Radiation Protection Glossary

A radiation protection glossary for Radiation Protection Supervisors (RPS), Radiation Protection Advisers (RPA) and anyone else interesting in radiation safety terms and definitions. The glossary is a mixture of health physics , phrases related to radiation protection legislation, transport, practical safety, technical terms and similar.

Search the Glossary by either clicking on a letter or typing a keyword into the search box. This glossary is relational so when looking at one term you can click through to other related terms as required.

For formal advice, see our Radiation Protection Adviser pages. 


    Cherenkov Radiation

    Cherenkov radiation is Electromagnetic (non-ionising) radiation emitted when a charged particle (e.g. an Electron) passes through a medium (e.g. water) at a speed greater than that of light in the medium. This can be seen as the characteristic "blue glow" where sources of large activity (e.g. irradiator sources or spent nuclear fuel) are stored under water.

    Chronic Exposure

    Exposure to sources of Ionising Radiation over a long period of time, possibly resulting in adverse health effects such as cancer or genetic disorders in offspring of exposed parents. Likely result of a Probabilistic / Stochastic effect of ionising radiation.

    Classified person

    A classified person is a designation given in the UK Ionising Radiations Regulations 2017 (IRR17). A classified person is someone who could be exposed to ionising radiation, through occupational exposure including reasonably foreseeable incidents, who could receive more than the following exposures: 6mSv/year whole body effective dose, 150 mSv/year equivalent dose to the extremities, and 15mSv/year to the lens of the eye. It also specifically applies to an employee who works with a source of ionising radiation where a dose rate could deliver a whole body exposure of 20mSv (or 500 mSv to the extremities or 20 mSv to the lens of the eye) within a few minutes from a reasonably foreseeable event.

    Closed Source

    With respect to Radiation Protection, a closed source is is a source of Ionising Radiation in the form of Radioactive material which is encapsulated or otherwise contained. The aim is that closed radioactive material can not escape and will not cause a Contamination hazard. Closed sources have many applications including use in irradiators (food and products), medical blood irradiators and density gauges. Whilst the term 'Closed Source' is comparable with 'Sealed Source' , it is defined in a particular way in some of the UK legislation.

    Collective Dose

    More accurately known as Collective Effective dose. This quantity is derived from summing the individual effective doses within an exposed population (or workforce). One type of unit to express this quantity is the man Sv. This quantity has been used to assess overall detriment and therefore as an aid to decision making techniques in optimising radiation protection (e.g. Risk Assessment). It is less well used nowadays where dose constraints are preferred instead .

    Consumer Products

    Any household product that contains a quantity of radioactive material yielding Ionising Radiation for reasons of functionality of that item. Examples include ionisation smoke detectors (e.g. Am-241) and luminising items such as watches and clocks which may contain, for example, radium. Consumer products from by-gone-days are a potentially significant radiation hazard if not carefully stored and protected from damage.


    Usually an undesirable situation where radioactive material in an Unsealed Source (open source) state is present in the working environment, or otherwise non-contained. Contamination can either be loose (easily removed) or fixed. Loose contamination is usually of more concern since intakes of radioactive material through Inhalation, Ingestion and Injection may occur.


    With respect to Radiation Protection, contingency means preparing for, and taking action, in the event of an unplanned release of Radioactive material or other unplanned Radiation incident which could lead to radiation exposure to the individual, the population or environment. Contingency may be determined by simple risk assessment or by a more comprehensive Probabilistic Safety Assessment (PSA) as part of a Safety Case. Usually contingency arrangements deal with Reasonably Foreseeable (credible) events, although for some industries (e.g. nuclear), the contingency plans have to be extendable.

    Controlled Area

    Controlled Area is defined in the Ionising Radiations Regulations 2017 (IRR17). A Controlled Area is an area where any person is likely to receive more than 6 mSv Effective dose (greater than 15mSv to the lens of the eye, or 3/10 of any other UK Dose Limit) and / or an area where specific and detailed procedures need to be followed in order to restrict exposure from Ionising Radiation and ensure that doses are ALARP. The area can also be designated on the basis of dose rate, such that a Controlled Area will generally be required where the dose rate exceeds 7.5 micro Sv/h when averaged over a working day. Conversely, an area may not need to be designated as Controlled where the dose rate is less than 7.5 micro Sv/h averaged over the working day, and where the instantaneous dose rate (IDR) does not exceed 100 micro Sv/h. However, where high IDR dose rates arise careful consideration of the ALARP concept is required.

    The above concise definition avoids over complicating this glossary entry, a more detailed explanation can be found here: IRR17 (17) - Designation of controlled or supervised areas . There are a number of important caveats and conditions and the reader is encouraged to visit this additional resource.

    Cosmic Rays

    Radiation originating from outside the Earth's atmosphere. The term 'cosmic ray' can actually include a number of classes of high energy radiation including Gamma Rays, Electrons and Ions.


    The term 'credible' is used in a number of areas of Radiation Protection, including Risk Assessments, Safety Cases and Probabilistic Safety Assessments. Credible can be taken to mean an incident or accident which is thought to be Reasonably Foreseeable. Credible can be expressed numerically and this value will differ depending on the situation being assessed (but perhaps in the range of 10-5 to 10-6).


    The Curie (Ci) is the traditional unit of Activity (where its SI equivalent in the Becquerel). 1 Ci is equivalent to 3.7 E10 disintegrations per second (dps) and since 1Bq=1dps it follows that 1Ci = 3.7 E10 Bq or 37GBq (approximately). The Ci was based on the activity found in 1 g of Radium.

Be less curious about people and more curious about ideas

– Marie Curie -