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. 


    Occupational Exposure

    With respect to Radiation Protection, occupational exposure is an exposure which occurs during work with sources of Ionising Radiation. For example, exposures received from working on a nuclear reactor, in nuclear reprocessing or by a dental nurse taking X-Rays would be classed as Occupational. This is distinct from Medical Exposures which are for the benefit of the patient.

    Open Source

    With respect to Radiation Protection, an open source is a source of Ionising Radiation in the form of Radioactive material which is not encapsulated or otherwise contained. The possibility is that open radioactive material can move around and if uncontrolled would lead to Contamination. If not controlled, this could lead to an intake of radioactive material by inhalation or ingestion. It should be noted that open sources are used extensively in biological research and medicine. See also Unsealed Source.


    Optimisation can be stated as '..a process or method used to make a system of protection as effective as possible within the given criteria and constraints..'. With respect to Radiation Protection, the ICRP 103 publication states '.consider how best to use resources in reducing radiation risks to individuals and populations ... so far as is reasonably achievable, social and economic factors being taken into account'. This is the basis of the ALARA principle, and the UK ALARP principle (but ALARP does not consider social and economic factors and is developed from Case Law)). Radiation protection uses optimisation to reduce exposures below Dose Limits.

    Outside worker

    An ‘outside worker’ is term used in the UK Ionising Radiations Regulations 2017 (IRR17).

    What is an outside worker?

    You will be an outside worker under IRR17 when both of the following apply:

    (A) You are a designated classified person or a non-classified person, working for an employer or you are self-employed, and

    (B) You are carrying out work (services) in a controlled area under the control of another employer.

    Under IRR17 you do not need to be a classified person to be an outside worker (which was the case under the previous IRR99 regulations).

    Example (outside worker)

    You are a service engineer and you are required to calibrate an x-ray source located in a controlled area of a customer site. Regardless of your classified person status, you will be an outside worker and you, your employer and the employer in charge of the controlled area will have specific duties under IRR17.

    Examples (where you are not an outside worker)

    You will not be an outside worker when one or more of the following apply:

    (A) You enter another employers controlled area only as a visitor (e.g. supervised entry and no practical work takes place)

    (B) You are employed by both employers

    (C) Where the controlled area is handed over to you (you take charge) so that you are now working in your own employer’s controlled area.

    (D) Where you go on to another employer’s site and set up a temporary controlled area

    In the above examples: (C) could be an engineer who takes charge of an x-ray room to undertake repair work, and (D) could be a NDT company setting up a temporary controlled area for site radiography.

    Over Exposure

    With respect to Radiation Protection, an overexposure means a person who has received an unexpected (non-routine) level of Ionising Radiation exposure above some permitted level (Dose Limits). The overexposure may result in a breach of regulations, but in severe cases adverse health effects or even death (see Deterministic Effects).

Be less curious about people and more curious about ideas

– Marie Curie -