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. 

    Ion Pair

    Ion pairs are created when an Atom or molecule undergoes the process of Ionisation. They either loose or gain Electrons leading a net positive or negative charge. In simple terms, if water (H2O) undergoes ionisation it will form OH- and H+ ions (an ion pair).


    Ionisation is the process whereby an Atom or molecule gains or loses an Electron and thus becomes an Ion. Ionising Radiation has sufficient energy to be able to ionise atoms and molecules and thus produce ions.

    Ionisation Chamber

    The ionisation chamber is a device designed to measure Ionising Radiation exposure. In simple terms, it consists of a chamber which may either be sealed (containing a gas) or vented to free air. Within the chamber are two electrodes to which a high potential difference (voltage) is applied. If an ionisation event occurs within the chamber it will create an Ion Pair within the fill gas. These ion pairs will migrate to the respective positive and negative electrodes which lead to a reduction in current which, can be detected and measured within external circuitry. Air-filled chambers made of tissue equivalent materials are particularly useful for measuring body radiation exposure (Absorbed Dose). They tend to have a slow response time and are susceptible to changes in climate.

    Ionising Radiation

    Ionising radiation can be described in a number of ways - the simplest being that it consists of Gamma Rays, X-rays, Alpha & Beta particles and other heavy Ions which have sufficient energy to cause ionisation in materials through which they interact . A less useful (but correct) definition can be found in the Ionising Radiations Regulations 2017 which states 'means transfer of energy in the form of particles or electromagnetic waves of a wavelength of 100 nanometers or less or a frequency of 3 E15 hertz or more capable of producing ions directly or indirectly'.

    Ionising Radiations Regulations 2017 (IRR17)

    The Ionising Radiations Regulations 2017 (IRR17) are a UK statutory law regarding work with ionising radiation. They apply to all UK employers (and the self-employed) and detail the requirements for protecting employees who work with ionising radiation, other person who may be affected and members of the public. Ionactive has produced an online guide to the regulations and can be found here: IRR17 Radiation Protection Guide by Ionactive.

    External link to Ionising Radiations Regulations 2017 (IRR17) (2017 No. 1075 on the gov.uk website)


    The process by which an article or body is exposed to a source of Ionising Radiation (either deliberate or by accident). A deliberate type of irradiation uses Gamma Rays or Electron beams to sterilize medical or food products. This type of irradiation is very different to the x-raying of items for security control where the exposures are much lower.


    An isotope represents Atoms of the same Element that have the same number of Protons but a different number of Neutrons. They therefore have different Atomic Masses but the same chemical properties. A radio-isotope is an isotope which is Radioactive.


    The Joule (J) is the SI unit of energy. One Joule is the energy expended when a force of one newton is applied over a displacement of one meter in the direction of the force. The use of the joule is probably limited in Radiation Protection but is used in the definition of Absorbed Dose and the Electron volt.


    Justification is one of the first three principles of the ICRP system of radiological protection. Justification, together with Optimisation and Limitation, are used as the basis of Radiation Protection internationally. Justification is defined in the ICRP 123 publication as 'no practice involving exposures to radiation should be adopted unless it produces sufficient benefit to the exposed individual or to society to offset the Detriment it causes'.


    The term is short for Kinetic Energy Released per Unit Mass (although many refer to the term 'Kinetic Energy Released in Material'). Either term can mean the same thing although by definition the former term is more accurate since the quantity is based on functions of energy and mass. KERMA is a quantity which expresses the initial kinetic energy of all charged Ionising Radiations (e.g. photo-electrons) which are produced as a result of indirectly ionising radiations (e.g. Photons) interaction with material. The quantity is material specific (e.g. air-KERMA or water-KERMA) and has the units J/kg or Gray. KERMA is related to Absorbed Dose but is not the same quantity.


    In general terms, LD-50 is used as an index to describe effectiveness of some entity producing a response in a subject, where that response occurs in 50% of the subjects. In Radiation Protection it is commonly used to describe the value of Absorbed Dose in humans which would lead to Deterministic Effects and therefore radiation sickness (leading to death without medical intervention). The quantity varies depending on the literature but is likely to be between 3 and 5 Gray (Gy) (with some variance related to medical treatments).


    With respect to Radiation Protection, lead together with perhaps concrete, is the most likely shielding material for attenuating X-Rays and Gamma Rays. It has a density approximately 11 times greater than that of water and is easily formed into sheets and interlocking bricks. For example, 4 cm lead will attenuate Co-60 gamma rays to 1/10th of the unshielded value (ignoring geometric and beam effects).


    LET is short for Linear Energy Transfer. This quantity actually reflects the linear rate of energy absorption, by the absorbing medium, as the Ionisation event traverses the medium. In simple terms low LET radiation (e.g. Beta Particles) transfer less energy per unit path length than high LET radiation (e.g. Alpha Particles) to absorbing mediums. LET has most use in radiobiology and has the units of KeV/micron.


    It is generally thought that Leukaemia is a likely form of malignancy which can result from whole body exposure to Ionising Radiation, the likelihood being Probabilistic in nature and increasing linearly with dose without threshold (although there does remain a controversy around the non-threshold hypothesis).


    Limitation is the last of the three principles of the ICRP system of protection. Limitation, together with Justification and Optimisation, are used as the basis of Radiation Protection internationally. Limitation is defined in ICRP 103 as '..effective dose to individuals shall not exceed the dose limits recommended..'. ICRP requires that Deterministic Effects are avoided and that Probabilistic / Stochastic effects are as low as reasonably achievable (ALARA).

    Line Source

    With respect to Ionising Radiation, a line source describes a Radioactive source which can be represented by a line between points A-B in any chosen plane. Calculations of Dose Rates from line sources are more complicated than those from Point Sources and in many cases, direct measurements are going to always be preferable. Such calculations usually involve knowing (or estimating) the Activity per unit length of the source, then calculating the dose rate at a chosen point by integrating contributions from integral points along the line. Commercial programs such as Microshield can undertake this process automatically.

    Linear Attenuation Coefficient

    The Linear Attenuation Coefficient is used in the description and calculation of exponential absorption of Gamma Rays. The quantity normally has the dimensions cm-1 and describes the fraction of gamma rays that are attenuated per unit thickness of absorber.

    Linear Dose Response

    Linear Dose Response in Radiation Protection relates to the zero-threshold model which predicts that every small addition of radiation exposure above background contributes to an increment in the probability of a Probabilistic / Stochastic effect (excess cancer). The response relies on the assumption that even one Photon has the ability to cause an Ionisation event in DNA which may initiate cancer (or other genetic effects). In adopting this model one has to remember that there is no certainty that an ionising radiation event actually leads to biological damage (and thus cancer etc), rather, it it the likelihood of induction which increases. LNT is the basis for Dose Limits, but many argue it exaggerates the risks from ionising radiation.

    Local Rules

    Local Rules are a requirement of the UK Ionising Radiations Regulations 2017 (IRR17) , particularly where you one or more designated areas (Controlled Area or Supervised Areas). Local Rules are the written instructions in order to comply with IRR17 and work safely with ionising radiation. They are closely related to the Radiation Protection Supervisor (RPS) who is a person appointed under IRR17 to ensure the Local Rules are implemented. They will detail RPS contact details, a description of the designated area, the dose investigation level, details of dosimetry or monitoring requirements and other written arrangements to restrict exposure (i.e. ALARP).

    Low Level Waste (LLW)

    Low-Level Waste (LLW) consists mainly of items such as protective clothing, laboratory equipment, packing materials, containers and site equipment which have come into contact with Radioactive material and are no longer useful. In the UK the definition of LLW is waste with a radioactive content not exceeding 4 GBq/tonne of Alpha activity or 12 GBq/tonne of Beta / Gamma activity. See Radioactive Waste for more information.

    Medical Exposure

    Medical exposures relate to planned and deliberate exposure of a patient to Ionising Radiation for medical diagnosis or treatment. E.g. the use of X-Rays by dentists, CT Scans and incorporation of Sealed or Unsealed sources in the body. There are no 'Dose Limits' but there are usually recommended upper levels (e.g. diagnostic reference levels). The ICRP states that only necessary exposures should be made and these should be justified on the basis that the '..medical benefits of the exposure outweigh the radiation risk..' - i.e. a minimum dose that would be consistent with the medical benefit. The technology being medical exposures is advancing all the time. The use of collimated beams from advanced linear accelerators and proton beam irradiation is providing ever better cancer treatment.


    General term for materials used in nuclear reactors to reduce the energy (speed) of Fast Neutrons to that of Thermal (slow) Neutrons. The thermal neutrons are then able to interact and Fission with U-235.


    The neutrino is a particle with no mass or charge. It is emitted during Beta decay during the emission of a beta particle. It has no great significance with respect to Radiation Protection but great interest still remains in its properties.


    The neutron is a constituent of the Nucleus of an Atom and has an Atomic Mass unit of 1 (identical mass to a Proton). Unlike protons, the neutron does not carry an electrical charge. Its electrical neutrality allows it to take part in many types of nuclear reactions because it is not deflected by the positive charge of the protons. Neutrons are not themselves Ionising, but the by-products of their interactions are (in simple terms their interactions take the form of collisions with other sub-atomic particles where energy is transferred).

    Non-Ionising Radiation

    Non-ionising radiation dose not have the ability to Ionise matter it interacts with. Examples include radio waves and microwaves. Non-ionising radiation is not considered in the Ionising Radiations Regulations 2017 (IRR17).


    The nucleus contains positively charged Protons and uncharged Neutrons which are bound by nuclear forces. The nucleus makes up the central portion of an Atom and is surrounded by orbiting Electrons. (This description is much simplified but serves its purposes for most practical Radiation Protection purposes).


    A nuclide describes an Atom who's properties are a function of the number of Protons and Neutrons contained in the Nucleus. A nuclide does not have to be Radioactive - if it is, then its known as a Radionuclide.

    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).

    Passive Dosimeter

    A passive dosimeter is a device used to record Personal Exposure to Radiation (and sometimes Environmental Exposure). Examples are the Film Badge and the Thermo Luminescent Dosimeter (TLD). A newer technique uses a solid state system to create a USB dosimeter. This is still passive but can be read by plugging into a computer. An alternative is to use an Active Dosimeter which provides a real-time instant measure of Dose accrued and Dose Rate. Active dosimetry is the preferred option and is becoming cheaper year by year.

    Passive Radiation Detector

    A passive detector is a device, usually in the form of a Dosimeter, which is used to measure levels of Ionising Radiation exposure. An example includes the Film Badge and the Thermo Luminescent Dosimeter (TLD). The detectors are passive because they need to be 'read' at a later stage in order to ascertain the level of exposure recorded. See Active Detector which can provide real-time information.

    Personal Decontamination

    Personal decontamination requires the removal of Radioactive Contamination from the clothes and skin (including eyes, ears mouth and hair) of a contaminated person. Whilst it is obviously advantageous to remove contamination quickly, it should be done so in such a way that restricts the possibility of radioactive materials being taken into the body (of both contaminated person and helper). For example, when removing contamination from the hair by washing, one should ensure that contaminated water does not run into the eyes or mouth (it might, for instance, be better to cut the hair off or use sticky tape). In addition, care should be taken to avoid corrosive cleaners or abrasive techniques which could break the skin causing discomfort, but also a route of entry into the body.

    Personal Exposure

    Personal Exposure is taken to mean Ionising Radiation exposure which is received by a person (as opposed to many people collectively or the environment)


    The photon is the elementary particle of Electromagnetic energy and represents quanta of energy. For Radiation Protection purposes they are represented by Gamma Rays and X-Ray Photons (Non-Ionising Radiation would, of course, include light Photons). They are taken to have no mass and no electrical charge.

    Planar Source

    With respect to Radiation Protection, a plane source describes a source of Ionising Radiation which can physically be represented by a flat area (e.g. area of contaminated land). Sources are often represented in this way when calculations of estimates of exposures are required. In many cases, the plane 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. Often the radioactivity distribution will be given as Bq per unit area (e.g. Bq/cm2)

    Other useful descriptions of sources include the Point Source, the Volume Source, the Line Source, and the Immersion (cloud) Source.

    Point Source

    A point source describes a source of Ionising Radiation which can physically be represented by a point in space. Whereas this is usually impossible in reality, commonly the point source is used to estimate radiation exposure since the mathematics lends itself well to 'back of envelope' calculations. The point source is related to fluency and mathematically is represented as a sphere surrounding the point source from which photons (or other particles) irradiate outwards. Quite good scoping estimates of exposure rates can be obtained by treating other types of source (e.g. Volume Source) as a point source, as long as the distance between the source and the point of interest is at least 10 times the largest dimension.

    Polonium - 210 (Po-210)

    Polonium 210 (Po-210) is an Alpha Emitter with a Half-Life of around 138 days. Po-210 is all around us in the environment at low concentrations (e.g. water, cigarette smoke, NORM, Radon etc). It is also used in industry, primarily in static elimination devices where the alpha particle creates intense Ionisation which can neutralize positive and negative static charges. There is a move away from radioactive sources - with a preference towards electrically driven static elimination. Since it has a massive specific activity (166500 GBq/g) its alpha emission actually causes it to be hot in gram quantities - this property has been used for power sources in satellites. The death of Alexander Litvinenko from Po-210 poisoning on the 23/11/2006 has raised its profile.


    The positron has properties which are identical to those of a negatively charged Electron, except that it has a positive charge. Positrons are unstable in matter and disappear by Annihilation with negatively charged electrons. This produces two photons, each of 0.511 MeV which move in opposite directions. In simple terms positron emitters (e.g. O-15 and F-18) decay by a Proton changing into a Neutron and releasing a positron. One can say that the Nucleus of the parent has too much energy, but not enough to release an Alpha Particle and so releases a positron instead.

    Probabilistic Effects

    With respect to Radiation Protection, probabilistic effects (also referred to as Stochastic) 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 Effects.

    Probabilistic Safety Assessment (PSA)

    Probabilistic Safety Assessment (PSA) is a reliability assessment method which analyses engineering or management systems where safety is critical. The technique is used to determine the reliability of a component, or system of components, based on the probability of a component or system failure. The technique is used extensively in the nuclear industry and usually forms a major part of Safety Cases. It is used less in the small user (non-nuclear) sector where more conventional Risk Assessments usually suffice.


    Probability can be generally defined as a measure of how likely some event will occur. The event could be an explosion, a lottery win or perhaps cancer induction. Mathematically speaking, the value of probability varies between 0 and 1 where 0 means an event will never occur whilst 1 means the event will definitely occur. The probability of a hazard such as cancer can be multiplied by the severity of that hazard (e.g. in terms of years of life lost, quality of life lost, effect on society) to give the Risk. In Radiation Protection, that risk can be expressed in terms of radiation Dose (by considering the overall Radiation Detriment).


    The proton is one of the basic particles that make up an Atom. The proton is found in the Nucleus and has a positive electrical charge equal to the negative charge of an Electron. It has a mass similar to that of a Neutron. The number of protons in an atom defines the Atomic Number and hence the type of Element present.

    Quality Factor

    The Quality Factor is used to modify the Absorbed Dose in Gray (Gy) by multiplying to obtain a quantity called the Equivalent Dose (Sv). It is 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 determined by the US Nuclear Regulatory Commission (NRC) and is the same basic quantity as the Radiation weighting factor as defined by the ICRP.


    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)?

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