Glossary of terms

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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.
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.
Alpha particle: A particle consisting of two protons plus two neutrons. Emitted by a radionuclide. More detailed information is available on the alpha particle web page.
Anthropogenic: As an adjective - caused by humans. Anthropogenic radiation is radiation caused by human activity.
Atom: The smallest portion of an element that can combine chemically with other atoms.
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.
Auger Electron: An electron, sometimes emitted from an atom instead of an x-ray that would otherwise carry away the excess energy. Auger electrons are usually of no significance as a radiation hazard but can contribute to radiation dose in the decay of some radionuclides while inside cells in the body, as in radiopharmaceuticals.
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. More detailed information is available on the beta particle web page.
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.
Cosmic rays: High energy ionising radiations from outer space. Complex composition at the surface of the earth.
Cosmogenic radionuclides: The cosmic radiation which strikes the earth induces radioactivity in the atmosphere. Most of this radioactivity is very short-lived. Some radionuclides however survive to eventually reach the surface of the earth. Among these are H (tritium), Be (beryllium-7) and C (carbon-14) which has the longest half-life (5730 years).
Decay: The process of spontaneous transformation of a radionuclide. The decrease in the activity of a radioactive substance.
Decay Mode: The type of decay a nuclide undergoes.
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.
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.
Electric field: Force field created between electrical charges. The electric field is measured in units of volt per metre, symbol Vm-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. More detailed information is available on the extremely low frequency electric and magnetic fields web page.
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.
EMF: Electromagnetic field. Not to be confused with the initials for electromotive force.
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.
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.
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.
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.
Gamma ray: A discrete quantity of electromagnetic energy without mass or charge. Emitted by a radionuclide. See x-ray. More detailed information is available on the gamma ray web page.
Gray: See absorbed dose.
Half-life: The time taken for the activity of a radionuclide to lose half its value by decay. Symbol t_½.
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.
Instability: Having the property of being unstable.
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.
Isotope: Nuclides with the same number of protons but different numbers of neutrons. Not a synonym for nuclide.
keV: Is equal to 1000 eV. See Electron volt .
Laser: Device which amplifies light and usually produces an extremely narrow intense beam of a single wavelength. More detailed information is available on the laser web page.
Light: Electromagnetic radiation capable of producing the sensation of vision and found between ultraviolet and infrared radiations in the electromagnetic spectrum.
Magnetic field: Force field created between moving electrical charges. The magnetic field is measured in units of amperes per meter, symbol Am-1.
Mass number: The number of protons plus neutrons in the nucleus of an atom. Symbol A.
Metastable: Nuclei that stay in an excited, unstable state for longer than about 10-10 seconds which is nevertheless an extremely short period of time. (i.e. 10-10 seconds equal to about 100 billionth of a second).
Molecule: The smallest portion of a substance that can exist by itself and retain the properties of the substance.
Morbidity: The presence of disease or medical condition. Morbidity rate is the number of new cases suffering from a particular disease or medical condition over the total population for a specific period of time.
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 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.
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.
Photon: A quantum of electromagnetic radiation.
Positron: See beta particle.
Probability: The mathematical chance that a given event will occur.
Proton: An elementary particle with unit atomic mass approximately and unit positive electric charge.
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. More detailed information is available on the radioactivity web page.
Radiofrequency radiation: Electromagnetic radiation used for telecommunications and found in the electromagnetic spectrum at longer wavelengths than infrared radiation. More detailed information is available on the radiofrequency radiation web page.
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.
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.
Sievert: See effective 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.
Stable: An isotope or nuclide is considered stable if it has a half-life longer than the age of the universe i.e. a half-life longer than about 13.7 billion years.
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.
Ultraviolet radiation: Electromagnetic radiation found between X-rays and light in the electromagnetic spectrum. Has subregions UVA, UVB, UVC. More detailed information is available on the ultraviolet radiation web page.
UV radiation: See ultraviolet radiation.
Unstable: An isotope or nuclide is considered to be unstable if it has a half-life less than the age of the universe i.e. a half-life less than about 13.7 billion years.
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. See gamma ray. More detailed information is available on the X-ray web page.; Top of Page