This glossary was first published in Living with Radiation. It is expected that more entries will be added from time to time. Courtesy National Radiological Protection Board.

Absorbed dose	Quantity of energy imparted by ionising radiation to unit mass of matter such as tissue. Unit gray, symbol Gy. 1 Gy = 1 joule per kilogram.
Actinides	A group of 15 elements with atomic number from that of actinium (89) to lawrencium (103) inclusive. All are radioactive. Group includes uranium, plutonium, americium, and curium.
Activity	Attribute of an amount of a radionuclide. Describes the rate at which transformations occur in it. Unit becquerel, symbol Bq. 1 Bq = 1 transformation per second.
Advanced gas cooled reactor	A development of the Magnox reactor, using enriched uranium oxide fuel in stainless steel cladding.
AGR	Advanced gas cooled reactor.
Alpha particle	A particle consisting of two protons plus two neutrons. Emitted by a radionuclide.
Atom	The smallest portion of an element that can combine chemically with other atoms.
Atomic bomb	See nuclear weapon.
Atomic mass	The mass of an isotope of an element expressed in atomic mass units, which are defined as one-twelfth of the mass of an atom of carbon-12.
Atomic number	The number of protons in the nucleus of an atom. Symbol Z.
Becquerel	See activity.
Beta particle	An electron emitted by the nucleus of a radionuclide. The electric charge may be positive, in which case the beta particle is called a positron.
Brachytherapy	Term applied to the use of radiation sources in or on the body for treating certain types of cancer.
Chromosomes	Rod-shaped bodies found in the nucleus of cells in the body. They contain the genes, or hereditary constituents. Human beings possess 23 pairs.
Collective dose	Frequently used for collective effective dose.
Collective effective dose	The quantity obtained by multiplying the average effective dose by the number of people exposed to a given source of ionising radiation. Unit man sievert, symbol man Sv. Frequently abbreviated to collective dose.
Consumer products	Personal and household goods such as timepieces, smoke alarms, and gas mantles that contain radioactive material for functional reasons.
Cosmic rays	High energy ionising radiations from outer space. Complex composition at the surface of the earth.
Current density	The electric current or flow of electric charge through a conducting medium, such as tissue, per unit cross-sectional area. Unit ampere per square metre, symbol A m-2.
Decay	The process of spontaneous transformation of a radionuclide. The decrease in the activity of a radioactive substance.
Decay product	A nuclide or radionuclide produced by decay. It may be formed directly from a radionuclide or as a result of a series of successive decays through several radionuclides.
Decommissioning	The process of closing down a nuclear reactor, removing the spent fuel, dismantling some of the other components, and preparing them for disposal. Term may also be applied to other major nuclear facilities.
Diagnostic radiology	Term usually applied to the use of X-rays in medicine for identifying disease or injury in patients.
Disposal	In relation to radioactive waste, dispersal or emplacement in any medium without the intention of retrieval.
DNA	Deoxyribonucleic acid. The compound that controls the structure and function of cells and is the material of inheritance.
Dose	General term for quantity of ionising radiation. See absorbed dose, equivalent dose, effective dose and collective effective dose. Frequently used for effective dose.
Effective dose	The quantity obtained by multiplying the equivalent dose to various tissues and organs by a weighting factor appropriate to each and summing the products. Unit sievert, symbol Sv. Frequently abbreviated to dose.
Electrical interaction	A force of repulsion acting between electric charges of like sign and a force of attraction acting between electric charges of unlike sign.
Electric field strength	A measure of the intensity of an electric field. Unit volt per metre, symbol V m-1.
Electromagnetic field	The region in which electromagnetic radiation from a source exerts an influence on another object with or without there being contact between them.
Electromagnetic radiation	Radiation that can be considered as a wave of electric and magnetic energy travelling through a vacuum or a material. Examples are gamma rays, X-rays, ultraviolet radiation, light, infrared radiation and radiofrequency radiation.
Electromagnetic spectrum	All electromagnetic radiations displayed as a continuum in order of increasing frequency or decreasing wavelength.
Electromagnetic wave	See electromagnetic radiation.
Electron	An elementary particle with low mass, 1/1836 that of a proton, and unit negative electric charge. Positively charged electrons, called positrons, also exist. See also beta particle.
Electron volt	Unit of energy employed in radiation physics. Equal to the energy gained by an electron in passing through a potential difference of 1 volt. Symbol eV. 1 eV = 1.6 × 10-19 joule approximately.
Element	A substance with atoms all of the same atomic number.
Emergency reference level	One of a dual set of doses likely to be averted by the introduction of countermeasures to protect the public from ionising radiation after a nuclear or other serious accident.
EMF	Electromagnetic field. Not to be confused with the initials for electromotive force.
Enriched uranium	Uranium in which the content of the isotope uranium-235 has been increased above its natural value of 0.7% by weight.
Equivalent dose	The quantity obtained by multiplying the absorbed dose by a factor to allow for the different effectiveness of the various ionising radiations in causing harm to tissue. Unit sievert, symbol Sv.
ERL	See emergency reference level of dose.
Erythema	Reddening of the skin caused by dilation of blood vessels.
Excitation	A process by which radiation imparts energy to an atom or molecule without causing ionisation. Dissipated as heat in tissue.
Fallout	The transfer of radionuclides produced by nuclear weapons from the atmosphere to earth; the material transferred.
Fast neutrons	Conventionally, neutrons with energies in excess of 0.1 MeV. Corresponding velocity of about 4 × 106 m s-1
Fast reactors	See nuclear reactor.
Fission	Nuclear fission. A process in which a nucleus splits into two or more nuclei and energy is released. Frequently refers to the splitting of a nucleus of uranium-235 into two approximately equal parts by a thermal neutron with emission of other neutrons.
Fission products	Nuclides or radionuclides produced as a result of fission.
Free radical	A grouping of atoms that normally exists in combination with other atoms but can sometimes exist independently. Generally very reactive in a chemical sense.
Frequency	The number of complete cycles of an electromagnetic wave in a second. Unit hertz, symbol Hz. 1 Hz = 1 cycle per second.
Fusion	Thermonuclear fusion. A process in which two or more light nuclei are formed into a heavier nucleus and energy is released.
Gamma ray	A discrete quantity of electromagnetic energy without mass or charge. Emitted by a radionuclide. Cf X-ray.
Geiger-Müller tube	A glass or metal envelope containing a gas at low pressure and two electrodes. Ionising radiation causes discharges, which are registered as electric pulses in a counter. The number of pulses is related to dose.
Genes	The biological units of heredity. They are arranged along the length of chromosomes.
Gray	See absorbed dose.
Half-life	The time taken for the activity of a radionuclide to lose half its value by decay. Symbol t½.
Infrared radiation	Electromagnetic radiation capable of producing the sensation of heat and found between light and radiofrequency radiations in the electromagnetic spectrum. Has subregions IRA, IRB and IRC.
Ion	Electrically charged atom or grouping of atoms.
Ionisation	The process by which a neutral atom or molecule acquires or loses an electric charge. The production of ions.
Ionising radiation	Radiation that produces ionisation in matter. Examples are alpha particles, gamma rays, X-rays and neutrons. When these radiations pass through the tissues of the body, they have sufficient energy to damage DNA.
Irradiance	The power per unit area of optical radiation. Unit watt per square metre, symbol W m-2.
Isotope	Nuclides with the same number of protons but different numbers of neutrons. Not a synonym for nuclide.
Laser	Device which amplifies light and usually produces an extremely narrow intense beam of a single wavelength.
Light	Electromagnetic radiation capable of producing the sensation of vision and found between ultraviolet and infrared radiations in the electromagnetic spectrum.
Magnetic flux density	A measure of the magnetic effect induced in a medium by an external field. Unit tesla, symbol T.
Magnox reactor	A thermal reactor named after the magnesium alloy in which the uranium metal fuel is contained. The moderator is graphite and the coolant is carbon dioxide gas.
Man sievert	See collective effective dose.
Mass number	The number of protons plus neutrons in the nucleus of an atom. Symbol A.
Maximum permissible exposure	The irradiance likely to cause detectable damage to the human eye or skin from exposure to optical radiation. Unit watt per square metre, symbol W m-2.
Moderator	A material used in nuclear reactors to reduce the energy and speed of the neutrons produced as a result of fission.
Molecule	The smallest portion of a substance that can exist by itself and retain the properties of the substance.
Mutation	A chemical change in the DNA in the nucleus of a cell. Mutations in sperm or egg cells or their precursors may lead to inherited effects in children. Mutations in body cells may lead to effects in the individual.
Neutron	An elementary particle with unit atomic mass approximately and no electric charge.
Non-ionising radiation	Radiation that does not produce ionisation in matter. Examples are ultraviolet radiation, light, infrared radiation and radiofrequency radiation. When these radiations pass through the tissues of the body they do not have sufficient energy to damage DNA directly.
Nuclear fuel cycle	The stages in which the fuel for nuclear reactors is first prepared, then used, and later reprocessed for possible use again. Waste management is also considered part of the cycle.
Nuclear medicine	Term usually applied to the use of radionuclides for diagnosing or treating disease in patients.
Nuclear power	Power obtained from the operation of a nuclear reactor.
Nuclear power industry	The industry associated with the production of nuclear power. In the United Kingdom, the preparation of fuel for nuclear reactors, the operation of reactors, the subsequent reprocessing of the fuel, and the disposal of radioactive wastes.
Nuclear reactor	A device in which nuclear fission can be sustained in a self-supporting chain reaction involving neutrons. In thermal reactors, fission is brought about by thermal neutrons.
Nuclear weapon	Explosive device deriving its power from fission or fusion of nuclei or from both.
Nucleus	The core of an atom, occupying little of the volume, containing most of the mass, and bearing positive electric charge.
Nucleus of a cell	The controlling centre of the basic unit of tissue. Contains the important material DNA.
Nuclide	A species of atom characterised by the number of protons and neutrons and, in some cases, by the energy state of the nucleus.
Optical radiation	Electromagnetic radiation comprising ultraviolet, visible and infrared radiations.
Order of magnitude	Quantity given to the nearest power of ten. A factor of ten or so.
Ozone	A form of oxygen gas which occurs naturally in very small quantities in air. Most of the ozone is in the stratosphere where it forms the ozone layer.
Photographic film	Film with emulsion sensitive to ionising radiation. The degree of blackening is related to dose.
Photon	A quantum of electromagnetic radiation.
Positron	See beta particle.
Power density	The power per unit cross sectional area in an electromagnetic field. Unit watt per square metre, symbol W m-2.
Pressurised water reactor	A thermal reactor using water as both a moderator and coolant. Uses enriched uranium oxide fuel.
Probability	The mathematical chance that a given event will occur.
Proton	An elementary particle with unit atomic mass approximately and unit positive electric charge.
PWR	Pressurised water reactor.
Radiation	The process of emitting energy as waves or particles. The energy thus radiated. Frequently used for ionising radiation except when it is necessary to avoid confusion with non-ionising radiation.
Radioactive	Possessing the property of radioactivity.
Radioactive waste	Useless material containing radionuclides. Frequently categorised in the nuclear power industry according to activity (and other criteria) as low level, intermediate level, and high level waste.
Radioactivity	The property of radionuclides of spontaneously emitting ionising radiation.
Radiobiology	The study of the effects of ionising radiation on living things.
Radiofrequency radiation	Electromagnetic radiation used for telecommunications and found in the electromagnetic spectrum at longer wavelengths than infrared radiation.
RF	See radiofrequency radiation.
Radiological protection	The science and practice of limiting the harm to human beings from radiation.
Radionuclide	An unstable nuclide that emits ionising radiation.
Radiotherapy	Term applied to the use of radiation beams for treating disease, usually cancers, in patients.
Reference accident	One of a range of accidents at a nuclear reactor or other nuclear installation that can reasonably be foreseen in safety analysis as giving rise to the most significant release of radionuclides from the site.
Risk	The probability of injury, harm or damage.
Risk factor	The probability of cancer and leukaemia or hereditary damage per unit equivalent dose. Usually refers to fatal malignant diseases and serious hereditary damage. Unit Sv-1.
Scintillation counter	A device containing material that emits light flashes when exposed to ionising radiation. The flashes are converted to electric pulses and counted. The number of pulses is related to dose.
Sievert	See effective dose.
Silicon diode	A device made of a silicon compound in which current flows when exposed to ionising radiation. The current is converted to electrical pulses and counted. The number of pulses is related to dose.
Specific energy absorption rate	The rate at which energy is absorbed by unit mass of tissue in an electromagnetic field. Unit watt per kilogram, symbol W kg-1.
SAR	See specific energy absorption rate.
Thermal neutrons	Neutrons that have been slowed to the degree that they have the same average thermal energy as the atoms or molecules through which they are passing. The average energy of neutrons at ordinary temperatures is about 0.025 eV, corresponding to an average velocity of 2.2 × 103 m s-1.
Thermal reactor	See nuclear reactor.
Thermoluminescent material	Material which, having been irradiated, releases light in proportion to the ionising radiation absorbed when it is subsequently heated.
Ultraviolet radiation	Electromagnetic radiation found between X-rays and light in the electromagnetic spectrum. Has subregions UVA, UVB, UVC.
UVR	See ultraviolet radiation.
Visible radiation	See light.
Waste management	The control of radioactive waste from creation to disposal.
Wavelength	The distance between successive crests of an electromagnetic wave passing through a given material. Unit metre, symbol m.
X-ray	A discrete quantity of electromagnetic energy without mass or charge. Emitted by an X-ray machine. Cf gamma ray.