Absorbed Dose

The energy deposited by ionising radiation per unit mass of irradiated material.  The unit is the gray (Gy), and is measured in joules per kilogram.


The number of disintegrations per unit time taking place in a radioactive material.  The unit of activity is the becquerel (Bq), which represents one disintegration per second.

Activity Concentration

The activity per unit volume or per unit mass of a substance (e.g. Bq/l or Bq/kg)

Alpha Particle

A positively-charged particle emitted from the nucleus of an atom during radioactive decay.  It consists of 2 protons and 2 neutrons (a helium-4 nucleus).  Although alpha particles are normally highly energetic, they travel only a few centimetres in air and are stopped by a sheet of paper or the outer layer of dead skin.

Background Radiation

The amount of radiation to which a member of the population is exposed from natural sources, such as terrestrial radiation due to naturally occurring radionuclides in the soil, cosmic radiation originating in outer space, and naturally occurring radionuclides deposited in the human body, through food, drink and breathing.

Becquerel (Bq)

A unit of activity, equal to one radioactive disintegration per second.

Beta Particle

A particle emitted from an atom during radioactive decay.  Beta particles are electrons with either a negative or a positive electric charge.  High energy beta particles may travel metres in air and several millimetres into the human body; low energy beta particles are unable to penetrate the skin.  Most beta rays can be stopped by a small thickness of light material, such as aluminium or plastic sheeting.

Collective Effective Dose

For a given source, the integrated product of effective dose and number of individuals in the population.  The unit is man-Sievert (or man-Sv).

Contamination                                                                                    Back to top

A deposit of dispersed radioactive material on or within any other medium, such as land, sea, air, structures, people, vehicles, food, water etc.

Contamination Meter

An assembly, including one or several radiation detectors and associated subassemblies, designed to measure radioactivity per unit surface area associated with the contamination of the examined object.

Decay (or Radioactive Decay)

The radioactive disintegration of an atomic nucleus resulting in the release of alpha or beta particles, or gamma radiation.


Removal or reduction of radioactive contamination.

Deterministic Health Effects

Health effects of which the probability rises rapidly from zero to unity as the dose increases above some threshold of dose.  The severity also rises with dose above the threshold.


An instrument that measure radiation dose.

Effective Dose                                                                                    Back to top

It is equal to the absorbed dose (in grays) multiplied by the radiation weighting factor which accounts for the harmfulness of different types of radiation (alpha, beta or gamma) and multiplied by the tissue weighting factors which account for the different sensitivities to radiation induced biological effects of different human organs or tissues.  Its unit is Sievert (Sv), or the more commonly used millisievert (mSv).  The effective dose represents the total detriment to an individual resulting from an exposure to ionising radiation.  On average, each individual receive an effective dose of 2.4 mSv every year from natural background radiation.  The effective dose received in a typical chest X-ray examination is about 0.05 mSv.

External Exposure

Exposure due to ionising radiation reaching the body from sources outside the body.

Gamma Radiation

Gamma radiation is short wavelength electromagnetic radiation, of the same physical nature as light, X-rays, radio waves, etc.  However, gamma radiation is highly penetrating (more so than X-rays) and, depending on its energy, may require a considerable thickness of lead or concrete to absorb it.  Since gamma radiation causes ionisation it constitutes a biological hazard.

Health Effects

The effects of an exposure on humans.  These may include deterministic and stochastic effects.

Half-life                                                                                                 Back to top

The time required for a radioactive substance to lose 50% of its activity by decay.  Each radionuclide has a unique half-life.

International Atomic Energy Agency (IAEA)

International Atomic Energy Agency, which came into being on 29 July 1957, is an independent intergovernmental organisation within the United Nations System.  Headquartered in Vienna, Austria, the Agency has more than 100 Member States who work together to carry out the main objectives of IAEA's Statute:  To accelerate and enlarge the contribution of atomic energy to peace, health, and prosperity throughout the world and to ensure so far as it is able that assistance provided by it, or at its request or under its supervision or control, is not used in such a way as to further any military purpose.

International Commission on Radiological Protection (ICRP)

An internationally recognised organisation whose primary function is to make recommendations on the various aspects of radiological protection, based upon which many nations formulate their policy and draft their legislation in relation to exposure to radiation.

Internal Dose

A dose of radiation delivered by radioactive materials that have been ingested, inhaled, or incorporated into the body.

Internal Exposure                                                                                 Back to top

Exposure due to radiation originating from radionuclides taken into the body through ingestion, inhalation, incorporation, or through wounds.

Ionising Radiation

Radiation capable of causing ionisation of the matter through which it passes.  Ionising radiation may damage living tissue.


Atoms of an element having the same number of protons but different number of neutrons in the nuclei.  Different isotopes have the same chemical properties, but somewhat different physical properties.  Isotopes of an element can be classified as radioisotopes (those undergoing decay with the emission of characteristic radiation) or stable isotopes.

Milli-Sievert (mSv)

One-thousandth of a Sievert.


The measurement of radiation or radioactivity, for assessment or control of exposure to radiation or radioactive materials, and the interpretation of such measurements.

Nuclide                                                                                                    Back to top

A species of atom characterized by the constitution of its nucleus, which is specified by its number of protons, number of neutrons and energy content.


A radioactive species of an atom characterized by the constitution of its nucleus.  Sometimes regarded as an alternative to radioisotope.

Stochastic Effects

In the context of radiation protection, radiation induced cancer or genetic effects.  The probability of these health effects, rather than their severity, is a function of radiation dose.  It is assumed that there is no threshold below which stochastic effects do not occur.

Thermoluminescent Dosemeter (TLD)

An integrating detector where radiation energy is absorbed (trapped) and can be read out later by thermal excitation of the detector.  It is used for estimating the total radiation dose received within a period.

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