Bobby's Radiation Glossary for Students

Prepared by Bobby R. Scott
Lovelace Respiratory Research Institute, P.O. Box 5890, Albuquerque, NM 87185

Updated: October 22, 1998

Absorbed dose: Radiation energy deposited in tissue or other material divided by the mass of the tissue or material. An often-used unit for absorbed dose is the gray (Gy). It is an SI unit that applies when radiation energy is in joules, and the target mass is in kilograms.

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.

Accelerator: A machine capable of accelerating charged particles in a vacuum and discharging the resultant particulate or other radiation into another medium.

Actinides: Series of elements beginning with actinium (element number 89) and continuing through lawrencium (element number 103). These elements occupy the same position in the periodic table. The series includes uranium (element 92) and all transuranic elements.

Activity: The number of nuclear transformations occurring in a given quantity of material per unit of time. Special units of activity are the curie (Ci) and becquerel (Bq).

Activated: Non-fissile material made radioactive by exposure to neutrons.

Alpha particle: A particle made up of two protons and two neutrons. Alpha particles are emitted by some heavy elements that include uranium, plutonium, and radon. Alpha particles can be stopped by a piece of paper and are only a concern when alpha-emitting isotopes are taken into the body (e.g., by inhalation). Many Russian nuclear workers inhaled large amounts of plutonium-239 while working at the Mayak plutonium production facility located in the Chelyabinsk region near Siberia. As a result, some of the Mayak workers developed serious health effects including lung cancer.

Annual Limit on Intake (ALI): The activity of a radionuclide taken into the body by inhalation or ingestion that commits a worker to receiving the relevant dose equivalent limit. The ALI is used to control worker intakes of radionuclides. Special problems arise for plutonium-238 and plutonium-236 in that inhaling a single particle can exceed the ALI. Such single particle intakes may go unnoticed.

Atmospheric fallout: Radioactive particles from a nuclear explosion. The particles gradually fall from the sky and deposit on the ground and on other surfaces.

Atom: An atom is the smallest particle of an element that has all of the chemical properties of that element. Atoms are made up of a nucleus that is surrounded by electrons. The nucleus contains protons and neutrons, which in turn are made up of other particles.

Background Radiation: Radiation arising from natural sources. These sources include: (1) terrestrial radiation from naturally occurring radioactive isotopes in the soil; (2) cosmic radiation originating in outer space; and (3) naturally occurring radioactive isotopes in the body. All of us were born with some naturally occurring radionuclides in our body (e.g., radioactive carbon).

Becquerel: Unit of radioactivity used internationally (i.e., SI unit). One becquerel equals one radioactive disintegration per second. To convert becquerels to microcuries, divide by 37,000.

Beryllium: The fourth lightest element. Beryllium is used to make some parts for nuclear weapons and can be harmful if taken into the body.

Beta particle: An electron (positive or negative) emitted during decay of some isotopes. Beta particles have a short range in air and even shorter range in more dense material. Beta-emitting isotopes (e.g. strontium-90) deposited on the skin can cause skin burns and other more serious effects. Japanese fishermen (in a boat called the Lucky Dragon) were exposed to atmospheric fallout from a U. S. thermonuclear test explosion conducted in 1954 in the Pacific Ocean and developed severe skin burns. Beta radiation burns to a large area of the skin can be lethal.

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.

Cesium: An element that is chemically similar to sodium and potassium. Some victims of the Chernobyl accident, which occurred in April 1986 in the Ukraine, inhaled cesium-137 along with other radionuclides. Cesium-137 emits highly penetrating gamma radiation.

Chain reaction: A reaction where fissile atoms (e.g., atoms of uranium-235) absorb neutrons, then split, releasing energy and more neutrons, which in turn cause additional atom splittings and neutron releases so that the process continues. Nuclear weapons that were exploded over Japanese cities of Nagasaki and Hiroshima were based on uncontrolled chain reactions. Radiation exposures resulting form the weapons mainly involved gamma rays but also neutrons. Although the radiation exposures occurred in the forties, radiation induced cancers are still occurring among A-bomb survivors.

Criticality: The conditions necessary for a sustained nuclear chain reaction.

Curie: The amount of radioactivity in 1 gram of radium-226. One curie equals 37 billion radioactive disintegrations per second. To convert curies to microcuries, multiply by one million. Conversely, to convert microcuries to curies, divided by one million. The curie is not an SI unit and is mainly used by U. S. scientists. The corresponding SI unit is the becquerel. One microcurie equals 37,000 becquerel.

Depleted uranium: Uranium residue left over after the process of uranium enrichment, which results from removing most of the uranium-235 from natural uranium. Natural uranium contains both uranium-235 and uranium-238. Depleted uranium has more uranium-238 than does natural uranium. Enriched uranium has more uranium-235 than natural uranium. Depleted uranium combusts spontaneously upon impact with a target and has been used to develop armor-penetrating munitions. Some Persian Gulf War veterans were exposed to depleted uranium (e.g., via inhalation and via wounds from shrapnel). Some of the health effects they are encountering (associated with lung and kidney) may be related to the chemical toxicity of depleted uranium.

Deuterium: A natural isotope of hydrogen that is present in heavy water. Deuterium has one neutron and one proton, while hydrogen has one proton but no neutrons.

Dose equivalent: A theoretical dose unit that makes different radiations appear equally effective in producing biological effects in a given organ or tissue. Special factors are used to convert absorbed dose to dose equivalent.

Electron: An elementary particle with a mass 1837 times smaller than for a proton. Negative charge electrons orbit around the nucleus and determine the chemical properties of an atom.

Fissile: Isotopes that can split (break up) when hit by a low-energy neutron. The fissile isotopes uranium-235 and plutonium-239 are used in nuclear weapons.

Fission: The splitting (breaking apart or fissioning) of the nucleus of a heavy atom such as uranium-235 or plutonium-239. The fission is usually caused by the absorption of a neutron.

Fusion: The process in which nuclei of lighter elements (e.g., deuterium and tritium) combined to form the nucleus of a heavier element, with the release of energy.

Gamma Radiation: High-energy, penetrating radiation emitted in the radioactive decay of many radionuclides. Gamma rays are similar to X rays rays, but X rays generally have lower energy. A dose of three gray of gamma rays delivered briefly to the total body would be lethal to about 50% of humans exposed because of severe damage to the hematopoietic system. Deaths would be expected to occur within about 60 days. However, spreading the 3 gray dose over a number of years would be expected to lead to far less net damage to the hematopoietic system. Some nuclear workers in Russia exposed over years, at low rates, to up to 10 gray of gamma rays survived. However, 10 gray delivered briefly to the total body would be considered lethal for all persons exposed.

Geologic repository: A place to dispose of radioactive waste deep beneath the surface of the earth (e.g., Waste Isolation Pilot Plant).

Gray: A unit of radiation dose equal to 1 joule of energy deposited in 1 kg of tissue or other material. The gray (Gy) is an SI unit and is equal to 100 rad.

Half-life: The average time it takes for one-half of any given number of unstable atoms to decay. Half lives of isotopes range from small fractions of a second to more than a billion years. Half-life as defined here is also called physical half-life.

High-LET radiation: Radiation that produces lots of damage over a short distance in tissue or other material is called high-LET radiation. In contrast, low-LET radiation produces only a small amount of damage when evaluated over a short distance. Alpha particles represent high-LET radiation. Gamma and x rays represent low-LET radiations. To produce a given amount of damage, it takes a larger absorbed dose of low-LET radiation than for high-LET radiation. Also, biological damage produced by low-LET radiation is usually more efficiently repaired than damage produced by high-LET radiation.

High-level waste: Highly radioactive material from the processing of spent nuclear fuel.

Irradiate: To expose to radiation.

Isotope: Forms of the same chemical element that have different numbers of neutrons. Many isotopes are produced in nuclear reactors and particle accelerators. The field of nuclear medicine depends on a constant supply of radioactive isotopes (i.e., radioisotopes).

LET: Short for linear energy transfer. LET represents the average amount of radiation energy lost when traversing a small distance. It has units of energy divided by the short distance.

Microcurie: One curie divided by one million. To convert microcuries to curies, divide by one million. To convert microcuries to becquerels, multiply by 37,000. To convert microcuries to kilobecquerels, multiply by 37. A kilobecquerel is 1000 becquerels.

Natural uranium: Uranium that has not been altered by humans. Natural uranium is made up of 99.3 percent uranium-238 and 0.7 percent uranium-235.

Neutron: An uncharged particle that makes up part of an atomic nucleus. Uranium-235 and plutonium-239 atoms split (fission) when they absorb neutrons. This causes additional neutrons and heat to be released, leading to a chain reaction. The controlled chain reactions that occur in nuclear power reactors and uncontrolled chain reactions that occur in nuclear weapons are due to neutrons being absorbed by atoms that then fission.

Nuclear Reactor: A complex device that sustains a controlled nuclear fission chain reaction. Nuclear power plants use nuclear reactors to produce electricity. Accidents sometimes occur at nuclear power plants placing workers and the public at risk to radiation exposure. In April of 1986, such an accident occurred at the Chernobyl power station in the Ukraine. Many children have developed thyroid cancer as a result of exposure to radioactive iodine released during the Chernobyl accident.

Photon: A quantum of electromagnetic radiation. Photons have no mass or electrical charge.

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

Plutonium: This is a man-made element. Pure plutonium (Pu) is a silvery metal that is heavier than lead. Plutonium found in the environment is often in the oxide form. This form is generally written as PuO2. The Rocky Flats plant near Denver has lots of PuO2-contaminated soil and buildings. A key question is how much cleanup of the site is needed so that future populations that may reside at or use the site for other purposes will be unlikely to be harmed from plutonium that remains.

Plutonium-238: This plutonium isotope is not used in nuclear weapons but is used in producing electricity for space travel. Plutonium-238 emits lots of alpha particles that generate heat that is used for producing electricity. Plutonium-238 is about 280 times more radioactive than plutonium-239. Some citizens are concerned that an accident related to space travel may lead to spreading plutonium-238 around our planet. The Cassini mission to Saturn involves a very large quantity of plutonium-238, and this led to protests by many concerned citizens. The possibility is remote that the Cassini spacecraft when it circles earth to produce a sling-shot effect to boost it toward Saturn will reenter the earth's atmosphere, burn up, and spread plutonium-238 around our planet. However, experts at NASA and elsewhere have assured us that this cannot happen.

Plutonium-239: This plutonium isotope can split (fission) when hit by neutrons. The splitting can release additional neutrons that can cause other plutonium-239 atoms to also split. This can lead to the chain reaction that takes place in nuclear weapons and in nuclear reactors used to produce electricity. Plutonium-239 was released in small amounts during the Chernobyl nuclear power plant accident that occurred in the Ukraine in April 1986.

Proton: An elementary particle found in the nucleus of atoms. Protons have positive charge and have a mass approximately 1837 times the mass of an electron. The atomic number of an atom is equal to the number of protons in the nucleus.

Quantum: An observable quantity is considered quantized when it only takes on discrete values. When the magnitude of the quantity is always a multiple of a definite unit, then the unit is called the quantum (of the quantity).

Quantum theory: The concept that energy is radiated intermittently in small units of definite magnitude called quanta, and absorbed in a like manner.

Rad: A unit of radiation dose equal to 100 ergs deposited in 1 gram of tissue or other material. The rad is not an SI unit and is mainly used by some U.S. scientists. Some scientific journals will not accept the rad unit when you submit a paper for publication. They prefer that you use the gray, which corresponds to 100 rad.

Radiation: Energy transferred as particles or waves through space or other media. Sunlight is a form of radiation. Without that radiation, we would not exist. Thus radiation is necessary for life on our planet, as we know it.

Radioactive: Exhibiting radioactivity.

Radioactivity: The spontaneous emission of radiation from unstable atoms. Radionuclides lose particles (e.g., alpha or beta) and energy through radioactive decay.

Radioisotope thermoelectric generator: A device that uses radionuclides such as plutonium-238 to produce heat for the purpose of generating electricity. Radioisotope thermoelectric generators (RTGs) are used to supply electricity in spacecraft, medical devices, and nuclear weapons.

Radionuclide: A radioactive species of an atom.

Radon: A radioactive inert gas that arises from the decay of radium. Radon occurs naturally in many minerals and is a chief hazard of uranium mill tailings. Some radon can also be found in our homes. Radon decays into other isotopes that emit alpha radiation. Inhaling the alpha-emitting isotopes may increase your risk for lung cancer. However, there is growing evidence that low doses of radiation to the lung may not cause lung cancer. There may be a threshold dose below which there is no risk for causing lung cancer.

Relative biological effectiveness: A factor that reflects how two different types of radiation differ quantitatively in producing biological effects. If one radiation type requires 10 Gy to produce a given level of biological effect and another type requires only 5 Gy for the same level of effect, the relative biological effectiveness (RBE) of the second relative to the first is 10 Gy/ 5 Gy or RBE = 2.

Sievert: The sievert (Sv) is the name of the SI unit of dose equivalent. The dose equivalent is a somewhat magical dose unit that in theory makes different radiations appear to be equally effective in producing biological effects in a given organ of the body even though the radiations may not be equally effective. Special factors are used to convert absorbed dose to dose equivalent. The factors include the relative biological effectiveness.

SI units: The international system of units. Special SI units for radiation include the becquerel (Bq), gray (Gy), and sievert (Sv).

Strontium-90: A beta-emitting isotope of strontium that is chemically similar to calcium. Strontium-90 has a half-life of 28 years and is a common isotopes produced in nuclear reactors.

Thorium: A radioactive element found in nature.

Transuranic elements: All elements beyond uranium on the periodic table. Transuranic elements are man-made.

Tritium: The heaviest isotope of hydrogen and a source of beta radiation. Using tritium gas as a booster enhances the power of modern nuclear weapons. Tritium has a half life of 12 years.

Uranium: A naturally occurring material used for nuclear technology. There are a variety of uranium isotopes that include uranium-235 and uranium-238.

Uranium-235: This isotopes is used in nuclear weapons. It is the only naturally occurring fissile element. Uranium-235 makes up 0.7% of the uranium that is mined from the ground. The half-life of uranium-235 is 700 million years.

Uranium-238: This isotope makes up over 99% of uranium that is mined from the ground. The isotope also makes up most of the uranium used in armor for military vehicles and in uranium munitions (penetrators) used by the U.S. military. The uranium material is called depleted uranium, as uranium-235 has been removed from natural uranium in producing the depleted uranium. The half-life of uranium-238 is 4.5 billion years.

Vitrification: A process of treating radioactive waste that stabilizes the waste by mixing it with molten glass.

X rays: Penetrating photon radiation often used in medical diagnosis. Sunlight is also a form of photon radiation but less hazardous than x rays.

Acknowledgments: The U.S. DOE Environmental Management Science Program provides support for developing and maintaining this glossary. Ms. Paula Bradley kindly provided editorial assistance for this version.