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. 


    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.

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