded by membrane. Inside the infected cell, the virus uses the synthetic capability of the host to produce progeny virus.

Voxel Phantom:

Computational anthropomorphic phantom based on medical tomographic images where the anatomy is described by small three-dimensional volume elements (voxels) specifying the density and the atomic composition of the various organs and tissues of the human body.

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W

Weighting Factor (WT):

A multiplier that is used for converting the equivalent dose to a specific organ or tissue into what is called the “effective dose.” The goal of this process was to develop a method for expressing the dose to a portion of the body in terms of an equivalent dose to the whole body that would carry with it an equivalent risk in terms of the associated fatal cancer probability. It applies only to the stochastic effects of radiation.

Whole Body Dose Equivalent (Hwb):

The dose equivalent associated with uniform irradiation of the whole body.

Working Level (WL):

A unit of air concentration of potential alpha energy released from radon and its daughters. One working level is the concentration of radon daughters that has a potential energy release of 1.3 X 105 MeV per liter of air or SI units of 2.08 X 10-5 Jh/m³.

Working Level Month (WLM):

Exposure of one working level from radon and its daughters for 170 hours. (3.5 X 10-3 Jh/m³).

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X

X-Ray:

Penetrating electromagnetic radiation having a range of wavelengths (energies) that are similar to those of gamma photons. X rays are usually produced by excitation of the electron field around certain nuclei. Although once formed, there is no difference in x rays and gamma photons; however, there is a difference in their origin. X rays are produced by shifts in the electrons between the rings outside the nucleus of an atom whereas gamma photons are produced by reactions within the nucleus of an atom.

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Y

Yeast Artificial Chromosome (YAC):

A vector used to clone DNA fragments (up to 400 Kb); it is constructed from the telomeric, centromeric, and replication origin sequences needed for replication in yeast cells.

Yield:

The energy released by a nuclear explosion.

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