Radiation Safety GlossaryThis is a featured page



Absorbed dose

Quantity of energy imparted by ionizing radiation to unit mass of matter such as tissue. Unit gray, symbol Gy. 1 Gy = 1 joule per kilogram.
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.
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.
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.

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 ionizing 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).




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.

In relation to radioactive waste, dispersal or emplacement in any medium without the intention of retrieval

Deoxyribonucleic acid. The compound that controls the structure and function of cells and is the material of inheritance.

General term for quantity of ionizing 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 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.


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 з 10-19 joule approximately.
A substance with atoms all of the same atomic number

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 ionizing radiations in causing harm to tissue. Unit sievert, symbol Sv.
A process by which radiation imparts energy to an atom or molecule without causing ionisation. Dissipated as heat in tissue.
FissionNuclear 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 radicalAgrouping of atoms that normally exists in combination with other atoms but can sometimes exist independently. Generally very reactive in a chemical sense.
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.
See absorbed dose.



The time taken for the activity of a radionuclide to lose half its value by decay. Symbol tН
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

Electrically charged atom or grouping of atom
The process by which a neutral atom or molecule acquires or loses an electric charge. The production of ions
Ionizing 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.
Nuclides with the same number of protons but different numbers of neutrons. Not a synonym for nuclide.

Device which amplifies light and usually produces an extremely narrow intense beam of a single wavelength.

Electromagnetic radiation capable of producing the sensation of vision and found between ultraviolet and infrared radiations in the electromagnetic spectrum.



Mass number

The number of protons plus neutrons in the nucleus of an atom. Symbol A.

The smallest portion of a substance that can exist by itself and retain the properties of the substance

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
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

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.
A quantum of electromagnetic radiation.

An elementary particle with unit atomic mass approximately and unit positive electric charge.

The process of emitting energy as waves or particles. The energy thus radiated. Frequently used for ionizing radiation except when it is necessary to avoid confusion with non-ionizing radiation.
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.
The property of radionuclides of spontaneously emitting ionizing radiation
Radiofrequency radiation
Electromagnetic radiation used for telecommunications and found in the electromagnetic spectrum at longer wavelengths than infrared radiation.
An unstable nuclide that emits ionizing radiation.
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.



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.

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 з 103 m s-1.
Ultraviolet radiation

Electromagnetic radiation found between x-rays and light in the electromagnetic spectrum. Has subregions UVA, UVB, UVC.
UV radiation


The distance between successive crests of an electromagnetic wave passing through a given material. Unit metre, symbol m.


A discrete quantity of electromagnetic energy without mass or charge. Emitted by an x-ray machine. See gamma ray.

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