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. 



    The rad is the old (non-SI) unit for Absorbed Dose, where 100 rad = 1 Gray (Gy). It follows that the rad represents an energy absorption of 0.01 Joules/kg of absorbing medium.


    Radiation is a general term for energy which radiates out from a source and which can be particulate or part of the Electromagnetic spectrum. It is more useful to specify the quality of the radiation, for example, Ionising Radiation or Non-Ionising Radiation.

    Radiation Protection

    Radiation Protection is a general term applied to the profession/science related to protecting man and the environment from Radiation hazards. Strictly speaking, it should represent all forms of radiation (e.g. Ionising and Non-Ionising) but is mostly applied to ionising radiations. See Health Physics for related definition.

    Radiation Protection Adviser (RPA)

    [Ionactive provides Radiation Protection Adviser services]

    A Radiation Protection Adviser (RPA) is a title used in the UK and is given to those who are competent to advise employers on the safe and compliant use of Ionising Radiations. The post is a legally recognised position and is a requirement of the Ionising Radiations Regulations 2017. The RPA needs to be appointed by the employer in writing, where the scope of the advice required is clearly defined. The employer also needs to determine if the RPA is suitable to advise on the types of sources of ionising radiation being used. The RPA is required to show the employer that they are 'competent' to be an RPA, this competence being formally and legally recognised (e.g. by RPA2000).

    Further advice and guidance can be found in our FAQ resources area: What is an RPA (Radiation Protection Adviser)?

    Radiation Protection Practitioner

    A Radiation Protection Practitioner is a general term applied to those who work in radiation protection at a professional level. The most obvious post holder in the UK would be the Radiation Protection Adviser (RPA). Other post holders under this definition might include the Radioactive Waste Adviser and the Qualified Expert (an internationally recognised position). Specialists in specific fields of radiation protection/health physics could also be included under this definition and might include Radiation Shielding Specialist, Dosimetry Specialist etc.

    Radiation Protection Supervisor (RPS)

    The Radiation Protection Supervisor (RPS) is an statutory appointment required by the UK Ionising Radiations Regulations 2017 (IRR17). The RPS is responsible for ensuring that work with ionising radiations is in compliance with the safety systems and procedures described in the local rules made under IRR17. For a more detailed explanation you may wish to read this resource: What is a Radiation Protection Supervisor (RPS)?

    Radiation weighting factor

    The Radiation Weighting Factor is used to modify the Absorbed Dose (Gray) by multiplying to obtain a quantity called the Equivalent Dose (Sv). It is defined by the ICRP and used because some types of radiation, such as Alpha Particles, are more biologically damaging internally than other types such as the Beta Particle. The factor is similar to the Quality Factor determined by the US Nuclear Regulatory Commission (NRC).


    Radioactive can generally describe the property of a substance (or more accurately atomic nuclei) which are unstable and spontaneously Decay (disintegrate) with the release of energy, the energy being either Electromagnetic Radiation, particulate or both. This process may occur in both naturally occurring radioactive material and man-made substances. For any given element there will be a number of Isotopes, some of which may be radioactive. The point at which a substance can be said to be radioactive requires careful interpretation of the law and may depend on particular circumstances. For example, in the UK exemption criteria is used to determine if something is considered radioactive.

    Radioactive Decay

    Radioactive decay describes the process whereby Radioactive substances decay spontaneously with the release of energy in the form of Electromagnetic Radiation or particulate radiation. The rate of radioactive decay will depend on Half-Life.

    Radioactive Waste

    For the purposes of Radiation Protection, radioactive waste can be defined as any Radioactive substances which is no longer required and has no further useful purposes. There are some exact definitions, some which relate to legal meaning. For example, in the Radioactive Substances Act 1993, radioactive waste is defined as '...a substance or article which, if it were not waste, would be radioactive material...' or '...a substance or article which has been contaminated in the course of the production, keeping or use of radioactive material, or by contact with or proximity to other waste..'. See Low-Level Waste (LLW).

    Radioactive Waste Adviser (RWA)

    A Radioactive Waste Adviser (RWA) is an appointed individual in the UK who is a recognised expert in radioactive waste accumulation and disposal. The role is complementary to, but not the same as, the Radiation Protection Adviser (RPA). One individual can take on both roles if they are recognised as competent by RPA 2000. Whereas the RPA advises on radiation safety of employees (occupationally exposed) and members of the public, the RWA advises on the compliant and safe use of radioactive sources, and in particular the accumulation and disposal of radioactive wastes. The role is required as a condition of holding environmental permits under the Environmental Permitting (England and Wales) Regulations 2016, and authorisations under the Environmental Authorisations (Scotland) Regulations 2018 in Scotland. The role of RWA is far-reaching and the duties required for a specific establishment should be specified in a written letter of appointment. Duties will include giving advice on compliance with the environmental legislation, interpretation of exemption or out-of-scope criteria, undertaking BAT assessments, advice on radioactive waste management systems, radioactive waste sampling and assessment, radioactive discharge dose assessments, radioactive source security and much more.


    A radionuclide is a type of Nuclide which is Radioactive and will undergo spontaneous Radioactive Decay.


    Radium consists of 16 isotopes, the most abundant being the Radioactive Radium-226. This is a metallic substance, has a Half-Life of 1602 years and Decays via a complicated chain, eventually leading to stable Lead-206. Along the way it decays to Radon Gas (Rn-222). Radium was isolated from pitchblende in 1898 by Marie and Pierre Curie. The activity of 1g of radium was used to define the activity unit, the Curie (Ci). Radium is difficult to shield needing significant quantities of lead. In addition, radium contaminated dust is a particular inhalation hazard due to its abundant Alpha Particle decay.

    Radon (Gas)

    Radon is a naturally occurring Radioactive gas which is derived from the Uranium and Thorium decay series. Radon (Rn-222) is a colourless, odourless, dense and chemically un-reactive substances and is the daughter of radium within the above-described series. It can be found in houses and workplace, more so where the ground contains Uranium decay series bearing rocks (e.g. granite). Radon is considered a health hazard because it decays to solid daughter products with the emission of Alpha Particles. For example, Rn-222 decays with a Half-Life of 3.8 days to Polonium-218, which itself then decays again (with a smaller half-life of 3 minutes) by an alpha particle to Lead-218. The decay series continues until stable Lead-206 is formed.

    Reasonably Foreseeable

    The term Reasonably Foreseeable is used in a number of areas of Radiation Protection, including Risk Assessment, Safety Cases and Probabilistic Safety Assessments (PSA). Reasonably foreseeable can be taken to mean an incident or accident which is thought to be Credible. It 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 term does not appear to be defined exactly in legislation and there is certainly an interchange in interpretation with 'Credible'.


    The rem is the old unit of Equivalent Dose (or more accurately Dose Equivalent) and is derived by multiplying Absorbed Dose (in Rads) by a Quality Factor. The modern SI unit is the Gray (Gy). 1 Gy = 100rem. See equivalent dose for a explanation of why the absorbed dose is modified to reflect the relative effectiveness of Ionising Radiations in causing biological damage.


    In general terms risk can be defined as the potential for unwanted, adverse consequences to human life, property, health, environment or society. The calculation (or estimation) of risk is usually based on the Probability of the event occurring multiplied by the consequence of the event given that it has occurred. In order to do this a Risk Assessment has to be made which looks at all the hazards, severity and conditional probabilities.

    Risk Assessment

    With respect to Radiation Protection, risk assessment is essentially about assessing risk of radiation exposure in order to mitigate that exposure, ensuring doses are as low as reasonably practicable (ALARP) and certainly below Dose Limits. In the UK risk assessment is a requirement of Regulation 7 of the Ionising Radiations Regulations 1999. A basic risk assessment requires that the radiation hazards are identified, an assessment of what can go wrong to realise those hazards is made, prediction of how likely something will go wrong and produce the hazard, and finally what are the consequences of the radiation exposure. Radiation risk assessments can be simple or complicated depending on the circumstances.


    RPA2000 is a non profit making company set up by The Society for Radiological Protection; The Institute of Physics and Engineering in Medicine; the Institute of Radiation Protection and the Association of University Radiation Protection Officers (the Societies), solely for the purpose of certifying competence in Radiation Protection practice. Certification is intended for members of the Societies however it is also open to non-members practising in the United Kingdom. You may want to visit the RPA 2000 website.

    RPS Training Course

    A formal training course attended by those who will be appointed as a Radiation Protection Supervisor. Generally, a new RPS will attend the course before being appointed and will then receive refresher training every 3-5 years. The course will teach basic radiation physics (dose, activity, x-ray, half-life etc), the principle of ALARP, the external and internal radiation hazard, principles of protection (time, distance and shielding), monitoring and dosimetry, risk assessment, an overview of the Ionising Radiations Regulations 2017 (IRR17) contingency planning and local rules.

    Ionactive provides online RPS training 24/7 on demand, to be taken at a pace of the delegates choosing. Live RPS training courses are also available.

Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less

– Marie Curie -