Dictionary of Radiation Terms

Alphabetical Listing of Terms:

Adapted from Glossary of Radiological Terms
(Centers for Disease Control and Prevention)

A | B | C | D | E | F | G | H | I | J | K | L | M
| O | P | Q | R | S | T | U | V | W | X | Y | Z


Absolute risk: the proportion of a population expected to get a disease over a specified time period. See also risk, relative risk.

Absorbed dose: (Animation) the amount of energy deposited by ionizing radiation in a unit mass of tissue. It is expressed in units of joule per kilogram (J/kg), and called “Gray” (Gy). For more information, see “Primer on Radiation Measurement” from CDC.

Actinides: elements in the periodic table with atomic numbers from 90 to 103 (thorium to lawrencium); i.e., elements with a higher atomic number than actinium, which has an atomic number of 89. These are also called "rare earth metals." They include most of the well-known elements found in nuclear reactions. Actinides with atomic numbers higher than 92 do not occur naturally but are produced artificially by bombarding other elements with particles. Some of the actinides include plutonium, curium, and californium.

Activity (radioactivity): the property of certain nuclides of emitting radiation by spontaneous transformation of their nuclei. Various units of (radio)activity have been used including curie (1 Ci = 3.7 x 1010 disintegrations per second) and becquerel ( 1 Bq = 1 disintegration per second). (Mettler FA Jr, Upton AC: Medical Effects of Ionizing Radiation, 3rd ed. Philadelphia, Pa: Saunders Elsevier, 2008, page 552)

Acute exposure: an exposure to radiation that occurred in a matter of minutes rather than in longer, continuing exposure over a period of time. See also chronic exposure, exposure, fractionated exposure.

Acute Radiation Syndrome (ARS): The Acute Radiation Syndrome (ARS) is also known as Radiation Sickness. A person exposed to radiation will develop ARS only if the radiation dose was high, penetrating (e.g., x-rays or gamma rays), encompassed most or all of the body, and was received in a short period of time. Clinical severity of the four subsyndromes of ARS (hematopoietic, cutaneous, gastrointestinal, and neurovascular) will vary with dose and host factors (e.g., young or old age, immunosuppression, and medical co-morbidity--especially extensive trauma and burns).

Air burst: a nuclear weapon explosion that is high enough in the air to keep the fireball from touching the ground. Because the fireball does not reach the ground and does not pick up any surface material, the radioactivity in the fallout from an air burst is relatively insignificant compared with a surface burst. For more information, see Chapter 2 of CDC’s fallout report.

Air kerma: the initial kinetic energy of the primary ionizing particles (photoelectrons, Compton electrons, positron/negatron pairs from photon radiation, and scattered nuclei from fast neutrons) produced by the interaction of the incident uncharged radiation in a small volume of air, when it is irradiated by an x-ray beam. Unit of measure is Gray. See also kerma.

ALARA: acronym for "As Low As Reasonably Achievable," means making every reasonable effort to maintain exposures to ionizing radiation as far below the dose limits as practical. This is a key principle in radiation protection and safety.

Alpha particle: (Animation) the nucleus of a helium atom, made up of two neutrons and two protons with a charge of +2. Certain radioactive nuclei emit alpha particles. Alpha particles generally carry more energy than gamma rays or beta particles, and deposit that energy very quickly while passing through tissue. Alpha particles can be stopped by a thin layer of light material, such as a sheet of paper, and cannot penetrate the outer, dead layer of skin (Illustration). Therefore, they do not damage living tissue when outside the body. When alpha-emitting atoms are inhaled or swallowed, however, they are especially damaging because they transfer relatively large amounts of ionizing energy to living cells. See also beta particles, gamma rays, neutron, x-ray.

Ambient air: the air that surrounds us.

Americium (Am): a silvery metal; it is a man-made element whose isotopes Am-237 through Am-246 are radioactive. Am-241 is formed spontaneously by the beta decay of plutonium-241. Trace quantities of americium are widely used in smoke detectors and as neutron sources in neutron moisture gauges.

Assigned Protection Factor: Assigned Protection Factor (APF) means the workplace level of respiratory protection that a respirator or class of respirators is expected to provide to employees enrolled in a continuing, effective respiratory protection program. (Source: Assigned Protection Factors for the Revised Respiratory Protection Standard, 2009. OSHA. U.S. Department of Labor Publication No. 3352-02 2009.)

Atom: the smallest particle of an element that can enter into a chemical reaction.

Atomic number: the total number of protons in the nucleus of an atom.

Atomic mass unit (amu): 1 amu is equal to one twelfth of the mass of a carbon-12 atom.

Atomic mass number: the total number of protons and neutrons in the nucleus of an atom.

Atomic weight: the mass of an atom, expressed in atomic mass units. For example, the atomic number of helium-4 is 2, the atomic mass is 4, and the atomic weight is 4.00026.

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Background radiation: ionizing radiation from natural sources, such as terrestrial radiation due to radionuclides in the soil or cosmic radiation originating in outer space.

Becquerel (Bq): (Animation) the amount of a radioactive material that will undergo one decay (disintegration) per second. For more information, see “Primer on Radiation Measurement” from CDC.

Beta burns: Energetic beta particles with high enough specific-activity, if left on the skin surface for a sufficient length of time, may cause erythema and dry (or even wet) desquamation. These are often called "beta burns." "Beta burns" have been described after a nuclear weapon detonation as a consequence of fallout on the skin. (Mettler FA Jr: Direct Effects of Radiation on Specific Tissues. In: Gusev IA, Guskova AK, Mettler FA Jr, eds.: Medical Management of Radiation Accidents, 2nd ed. Boca Raton, Fl: CRC Press, 2001, page 77)

Beta particles: (Animation) (Image) electrons ejected from the nucleus of a decaying atom. Although they can be stopped by a thin sheet of aluminum, beta particles can penetrate the dead skin layer, potentially causing burns. They can pose a serious direct or external radiation threat and can be lethal depending on the amount received. They also pose a serious internal radiation threat if beta-emitting atoms are ingested or inhaled. See also alpha particle, gamma rays, neutron, x-ray.

Bioassay: a measurement of radioactive materials present inside a person’s body through analysis of the person’s blood, urine, feces, or sweat.

Biodosimetry: The use of physiological, chemical or biological markers of exposure of human tissues to ionizing radiation for the purpose of reconstructing doses to individuals or populations. (Adapted from Radiation Dose Reconstruction: Principles and Practices, NCRP Report No. 163, page 182, Bethesda, MD, 2009) (Interactive Tool)

Biological Effects of Ionizing Radiation (BEIR) Reports: reports of the National Research Council's committee on the Biological Effects of Ionizing Radiation. For more information, see http://www.nap.edu/books/0309039959/html/.

Biological half-life: the time required for one half of the amount of a substance, such as a radionuclide, to be expelled from the body by natural metabolic processes, not counting radioactive decay, once it has been taken in through inhalation, ingestion, or absorption. See also radioactive half-life, effective half-life.

Burn: the partial or complete destruction of skin caused by some form of energy, usually thermal energy.

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Carcinogen: a cancer-causing substance.

Cesium-137 (Cs-137): has a half-life of 30.17 years and decays by beta and gamma radiation. Cs-137 is produced by nuclear fission for use in medical devices and gauges and is one of the byproducts of nuclear fission processes in nuclear reactors and nuclear weapons testing. Small quantities of Cs-137 can be found in the environment from nuclear weapons tests that occurred in the 1950s and 1960s and from nuclear reactor accidents, such as the Chernobyl power plant accident in 1986, which distributed Cs-137 to many countries in Europe. (HHS/CDC)

Chain reaction: a process that initiates its own repetition. In a fission chain reaction, a fissile nucleus absorbs a neutron and fissions (splits) spontaneously, releasing additional neutrons. These, in turn, can be absorbed by other fissile nuclei, releasing still more neutrons. A fission chain reaction is self-sustaining when the number of neutrons released in a given time equals or exceeds the number of neutrons lost by absorption in non-fissile material or by escape from the system.

Chronic exposure: exposure to a substance over a long period of time, possibly resulting in adverse health effects. See also acute exposure, fractionated exposure.

Cobalt (Co): a gray, hard, magnetic, and somewhat malleable metal. Cobalt is relatively rare and generally obtained as a byproduct of the production of other metals, such as copper. Its most common radioisotope, cobalt-60 (Co-60), is used in radiography and medical applications. Co-60 emits beta particles and gamma rays during radioactive decay.

Collective dose: (Animation) The sum of the individual doses received in a given time period by a specified population from exposure to a specified source of radiation (Adapted from HPS).

Combined injury: physical, thermal, and/or chemical trauma combined with radiation exposure at a dose sufficient to diminish the likelihood of overall survival or functional recovery.

Committed dose: a dose that accounts for continuing exposures expected to be received over a long period of time (such as 30, 50, or 70 years) from radioactive materials that were deposited inside the body. For more information, see “Primer on Radiation Measurement” from CDC.

Committed dose equivalent (CDE): 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. (Radiation Terms, Health Physics Society)

Committed effective dose equivalent (CEDE): The committed dose equivalent for a given organ multiplied by a weighting factor (see the definition of Weighting Factor). (Radiation Terms, Health Physics Society)

Concentration: the ratio of the amount of a specific substance in a given volume or mass of solution to the mass or volume of solvent.

Conference of Radiation Control Program Directors (CRCPD): an organization whose members represent state radiation protection programs. For more information, see the CRCPD Web site at http://www.crcpd.org.

Contamination (radioactive): the deposition of unwanted radioactive material on the surfaces of structures, areas, objects, or people where it may be external or internal. [Animations of Contamination] See also decontamination, incorporation.

Contamination means that radioactive materials in the form of gases, liquids, or solids are released into the environment and contaminate people externally, internally, or both. An external surface of the body, such as the skin, can become contaminated, and if radioactive materials get inside the body through the lungs, gut, or wounds, the contaminant can become deposited internally. (REAC/TS)

Contamination, fixed: Fixed skin contamination is that which remains after bathing or attempted decontamination. Contamination is assumed to be removed by natural processes within 336 hours (14 days) after deposition on the skin. (Source: Contamination Monitoring Guidance for Portable Instruments Used for Radiological Emergency Response to Nuclear Power Plant Accidents (PDF - 643 KB) (FEMA, October 2002))

Contamination, loose: Loose skin contamination is that which is removable by bathing or decontamination. (Source: Background information on FEM-REP-22: Contamination Monitoring Guidance for Portable Instruments Used for Radiological Emergency Response to Nuclear Power Plant Accidents (FEMA, October 2002))

Controlled area: An area where entry, activities, and exit are controlled to help ensure radiation protection and prevent the spread of contamination. (Basics of Radiation: Definitions, REAC/TS)

Cosmic radiation: radiation produced in outer space when heavy particles (nuclei of all known natural elements) bombard the earth. See also background radiation, terrestrial radiation.

Coulomb: the international system (SI) unit of electric charge. A coulomb is the quantity of charge passing a cross section of conductor in one second when the current is one ampere.

Criticality: a fission process where the neutron production rate equals the neutron loss rate to absorption or leakage. A nuclear reactor is "critical" when it is operating.

Critical mass: the minimum amount of fissile material that can achieve a self-sustaining nuclear chain reaction.

Cumulative dose: the total dose resulting from repeated or continuous exposures of the same portion of the body, or of the whole body, to ionizing radiation. For more information, see “Primer on Radiation Measurement ” from CDC.

Curie (Ci): the traditional measure of radioactivity based on the observed decay rate of 1 gram of radium. One curie of radioactive material will have 37 billion disintegrations in 1 second. For more information, see “Primer on Radiation Measurement” from CDC.

Cutaneous Radiation Syndrome (CRS): the complex syndrome resulting from radiation exposure of more than 200 rads to the skin. The immediate effects can be reddening and swelling of the exposed area (like a severe burn), blisters, ulcers on the skin, hair loss, and severe pain. Very large doses can result in permanent hair loss, scarring, altered skin color, deterioration of the affected body part, and death of the affected tissue (requiring surgery). For more information, see CDC’s fact sheet “Acute Radiation Syndrome” at http://emergency.cdc.gov/radiation/ars.asp.

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Decay chain (decay series): the series of decays that certain radioisotopes go through before reaching a stable form. For example, the decay chain that begins with uranium-238 (U-238) ends in lead-206 (Pb-206) after forming isotopes, such as uranium-234 (U-234), thorium-230 (Th-230), radium-226 (Ra-226), and radon-222 (Rn-222).

Decay constant: the fraction of a number of atoms of a radionuclide that disintegrates in a unit of time. The decay constant is inversely proportional to the radioactive half-life.

Decay products (or daughter products): the isotopes or elements formed and the particles and high-energy electromagnetic radiation emitted by the nuclei of radionuclides during radioactive decay. Also known as "decay chain products" or "progeny" (the isotopes and elements). A decay product may be either radioactive or stable.

Decay, radioactive: disintegration of the nucleus of an unstable atom by the release of radiation.

Decontamination (radioactive): the reduction or removal of radioactive contamination from a structure, object, or person.

Decorporation: removal of radioactive isotopes from the body using specific drugs called "decorporation agents." (List of Countermeasures for Treatment)

Depleted uranium: uranium containing less than 0.7% uranium-235, the amount found in natural uranium. See also enriched uranium.

Deposition density: the activity of a radionuclide per unit area of ground. Reported as becquerels per square meter or curies per square meter.

Detector: A device that is sensitive to radiation and can produce a response signal suitable for measurement or analysis. A radiation detection instrument. (Basics of Radiation: Definitions, REAC/TS)

Deterministic effect: an effect that can be related directly to the radiation dose received. The severity increases as the dose increases. A deterministic effect typically has a threshold below which the effect will not occur. See also stochastic effect, non-stochastic effect.

Deuterium: a non-radioactive isotope of the hydrogen atom that contains a neutron in its nucleus in addition to the one proton normally seen in hydrogen. A deuterium atom is twice as heavy as normal hydrogen. See also tritium.

Dirty bomb: a device designed to spread radioactive material by conventional explosives when the bomb explodes. A dirty bomb kills or injures people through the initial blast of the conventional explosive and spreads radioactive contamination over possibly a large area—hence the term “dirty.” Such bombs could be miniature devices or large truck bombs. A dirty bomb is much simpler to make than a true nuclear weapon. See also radiological dispersal device.

Dose (radiation): radiation absorbed by a person’s body. Several different terms describe radiation dose. For more information, see “Primer on Radiation Measurement” from CDC.

Dose coefficient: the factor used to convert radionuclide intake to dose. Usually expressed as dose per unit intake (e.g., sieverts per becquerel).

Dose equivalent: (Animation) The product of absorbed dose to a given organ or tissue multiplied by a quality factor (also known as a weighting factor [WF]), 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 rem (traditional units) or sieverts (SI units). (Radiation Terms, Health Physics Society)

Dose rate: the radiation dose delivered per unit of time.

Dose reconstruction: scientific procedures that assist with 4 activities - managing victims of radiation emergencies, such as providing input to decisions on protection of emergency workers and members of the public or medical treatment of exposed individuals; providing exposed individuals or populations with information about the doses they received; investigating dose-response relationships in epidemiologic studies; determining whether individuals whose disease might have been induced by radiation qualify for compensation. (Adapted from Radiation Dose Reconstruction: Principles and Practices, NCRP Report No. 163, page 21, Bethesda, MD, 2009)

Dosimeter: a small portable instrument (such as a film badge, thermoluminescent dosimeter [TLD], or pocket dosimeter) for measuring and recording the total accumulated dose of ionizing radiation a person receives.

Dosimetry: assessment (by measurement or calculation) of radiation dose.

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Effective dose: (Animation) a calculated quantity developed by the ICRP (1991) for purposes of radiation protection. The effective dose is assumed to be related to the risk of a radiation-induced cancer or a severe hereditary effect. It takes into account: (1) the absorbed doses that will be delivered to the separate organs or tissues of the body during the lifetime of an individual due to intakes of radioactive materials; (2) the absorbed doses due to irradiation by external sources; (3) the relative effectiveness of different radiation types in inducing cancers or severe hereditary effects; (4) the susceptibility of individual organs to develop a radiation-related cancer or severe hereditary effect; (5) considerations of the relative importance of fatal and non-fatal effects; and, (6) the average years of life lost from a fatal health effect. (HPS 005-3) Thus, the effective dose is a quantity calculated by multiplying the equivalent dose received by every significantly irradiated tissue in the body by a respective tissue weighting factor (this factor reflects the risk of radiation-induced cancer to that tissue) and summing together the individual tissue results to obtain the effective dose. Such a dose, in theory, carries with it the same risk of cancer as would an equal equivalent dose delivered uniformly to the whole body. (from HPS) See also What is Effective Dose?

Effective half-life: the time required for the amount of a radionuclide deposited in a living organism to be diminished by 50% as a result of the combined action of radioactive decay and biological elimination. See also biological half-life, decay constant, radioactive half-life.

Electromagnetic radiation: A traveling wave motion that results from changing electric and magnetic fields. Types of electromagnetic radiation range from those of short wavelength, like x-rays and gamma rays, through the ultraviolet, visible, and infrared regions, to radar and radio waves of relatively long wavelengths. (Basics of Radiation: Definitions, REAC/TS)

Electron: an elementary particle with a negative electrical charge and a mass 1/1837 that of the proton. Electrons surround the nucleus of an atom because of the attraction between their negative charge and the positive charge of the nucleus. A stable atom will have as many electrons as it has protons. The number of electrons that orbit an atom determine its chemical properties. See also neutron.

Electron volt (eV): a unit of energy equivalent to the amount of energy gained by an electron when it passes from a point of low potential to a point one volt higher in potential.

Element: 1) all isotopes of an atom that contain the same number of protons. For example, the element uranium has 92 protons, and the different isotopes of this element may contain 134 to 148 neutrons. 2) In a reactor, a fuel element is a metal rod containing the fissile material.

Emergency Planning Zone (EPZ): the area surrounding a nuclear power plant for which plans required by the NRC have been made in advance to ensure that prompt and effective actions are taken to protect the health and safety of the public in case of an incident. There is a plume exposure pathway EPZ which extends about 10 miles in radius around a plant. Its primary concern is the exposure of the public to, and the inhalation of, airborne radioactive contamination. The ingestion pathway EPZ extends about 50 miles in radius around a plant. Its primary concern is the ingestion of food and liquid that is contaminated by radioactivity.

Enriched uranium: uranium in which the proportion of the isotope uranium-235 has been increased by removing uranium-238 mechanically. See also depleted uranium.

Epidemiology: the study of the distribution and determinants of health-related states or events in specified populations; and the application of this study to the control of health problems.

Epoetin: a recombinant version of human erythropoietin.

Event (planned event): Examples: a scheduled nonemergency activity (e.g., sporting event, concert, parade, training exercise, large convention, fair, large gathering, etc.). (Source: Introduction to the Incident Command System (ICS 100.b), Student Manual, August 2010, (PDF - 8.99 MB). See glossary.)

Exposure (radiation): a measure of ionization in air caused by x-rays or gamma rays only. The unit of exposure most often used is the roentgen. See also contamination.

Exposure pathway: a route by which a radionuclide or other toxic material can enter the body. The main exposure routes are inhalation, ingestion, absorption through the skin, and entry through a cut or wound in the skin.

Exposure rate: a measure of the ionization produced in air by x-rays or gamma rays per unit of time (frequently expressed in roentgens per hour).

External irradiation (or external exposure): External irradiation occurs when all or part of the body is exposed to penetrating radiation from an external source. During exposure, this radiation can be absorbed by the body or it can pass completely through. A similar thing occurs during an ordinary chest x-ray. Following external exposure, an individual is not radioactive and can be treated like any other patient. Gamma or photon radiation exposure from a terrorist nuclear event or radiation dispersal device would make the victim at risk for Acute Radiation Syndrome, depending on the dose received. [Animations of Exposure] (REAC/TS)

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Fallout, nuclear: minute particles of radioactive debris that descend slowly from the atmosphere after a nuclear explosion. For more information, see Chapter 2 of CDC’s fallout report.

First receiver: Healthcare workers in a hospital or other facility where victims arrive for treatment. First receivers provide medical care at locations remote from the incident and not at the site of a hazardous materials release. Since victims may arrive for treatment still contaminated with hazardous materials, first receivers must also protect themselves by putting on appropriate PPE before delivering medical care. (Source: OSHA BEST PRACTICES for HOSPITAL-BASED FIRST RECEIVERS OF VICTIMS from Mass Casualty Incidents Involving the Release of Hazardous Substances. (PDF - 1.93 MB) (OSHA, January 2005))

First responder: An individual responsible for protecting and preserving life, property, evidence, or the environment during the earliest stages of a mass casualty event or other emergency. First responders generally work at or near the incident site. (Source: Homeland Security Presidential Directive / HSPD-8 (December 17, 2003))

Fissile material: any material in which neutrons can cause a fission reaction. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239.

Fission (fissioning): the splitting of a nucleus into at least two other nuclei that releases a large amount of energy. Two or three neutrons are usually released during this transformation. See also fusion.

Fluorescent in situ hybridization (FISH): a cytogenetic technique used to detect and localize the presence or absence of specific DNA sequences on chromosomes. (Wikipedia)

Fractionated exposure: exposure to radiation that occurs in several small acute exposures, rather than continuously as in a chronic exposure.

Fusion: a reaction in which two lighter nuclei unite to form a heavier one, releasing energy in the process. Reactions of this type are responsible for the release of energy in stars or in thermonuclear devices.

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Gamma rays: (Image) (Animation) high-energy electromagnetic radiation emitted by certain radionuclides when their nuclei transition from a higher to a lower energy state. These rays have high energy and a short wave-length. All gamma rays emitted from a given isotope have the same energy, a characteristic that enables scientists to identify which gamma emitters are present in a sample. Gamma rays penetrate tissue farther than do beta or alpha particles but leave a lower concentration of ions in their path to potentially cause cell damage. Gamma rays are very similar to x-rays. See also neutron.

Geiger counter: a radiation detection and measuring instrument consisting of a gas-filled tube containing electrodes, between which an electrical voltage but no current flows. When ionizing radiation passes through 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 measures the intensity of the radiation field. Geiger counters are the most commonly used portable radiation detection instruments.

Genetic effects: hereditary effects (mutations) that can be passed on through reproduction because of changes in sperm or ova. See also teratogenic effects, somatic effects.

Gray (Gy): The new international system (SI) unit of radiation dose, expressed as absorbed energy per unit mass of tissue. The SI unit "Gray" has replaced the older "rad" designation. (1 Gy = 1 joule/kilogram = 100 rad). Gray can be used for any type of radiation (e.g., alpha, beta, neutron, gamma), but it does not describe the biological effects of different radiations. Biological effects of radiation are measured in units of "Sievert" (or the older designation "rem"). Sievert is calculated as follows: Gray multiplied by the "radiation weighting factor" (also known as the "quality factor") associated with a specific type of radiation.

Groundshine: Gamma and/or beta radiation from radioactive material deposited on the ground, including as fallout after a nuclear detonation.

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Half-life: the time any substance takes to decay by half of its original amount. See also biological half-life, decay constant, effective half-life, radioactive half-life.

Health physics: a scientific field that focuses on protection of humans and the environment from radiation. Health physics uses physics, biology, chemistry, statistics, and electronic instrumentation to help protect individuals from any damaging effects of radiation. For more information, see the Health Physics Society Web site at http://www.hps.org/.

High-Efficiency Particulate Air Filter (HEPA): a filter that is at least 99.97% efficient in removing monodisperse particles of 0.3 micrometers in diameter. The equivalent NIOSH 42 CFR 84 particulate filters are the N100, R100, and P100 filters. (Source: NIOSH Respirator Selection Logic, 2004. DHHS(NIOSH) Publication No. 2005-100. and Assigned Protection Factors for the Revised Respiratory Protection Standard, 2009. OSHA U.S. Department of Labor Publication No. 3352-02 2009.)

High-level radioactive waste: the radioactive material resulting from spent nuclear fuel reprocessing. This can include liquid waste directly produced in reprocessing or any solid material derived from the liquid waste having a sufficient concentration of fission . Other radioactive materials can be designated as high-level waste if they require permanent isolation. This determination is made by the U.S. Nuclear Regulatory Commission on the basis of criteria established in U.S. law. See also low-level waste, transuranic waste.

Highly enriched uranium (HEU): uranium that is enriched to above 20% uranium-235 (U-235). Weapons-grade HEU is enriched to above 90% in U-235.

Hot spot: any place where the level of radioactive contamination is considerably greater than the area around it.

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Immediately Dangerous to Life or Health (IDLH): A level of exposure to airborne contaminants likely to cause (1) death; (2) immediate or delayed permanent adverse health effects; or (3) prevent escape from such an environment. IDLH values are considered a maximum level above which only a highly reliable breathing apparatus providing maximum worker protection is permitted. (Source: Documentation for Immediately Dangerous to Life or Health Concentrations (IDLH), NIOSH)

Incident (unplanned event): Examples: An occurrence or event, natural or manmade that requires a response to protect life or property. Incidents can include major disasters, emergencies, terrorist attacks, terrorist threats, civil unrest, wild land and urban fires, floods, hazardous materials spills, nuclear accidents, aircraft accidents, earthquakes, hurricanes, tornadoes, tropical storms, tsunamis, war-related disasters, public health and medical emergencies, and other occurrences requiring an emergency response. (Source: Introduction to the Incident Command System (ICS 100) Student Manual, (PDF - 12 KB), see glossary. )

Incorporation: Incorporation refers to the uptake of radioactive materials by body cells, tissues, and target organs such as bone, liver, thyroid, or kidney. In general, radioactive materials are distributed throughout the body based upon their chemical properties. Incorporation cannot occur unless contamination has occurred. Incorporation is also called internal contamination. (REAC/TS)

Ingestion: 1) the act of swallowing; 2) in the case of radionuclides or chemicals, swallowing radionuclides or chemicals by eating or drinking.

Inhalation: 1) the act of breathing in; 2) in the case of radionuclides or chemicals, breathing in radionuclides or chemicals.

Internal exposure: exposure to radioactive material taken into the body.

Inverse square law: the relationship that states that electromagnetic radiation intensity is inversely proportional to the square of the distance from a point source. (Basics of Radiation: Definitions, REAC/TS)

Iodine: a nonmetallic solid element. There are both radioactive and non-radioactive isotopes of iodine. Radioactive isotopes of iodine are widely used in medical applications. Radioactive iodine is a fission product and is the largest contributor to people’s radiation dose after an accident at a nuclear reactor.

Ion: an atom that has fewer or more electrons than it has protons, causing it to have an electrical charge and, therefore, be chemically reactive.

Ionization: (Animations) the process of adding one or more electrons to, or removing one or more electrons from, atoms or molecules, thereby creating ions. High temperatures, electrical discharges, or nuclear radiation can cause ionization.

Ionizing radiation: any radiation capable of displacing electrons from atoms, thereby producing ions. High doses of ionizing radiation may produce severe skin or tissue damage. See also alpha particle, beta particles, gamma rays, neutron, x-ray. [Illustration]

Iridium-192: A gamma-ray emitting radioisotope used for gamma radiography. The half-life is 73-83 days.

Irradiation: exposure to radiation.

Isotope: a nuclide of an element having the same number of protons but a different number of neutrons.

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Kerma: the initial kinetic energy of the primary ionizing particles (photoelectrons, Compton electrons, positron/negatron pairs from photon radiation, and scattered nuclei from fast neutrons) produced by the interaction of the incident uncharged radiation, per unit mass of interacting medium. Unit of measure is gray. See also air kerma.

Kiloton (Kt): the energy of an explosion that is equivalent to an explosion of 1,000 tons of TNT. See also megaton.

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Latent period: the time between exposure to a toxic material and the appearance of a resultant health effect.

Lead (Pb): a heavy metal. Several isotopes of lead, such as Pb-210 which emits beta particles, are in the uranium decay chain.

Lead federal agency (LFA): the federal agency that leads and coordinates the emergency response activities of other federal agencies during a nuclear emergency. After a nuclear emergency, the Federal Radiological Emergency Response Plan (FRERP, available at http://www.fas.org/nuke/guide/usa/doctrine/national/frerp.htm) will determine which federal agency will be the LFA.

Lethal dose (50/30): the dose of radiation expected to cause death within 30 days to 50% of those exposed without medical treatment. The generally accepted dose is about 400 rem received over a short period of time.

Leukocyte reduction: a process used to filter and remove white blood cells from whole blood before transfusion. Leukocytes are removed from blood because they provide no benefit to the recipient but may carry bacteria and viruses to the recipient. Patients who receive blood that has not been leuko-reduced may have adverse effects, including fever with chills; alloimmunization, an immune system reaction that can compromise a later transfusion; and the transmission of viruses, including cytomegalovirus, which can be dangerous for low-birth weight infants and to immunosuppressed patients.

Local radiation injury (LRI): acute radiation exposure (more than 1,000 rads) to a small, localized part of the body. Most local radiation injuries do not cause death. However, if the exposure is from penetrating radiation (neutrons, x-rays, or gamma rays), internal organs may be damaged and some symptoms of Acute Radiation Syndrome (ARS), including death, may occur. Local radiation injury invariably involves skin damage, and a skin graft or other surgery may be required. See also CDC’s fact sheet “Acute Radiation Syndrome” at http://emergency.cdc.gov/radiation/ars.asp.

Low-level waste (LLW): radioactively contaminated industrial or research waste, such as paper, rags, plastic bags, medical waste, and water-treatment residues. It is waste that does not meet the criteria for any of three other categories of radioactive waste: spent nuclear fuel and high-level radioactive waste; transuranic radioactive waste; or uranium mill tailings. Its categorization does not depend on the level of radioactivity it contains.

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Megaton (Mt): the energy of an explosion that is equivalent to an explosion of 1 million tons of TNT. See also kiloton.

Molecule: a combination of two or more atoms that are chemically bonded. A molecule is the smallest unit of a compound that can exist by itself and retain all of its chemical properties.

Monitoring: determining the amount of ionizing radiation or radioactive contamination present. Also referred to as surveying. (Basics of Radiation: Definitions, REAC/TS)

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Neoplastic: pertaining to the pathologic process resulting in the formation and growth of an abnormal mass of tissue.

Neutron: (Illustration) a small atomic particle possessing no electrical charge, typically found within an atom's nucleus. Neutrons are, as the name implies, neutral in their charge. That is, they have neither a positive nor a negative charge. A neutron has about the same mass as a proton. See also alpha particle, beta particles, gamma rays, nucleon, x-ray.

Non-ionizing radiation: radiation that has lower energy levels and longer wavelengths than ionizing radiation. It is not strong enough to affect the structure of atoms it contacts but is strong enough to heat tissue and can cause harmful biological effects. Examples include radio waves, microwaves, visible light, and infrared from a heat lamp. [Illustration]

Non-stochastic effect: an effect that can be related directly to the radiation dose received. The effect is more severe with a higher dose. It typically has a threshold, below which the effect will not occur. It is sometimes called deterministic effect. For example, a skin burn from radiation is a non-stochastic effect that worsens as the radiation dose increases. See also stochastic effect.

Nuclear energy: the heat energy produced by the process of nuclear fission within a nuclear reactor or by radioactive decay.

Nuclear fuel cycle: the steps involved in supplying fuel for nuclear power plants. It can include mining, milling, isotopic enrichment, fabrication of fuel elements, use in reactors, chemical reprocessing to recover the fissile material remaining in the spent fuel, re-enrichment of the fuel material refabrication into new fuel elements, and waste disposal.

Nuclear reactor: A device in which a controlled, self-sustaining nuclear chain reaction can be maintained with the use of cooling to remove generated heat.

Nuclear tracers: radioisotopes that give doctors the ability to "look" inside the body and observe soft tissues and organs, in a manner similar to the way x-rays provide images of bones. A radioactive tracer is chemically attached to a compound that will concentrate naturally in an organ or tissue so that an image can be taken.

Nucleon: a proton or aneutron; a constituent of the nucleus of an atom.

Nucleus: the central part of an atom that contains protons and neutrons. The nucleus is the heaviest part of the atom.

Nuclide: a general term applicable to all atomic forms of an element. Nuclides are characterized by the number of protons and neutrons in the nucleus, as well as by the amount of energy contained within the atom.

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P-100 Filter: P-100 filtering face piece disposable particulate respirators are capable of filtering at least 99.97% of airborne particles measuring ≥0.3 micron in diameter. Filtering face piece respirators designated with a "P" are strongly resistant to oil.

Pathways: the routes by which people are exposed to radiation or other contaminants. The three basic pathways are inhalation, ingestion, and direct external exposure. See also exposure pathway.

Penetrating radiation: radiation that can penetrate the skin and reach internal organs and tissues. Photons (gamma rays and x-rays), neutrons, and protons are penetrating radiations. However, alpha particles and all but extremely high-energy beta particles are not considered penetrating radiation.

Personal protective equiptment: Clothing and/or equipment worn by workers (including first responders and first receivers) to prevent or mitigate job-related illness or injury. Individual PPE elements can include respiratory and percutanous protective equipment.

Photon: a discrete "packet" of pure electromagnetic energy. Photons have no mass and travel at the speed of light. The term "photon" was developed to describe energy when it acts like a particle (causing interactions at the molecular or atomic level), rather than a wave. Gamma rays and x-rays are photons.

Pitchblende: a brown to black mineral that has a distinctive luster. It consists mainly of uraninite (UO2), but also contains radium (Ra). It is the main source of uranium (U) ore.

Plume: the material spreading from a particular source and traveling through environmental media, such as air or ground water. For example, a plume could describe the dispersal of particles, gases, vapors, and aerosols in the atmosphere, or the movement of contamination through an aquifer (for example, dilution, mixing, or adsorption onto soil).

Plutonium (Pu): a heavy, man-made, radioactive metallic element. The most important isotope is Pu-239, which has a half-life of 24,000 years. Pu-239 can be used in reactor fuel and is the primary isotope in weapons. One kilogram is equivalent to about 22 million kilowatt-hours of heat energy. The complete detonation of a kilogram of plutonium produces an explosion equal to about 20,000 tons of chemical explosive. All isotopes of plutonium are readily absorbed by the bones and can be lethal, depending on the dose and exposure time.

Polonium (Po): a radioactive chemical element and a product of radium (Ra) decay. Polonium is found in uranium (U) ores.

Post-Transfusion Graft-vs.-Host Disease: PT-GVHD occurs when donor lymphocytes in transfused blood attack recipient organs and tissues recognizing recipient HLA and are not eliminated by host immunological defense.

Prenatal radiation exposure: radiation exposure to an embryo or fetus while it is still in its mother’s womb. At certain stages of the pregnancy, the fetus is particularly sensitive to radiation and the health consequences could be severe above 5 rads, especially to brain function. For more information, see CDC’s fact sheet, “Possible Health Effects of Radiation Exposure on Unborn Babies,” at http://emergency.cdc.gov/radiation/prenatal.asp.

Protective Actions: During a radiation emergency incident with an uncontrolled source of radiation, protection of the public from unnecessary exposure to radiation may require some form of intervention that will disrupt normal living. Such intervention is termed a “Protective Action”. Examples of primary protective actions include: sheltering-in-place, evacuation, relocation, interdiction of food and water. Examples of secondary protective actions include medical countermeasure administration, decontamination of people and places, restrictions of food and/or water, access control, and victim extraction. (Source: PAG Manual: Protective Action Guides and Planning Guidance for Radiological Incidents, EPA-400/R-17/001 (PDF - 1.48 MB) (EPA, January 2017))

Protective Action Guide (PAG): A PAG is defined in the EPA PAG Manual as the projected dose to an individual from a release of radioactive material at which a specific “protective action” to reduce or avoid that dose is recommended. (See PAG Manual Section 2.3 for a discussion of projected dose.) PAGs are guides to help officials select protective actions under emergency conditions during which exposures would occur for relatively short time periods. They are not meant to be applied as strict numeric criteria, but rather as guidelines to be considered in the context of incident- specific factors. PAGs do not establish an acceptable level of risk for normal, non-emergency conditions, nor do they represent the boundary between safe and unsafe conditions. The PAGs are not legally binding regulations or standards and do not supersede any environmental laws. (Source: PAG Manual: Protective Action Guides and Planning Guidance for Radiological Incidents, EPA-400/R-17/001 (PDF - 1.48 MB) (EPA, January 2017))

Proton: a small atomic particle, typically found within an atom's nucleus, that possesses a positive electrical charge. Even though protons and neutrons are about 2,000 times heavier than electrons, they are tiny. The number of protons is unique for each chemical element. See also nucleon.

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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. (Radiation Terms, Health Physics Society)

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Rad (radiation absorbed dose): a basic unit of absorbed radiation dose. It is a measure of the amount of energy absorbed by the body. The rad is the traditional unit of absorbed dose. It is being replaced by the unit gray (Gy), which is equivalent to 100 rad. One rad equals the dose delivered to an object of 100 ergs of energy per gram of material. For more information, see “Primer on Radiation Measurement” from CDC.

Radiation: energy moving in the form of particles or waves. Familiar radiations are heat, light, radio waves, and microwaves. Ionizing radiation is a very high-energy form of electromagnetic radiation.

Radiation protection: sometimes known as radiological protection, is the science of protecting people and the environment from the harmful effects of ionizing radiation, which includes both particle radiation and high energy electromagnetic radiation. (source: Wikipedia)

Radiation sickness: See also acute radiation syndrome (ARS), or the CDC fact sheet “Acute Radiation Syndrome” at http://emergency.cdc.gov/radiation/ars.asp.

Radiation warning symbol: a symbol prescribed by the Code of Federal Regulations. It is a magenta or black trefoil on a yellow background. It must be displayed where certain quantities of radioactive materials are present or where certain doses of radiation could be received.

Radioactive contamination: the deposition of unwanted radioactive material on the surfaces of structures, areas, objects, or people. It can be airborne, external, or internal. See also contamination, decontamination.

Radioactive decay: the spontaneous disintegration of the nucleus of an atom.

Radioactive half-life: the time required for a quantity of a radioisotope to decay by half. For example, because the half-life of iodine-131 (I-131) is 8 days, a sample of I-131 that has 10 mCi of activity on January 1, will have 5 mCi of activity 8 days later, on January 9. See also biological half-life, decay constant, effective half-life. [Illustrations]

Radioactive material: material that contains unstable (radioactive) atoms that give off radiation as they decay.

Radioactivity: the process of spontaneous transformation of the nucleus, generally with the emission of alpha or beta particles often accompanied by gamma rays. This process is referred to as decay or disintegration of an atom.

Radioactive waste: Disposable, radioactive materials resulting from nuclear operations. Wastes are generally classified into two categories, high-level and low-level waste.

Radioassay: a test to determine the amounts of radioactive materials through the detection of ionizing radiation. Radioassays will detect transuranic nuclides, uranium, fission and activation products, naturally occurring radioactive material, and medical isotopes.

Radiogenic: health effects caused by exposure to ionizing radiation.

Radiography: 1) medical: the use of radiant energy (such as x-rays and gamma rays) to image body systems; 2) industrial: the use of radioactive sources to photograph internal structures, such as turbine blades in jet engines. A sealed radiation source, usually iridium-192 (Ir-192) or cobalt-60 (Co-60), beams gamma rays at the object to be checked. Gamma rays passing through flaws in the metal or incomplete welds strike special photographic film (radiographic film) on the opposite side.

Radioisotope (radioactive isotope): isotopes of an element that have an unstable nucleus. Radioactive isotopes are commonly used in science, industry, and medicine. The nucleus eventually reaches a stable number of protons and neutrons through one or more radioactive decays. Approximately 3,700 natural and artificial radioisotopes have been identified.

Radiological or radiologic: related to radioactive materials or radiation. The radiological sciences focus on the measurement and effects of radiation.

Radioluminescence: The luminescence produced by particles emitted during radioactive decay.

Radiological dispersal device (RDD): a device that disperses radioactive material by conventional explosive or other mechanical means, such as a spray. See also dirty bomb.

Radiological exposure device (RED): also called a "hidden sealed source." An RED is a terrorist device intended to expose people to significant doses of ionizing radiation without their knowledge. Constructed from partially or fully unshielded radioactive material, an RED could be hidden from sight in a public place (e.g., under a subway seat, in a food court, or in a busy hallway), exposing those who sit or pass close by. If the seal around the source were broken and the radioactive contents released from the container, the device could become a radiological dispersal device (RDD), capable of causing radiological contamination. [Illustration]

Radionuclide: an unstable and therefore radioactive form of a nuclide.

Radium (Ra): a naturally occurring radioactive metal. Radium is a radionuclide formed by the decay of uranium (U) and thorium (Th) in the environment. It occurs at low levels in virtually all rock, soil, water, plants, and animals. Radon (Rn) is a decay product of radium.

Radon (Rn): a naturally occurring radioactive gas found in soil, rock, and water throughout the United States. Radon causes lung cancer and is a threat to health because it tends to collect in homes, sometimes to very high concentrations. As a result, radon is the largest source of exposure to people from naturally occurring radiation.

Relative risk: the ratio between the risk for disease in an irradiated population to the risk in an unexposed population. A relative risk of 1.1 indicates a 10% increase in cancer from radiation, compared with the "normal" incidence. See also risk, absolute risk.

Relative biologic effectiveness (RBE): The RBE of some test radiation (r) compared with x-rays is defined by the ratio D250/Dr where D250 and Dr are, respectively, the doses of x-rays and the test radiation required for equal biologic effect. (National Bureau of Standards, 1954)

Rem (roentgen equivalent, man): a unit of equivalent dose. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Rem relates the absorbed dose in human tissue to the effective biological damage of the radiation. It is determined by multiplying the number of rads by the quality factor, a number reflecting the potential damage caused by the particular type of radiation. The rem is the traditional unit of equivalent dose, but it is being replaced by the sievert (Sv), which is equal to 100 rem. For more information, see “Primer on Radiation Measurement” from CDC.

Risk: the probability of injury, disease, or death under specific circumstances and time periods. Risk can be expressed as a value that ranges from 0% (no injury or harm will occur) to 100% (harm or injury will definitely occur). Risk can be influenced by several factors: personal behavior or lifestyle, environmental exposure to other material, or an inborn or inherited characteristic known from scientific evidence to be associated with a health effect. Because many risk factors are not exactly measurable, risk estimates are uncertain. See also absolute risk, relative risk.

Risk assessment: an evaluation of the risk to human health or the environment by hazards. Risk assessments can look at either existing hazards or potential hazards.

Roentgen (R): a unit of exposure to x-rays or gamma rays. One roentgen is the amount of gamma or x-rays needed to produce ions carrying 1 electrostatic unit of electrical charge in 1 cubic centimeter of dry air under standard conditions.

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Sealed source: A radioactive source, sealed in an impervious container that has sufficient mechanical strength to prevent contact with and dispersion of the radioactive material under the conditions of use and wear for which it was designed. Generally used for radiography or radiation therapy. May be classified "Special Form" on shipping papers and packages. (Basics of Radiation: Definitions, REAC/TS)

Sensitivity: ability of an analytical method to detect small concentrations of radioactive material.

Shielding: (Animation) (Illustration) the material between a radiation source and a potentially exposed person that reduces exposure.

Sievert (Sv): 1 Sv = 1 Joule per kilogram. Used to measure the health effects of ionizing radiation (usually low doses), both external and internal. Sievert is the special name for the Standard International (SI) unit of dose equivalent (H), equivalent dose (HT), effective dose (E), weighted dose, and organ dose equivalent. J per kilogram is also the SI unit for the ICRU operational quantities (see operational quantity). For additional discussion, see NCRP glossary and Wikipedia “Sievert” page.

SI units: the Systeme Internationale (or International System) of units and measurements. This system of units officially came into being in October 1960 and has been adopted by nearly all countries, although the amount of actual usage varies considerably. For more information, see “Primer on Radiation Measurement” from CDC.

Somatic effects: effects of radiation that are limited to the exposed person, as distinguished from genetic effects, which may also affect subsequent generations. See also teratogenic effects.

Source term: Types and amounts of radioactive or hazardous material released to the environment following an accident. (NRC)

Special Nuclear Material (SNM): Plutonium and uranium enriched in the isotope uranium-233 or uranium 235.

Specific activity: Unit pertaining to the disintegrations per gram of a radioisotope (Mettler FA Jr, Upton AC: Medical Effects of Ionizing Radiation, 3rd ed. Philadelphia, Pa: Saunders Elsevier, 2008, page 554)

Stable nucleus: the nucleus of an atom in which the forces among its particles are balanced. See also unstable nucleus.

Stochastic effect: an effect that occurs on a random basis independent of the size of dose. The effect typically has no threshold and is based on probabilities, with the chances of seeing the effect increasing with dose. If it occurs, the severity of a stochastic effect is independent of the dose received. Cancer is a stochastic effect. See also non-stochastic effect, deterministic effect.

Strontium (Sr): a silvery, soft metal that rapidly turns yellow in air. Sr-90 is one of the radioactive fission materials created within a nuclear reactor during its operation. Stronium-90 emits beta particles during radioactive decay.

Surface burst: a nuclear weapon explosion that is close enough to the ground for the radius of the fireball to vaporize surface material. Fallout from a surface burst contains very high levels of radioactivity. See also air burst. For more information, see Chapter 2 of CDC’s fallout report.

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Tailings: waste rock from mining operations that contains concentrations of mineral ore that are too low to make typical extraction methods economical.

Thermonuclear device: a “hydrogen bomb.” A device with explosive energy that comes from fusion of small nuclei, as well as fission.

Teratogenic effects: birth defects that are not passed on to future generations, caused by exposure to a toxin as a fetus. See also genetic effects, somatic effects.

Terrestrial radiation: radiation emitted by naturally occurring radioactive materials, such as uranium (U), thorium (Th), and radon (Rn) in the earth.

Thorium (Th): a naturally occurring radioactive metal found in small amounts in soil, rock, water, plants, and animals. The most common isotopes of thorium are thorium-232 (Th-232), thorium-230 (Th-230), and thorium-238 (Th-238).

Total Effective Dose Equivalent (TEDE): The sum of effective dose equivalent from external radiation and the committed effective dose equivalent from inhaled and ingested radioactive material. Quoted in units of rem.

Transuranic: pertaining to elements with atomic numbers higher than uranium (92). For example, plutonium (Pu) and americium (Am) are transuranics.

Triage: The use of simple procedures for rapidly sorting affected people into groups so as to expedite treatment and maximize the effective use of medical and monitoring supplies. (Source: Use of Prussian Blue (Ferric Hexacyanoferrate) for Decorporation of Radiocaesium, page 34, Public Health England [PHE], formerly Health Protection Agency [HPA])

Triage, radiological: actions intended to sort people according to whether they have been exposed to radioactive material at a level that will definitely have an effect on their health (i.e., causing deterministic injuries like tissue reactions or acute radiation syndrome); or whether they may have been exposed to radioactive material at lower levels that might impart a long-term health risk (i.e., stochastic health effects or cancer induction in the future); or whether they are in the potentially large group of people whose exposures are very unlikely to have any effect on health, or who were not exposed at all. (Source: Use of Prussian Blue (Ferric Hexacyanoferrate) for Decorporation of Radiocaesium, page 34, Public Health England [PHE], formerly Health Protection Agency [HPA])

Tritium: (chemical symbol H-3) a radioactive isotope of the element hydrogen (chemical symbol H). See also deuterium.

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Unstable nucleus: a nucleus that contains an uneven number of protons and neutrons and seeks to reach equilibrium between them through radioactive decay (i.e., the nucleus of a radioactive atom). See also stable nucleus.

UNSCEAR: United Nations Scientific Committee on the Effects of Atomic Radiation. See also http://www.unscear.org/.

Uranium (U): a naturally occurring radioactive element whose principal isotopes are uranium-238 (U-238) and uranium-235 (U-235). Natural uranium is a hard, silvery-white, shiny metallic ore that contains a minute amount of uranium-234 (U-234).

Uranium mill tailings: naturally radioactive residue from the processing of uranium ore. Although the milling process recovers about 95% of the uranium, the residues, or tailings, contain several isotopes of naturally occurring radioactive material, including uranium (U), thorium (Th), radium (Ra), polonium (Po), and radon (Rn).

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Weighting factor: 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. (Radiation Terms, Health Physics Society)

Whole-body count: the measure and analysis of the radiation being emitted from a person’s entire body, detected by a counter external to the body.

Whole-body exposure: an exposure of the body to radiation, in which the entire body, rather than an isolated part, is irradiated by an external source.

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X-ray: electromagnetic radiation caused by deflection of electrons from their original paths, or inner orbital electrons that change their orbital levels around the atomic nucleus. X-rays, like gamma rays can travel long distances through air and most other materials. Like gamma rays, x-rays require more shielding to reduce their intensity than do beta or alpha particles. X-rays and gamma rays differ primarily in their origin: x-rays originate in the electron shell; gamma rays originate in the nucleus. See also neutron.