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 International Commission on Radiological Protection (ICRP) is an independent registered charity, established to advance, for the public benefit, the science of radiation protection, in particular by providing recommendations and guidance on all aspects of protection against ionising radiation. Follow this link to the ICRP website.

    Immersion Source

    An immersion source represents a 'cloud source' or similar where the body being exposed (e.g. a person) is vulnerable because they are immersed in the activity. Exposure can occurred from direct radiation (e.g. External Radiation hazard) of by breathing in the radioactive material (Internal Radiation hazard).

    Industrial Radiography

    Industrial radiography is specifically the use of ionising radiation in non destructive testing (NDT). NDT is a process where an article is "tested", and in the case of industrial radiography uses ionising radiation (via a radioactive source, x-ray tube or accelerator) to form an image on radiation sensitive film or real time imaging systems, to detect potential or actual defects. Examples of this would include testing for a crack in a gas pipe, a defective pipe weld, integrity of a pressure vessel or similar. The UK IRR17 defines this exactly as follows 'means the use of ionising radiation for non-destructive testing purposes where an image of the item under test is formed (but excluding any such testing which is carried out in a cabinet which a person cannot enter)' (Reg 2-1). Note here the word "test" (of an item). It follows that Industrial Radiography does not include any of the following:

    • X-ray screening for security purposes (e.g. in cargo and freight etc)
    • X-ray diagnostic imaging or patients (this is a medical exposure - diagnostic radiology).
    • X-ray imaging of a painting, museum specimen or similar.
    • X-ray of animals (e.g. veterinary x-ray) - note this is not diagnostic radiology as that only applies to medical x-rays of humans.

    For a detailed discussion of industrial radiography, check out the following link (December 2023): Potential occupational, non-occupational and accidental radiation exposures in industrial radiography using radioactive sources.


    With respect to Radiation Protection, ingestion describes one possible mode by which Radioactive materials may enter the body and therefore present an Internal Radiation protection hazard. Ingestion may occur where ever loose Contamination exists, either in the work place, where it can be picked up on the hands and transferred to the mouth, or in the environment where foodstuffs are contaminated.


    With respect to Radiation Protection, inhalation describes one possible mode by which Radioactive materials may enter the body and therefore present an Internal Radiation hazard. Inhalation can occur where the radioactive material is airborne, volatile or being processed in such a way as to make the inhalation route possible.


    With respect to Radiation Protection, injection describes a route by which Radioactive materials may enter the body - thus presenting an Internal Radiation hazard. Injection routes can obviously occur where needles are used to handle or administer radioactive materials, but may also be present where any sharp object is contaminated with radioactive materials which then causes a wound.

    Internal Radiation

    The internal radiation (hazard) exists where radioactive materials enter the body. The materials can enter via inhalation, ingestion, absorption or injection. Once in the body they will enter the systemic system where they will then migrate to the organs of accumulation (for example I-125 will go to the thyroid and Ca-45 will migrate to the bone). The actual radiation dose delivered will depend on the radioactive nature of the material (its half-life and principal emitters) and its physical and chemical properties (its particle size and the biological half-life).

    Inverse Square law

    The inverse square law applies to any process which radiates out from a point in space (see Point Source and Fluence Rate to expand on this concept). With respect to Radiation Protection, the law says if you double your distance from a source of Ionising Radiation you will reduce your exposure by 4. It follows that if you triple your distance from the source, the exposure will reduce to 1/9 of the original value. This concept is one of the cornerstones of radiation protection and is used with the Distance Rule. It is not always reliable where the distance between source and measurement position is small compared to the size of the source. Generally, the rule will work mathematically where the distance between the source and the calculation point, is at least 10 times the largest dimension of the source.

    If r is the distance from the source, the dose dose (or dose rate) D is proportional to 1/r2 as shown below.

    \[\text{ D (Dose or Dose rate)}\propto \frac{1}{r^{2}}\]

    This can be expanded to consider two dose rates at two different distances in the following form:


    Here D1 is dose rate at distance r1 and D2 is dose rate at r2 (some greater distance from the source). This can be rearranged in several ways, the example below allows us to find D2 if we known D1 and both distances r1 and r2 .

    \[ D_{2}=D_{1}\frac{(r_{1})^{2}}{(r_{2})^{2}}\]

    The graphic below is from our radiation protection widget resource where you can see this process interactively.

    Inverse square law radiation protection widget

    Try out the inverse square law widget now!

    [Updated March 2024]


    An ion is a charged Atom or molecule. The ion is charged because the number of Electrons do not equal the number of Protons in the atom or molecule. If the atom or molecule contains an excess of electrons it will be negatively charged (e.g. OH-) where as if it is depleted in electrons it will have a net positive charge (e.g. H+).

    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.

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