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. 

    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.

    Safety Case

    Safety Cases are used with in the nuclear industry,in other high risk industries, and where the public are exposed to systematic risks on a daily basis (e.g. railway transport systems). A safety case will set out how operators manage and control the health and safety of employees, members of the public and the environment. The safety case will also detail Contingency plans for dealing with incidents and accidents by assessing all Reasonably Foreseeable adverse events. In essence, the safety case provides a 'safe operating envelope' which considers things like safety policy, Risk Assessment, safety management systems, risk control measures, reliability assessments and contingency.


    With respect to Radiation Protection, scintillation is the process where by a material will emit light Photons when exposed to Ionising Radiation. The light photons can be measured with a photo-multiplier tube which will multiply the events to produce an electrical signal or pulse. The pulse can be counted to give an indication of the magnitude of the incoming radiation. Materials that have scintillation properties include Zinc Sulphide (used in many Alpha emission detectors), Sodium Iodide (used in many X-Rays / Gamma emitter surveying probes), and scintillant cocktails (used in Liquid Scintillation Counting).

    Sealed Source

    With respect to Radiation Protection, a sealed source is a source of Ionising Radiation in the form of Radioactive material which is encapsulated. Sealed radioactive material can not escape and will not cause a Contamination hazard. Sealed sources are used in irradiators (food, products, blood), and thickness & level gauges. The activity of the sealed source can vary from a few Bq to many 10's of TBq. See Closed Source for additional definitions and Unsealed Source for the alternative type of source. Also see HASS sources.

    Shielding (Radiation)

    Shielding is a major protection principle for reducing exposure to Ionising Radiation (the two other related principles being Time and Distance). See Lead for a shielding example.


    The Sievert (Sv) is the SI unit of Equivalent Dose & Effective dose. The equivalent older unit is the Rem where 1Sv = 100 rem.

    Specific Activity

    Specific activity is taken to be the Activity (Bq) per unit mass (grams),normally expressed as Bq/g. Specific activity is related to Half-Life such that the shorter the half life the higher the specific activity. For example, the specific activity of natural Uranium is 25.4 kBq/g and has a half life of 4.51 E9 years (4510 million years). Compare this to Polonium - 210 which has a specific activity of 166500 GBq/g and a half life of 138 days.

    Specified practice

    A specified practice is a term defined in Regulation 7 of the Ionising Radiations Regulations 2017 (IRR17). A specified practice is work with ionising radiation in the UK, which is placed into the highest risk category, and therefore requires a 'consent' from the Health and Safety Executive (HSE). A list of specified practices will include the following:

    (a) the deliberate administration of radioactive substances to persons and, in so far as the radiation protection of persons is concerned, animals for the purpose of medical or veterinary diagnosis, treatment or research

    (b) the exploitation and closure of uranium mines

    (c) the deliberate addition of radioactive substances in the production or manufacture of consumer products or other products, including medicinal products

    (d) the operation of an accelerator (except when operated as part of a practice within sub-paragraph (e) or (f) below and except an electron microscope);

    (e) industrial radiography

    (f) industrial irradiation

    (g) any practice involving a high-activity sealed HASS source (other than one within sub-paragraph (e) or (f) above);

    (h) the operation, decommissioning or closure of any facility for the long-term storage or disposal of radioactive waste (including facilities managing radioactive waste for this purpose) but not any such facility situated on a site licensed under section 1 of the Nuclear Installations Act 1965

    (i) practices discharging significant amounts of radioactive material with airborne or liquid effluent into the environment.


    With respect to Radiation Protection, stochastic effects (also referred to as Probabilistic) represent radiation harm for which there is no threshold (see Linear Dose Response). Even the smallest quantity of Ionising Radiation exposure can be said to have a finite probability of causing an effect, and this effect being either cancer in the individual or genetic damage. Dose Limits are set to ensure that these effects are minimised to broadly acceptable levels. Also see Deterministic Effect.

    Thermal Neutrons

    Thermal neutrons are a class of Neutron which are said to be in 'thermodynamic equilibrium' which means they are moving with the same kinetic energy as their surroundings. At room temperature the average energy of a thermal neutron is around 0.025 eV. Their significance in Radiation Protection is quite different to that of Fast Neutrons.

    Thermo Luminescent Dosimeter (TLD)

    The thermo Luminescent Dosimeter (TLD) is a type of Passive Dosimeter which is used to measure exposure from Ionising Radiation. It is one of a number of methods used in the area of Dosimetry. The TLD consists of a crystal (e.g. CaSO4) which gives off light when heated, the light being proportional to the degree of exposure seen by the TLD. The crystal is usually placed in a holder which contains filters which can be used to differentiate between skin doses and penetrating doses of ionising radiation. The TLD is normally worn on the trunk of the body but can also be worn on the extremities (e.g. for measuring doses to the fingers).


    The thyroid is a butterfly-shaped gland in the neck that secretes thyroid hormones which regulate a number of physiologic processes, including growth, development and metabolism. With respect to Ionising Radiation, the thyroid is vulnerable to intakes of radioactive Iodine, for example I-125, by either Inhalation or Ingestion. In the event that an intake of I-125 occurs the thyroid will be the organ or accumulation and will preferentially uptake the iodine from the systemic system. This creates an Internal Radiation hazard.

    Whilst an intake of radioactive iodine into the thyroid occupationally is undesirable, it is also used in medicine. For example, I-131 can be used in radioiodine therapy for thyroid cancer where somewhere between 1-5 GBq will be used. It can also be used in thyroid diagnostic scans where much less activity is used. In medical use the radiation exposure is justified so that the risk of the exposure is outweighed by the benefits (of treatment and diagnostic information).


    In Radiation Protection 'time' is still considered one of the key principles for protection from External Radiation sources of Ionising Radiation. For a given Dose Rate, the exposure from the source can be minimised by minimising the time spent near the source. Whilst this concept is still valid, and in some circumstances its vital, it is usually easier to comply with the principles of ALARP by using the other related methods such as maximising Distance, using Shielding or using an alternative to ionising radiation.

    Tissue Weighting Factor

    The tissue weighing factor is an ICRP multiplier use to determine the Effective dose from the Equivalent Dose in one or more organs or tissues. The factor takes account of the different sensitivities of different organs and tissues for induction of Probabilistic Effects from exposure to Ionising Radiation (principally induction of cancer).

    Transport Index (TI)

    The transport index (TI) is a special number applied to the labels of Type A and Type B radioactive packages for transport (as specified by the ADR legislation and IAEA - "Regulations for the Safe Transport of Radioactive Material", 2018, SSR-6). The formal way of determining the TI is to measure the dose rate at 1m from the package, (from all sides that are reasonably accessible to obtain the highest value), in mSv/h. This value is then multiplied by 100 and rounded up to the first decimal place. Example: The maximum dose rate at 1m from a package is found to be 0.0127 mSv/h. This is multiplied by 100 to give 1.27, and this is then rounded up to give a TI of 1.3.

    An alternative method is to measure the dose rate in micro Sv/h and then divide this value by 10 and then round up as before. Example. The maximum dose rate at 1m from a package is found to be 12.7 micro Sv/h. This value is then divided by10 to give 1.27, and this is then rounded up to give a TI of 1.3 (as above).

    The TI should be made up of the sum of all radiations present, which in the main will be gamma radiation but could also include neutrons (in the case of an AmBe source or a radionuclide such as Cf-252). Where possible the neutron dose rate should be measured with a neutron monitor, a TI calculated, and then summed with the gamma TI to provide the full transport TI. However, it is often common practice to infer the neutron dose rate from a known ratio of neutron to gamma dose rates (this having being determined by the source supplier or by the user where both gamma and neutron monitors are available periodically).

    Unsealed Source

    With respect to Radiation Protection, an unsealed source is is a source of Ionising Radiation in the form of Radioactive material which is not encapsulated or otherwise contained. The implication is that unsealed radioactive material can move around and if uncontrolled would lead to Contamination. It should be noted that unsealed sources are used extensively in biological research and medicine. See Open Source for a related definition, and Sealed or Closed Source for alternative types. For a more detailed explanation please head over to the following Ionactive technical guidance: What is the difference between sealed and unsealed radioactive source?


    Uranium is a heavy, metallic, naturally Radioactive Element of Atomic Number 92. It has two principle isotopes of uranium-235 and uranium-238 (and very small quantities of Uranium-234). The proportions of natural uranium by weight are about 0.01% Uranium-234, 0.072% Uranium-235, and 99.27% U-238. Uranium-235 is used in the the nuclear industry because it is fissionable by Thermal Neutrons . Natural uranium is extracted from uranium ore where it starts life as a processed uranium oxide (Yellow Cake). Following this the uranium will undergo a process of enrichment where the concentration of U-235 is increased. A byproduct of this process is depleted uranium which can be used for radiation shielding due to its high density.

    Volume Source

    With respect to Radiation Protection, a volume source describes a source of Ionising Radiation which can physically be represented by a solid area. For example a drum containing radioactive waste can be considered a volume source. Sources of ionising radiation are often represented in this way when calculations of estimates of exposures from the source are required. In many cases the volume source will only represent an approximation of the real source, but this is usually adequate if sufficient factors of safety are built into the model. Other useful descriptions of sources include the Point Source, the Planar Source, and the Line Source.

    If you have a volume source the use of the inverse square law will provide unreliable data, since this law only mathematically applies to a point source. The volume source can also self shield, especially if the radioactive material is not uniformly distributed within the volume (this is typical in radioactive waste drums).

    A person undergoing a nuclear medicine procedure (such as a PET scan using radioactive F-18) will also approximate to a walking, talking volume source.

    Whole Body Monitor

    Whole body monitoring is a technique which uses large Scintillation detectors to measure Radionuclide accumulation (natural or artificial). The measurements take place in heavily shielded enclosures. The subject is usually placed on their back and the detectors are brought down onto the surface of the body trunk where counting commences (typically count times might be 1 hour). Whilst the technique can be used for counting many Beta and Gamma emitters, its particularly useful in detecting the low energy x-ray / gamma emissions from Actinides (where other forms of Biological Dosimetry may not be so effective).

    X-ray Fluorescence (XRF)

    If an Element is exposed to X-Rays of a certain energy (or wavelength) it is possible to transfer the x-ray energy to the orbital Electron making up the atoms of the element. The electrons move up an 'energy level' in the process. As these energised electrons fall back to their normal state, energy is released in the form of characteristic discrete x-ray Photons which are unique to the element in question. The process is used in analytical techniques where element identification is required.

    The process is especially useful with materials which are made up of multiple elements (steel for example). XRF can be used to grade the steel by looking for % composition of certain elements such as chromium, molybdenum, and nickel. This can often be achieved with handheld XRF devices which are brought close to the specimen requiring analysis, and provides a near instantaneous results.


    X-rays are part of the Electromagnetic spectrum. They are a penetrating form of electromagnetic radiation and consist of quantum's of energy (Photon). X-rays are commonly produced by the excitation of atomic Electrons, by firing electrons between a high potential difference towards a target (which is the principle of an x-ray machine).

    The target electrons are excited, and as they de-excite x-ray photons are produced. X-rays can also be produced as a result of Bremsstrahlung or by nuclear reactions. X-rays have many uses including medical imaging and industrial quality assurance.

    Whilst commonly stated (or assumed) that gamma rays are emitted by certain radioactive materials, x-rays can also be emitted if the process involves photons being produced within the electron orbit (cloud) around the nucleus (such as in the case of electron capture). An example is the radioactive substance I-125 which emits gamma rays during decay and x-rays from the post decay product(Te-125).

    Yellow Cake

    Processed crude Uranium oxide. This is the form in which most uranium is shipped around the world before processing into metallic metal and other forms.

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 -