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. 


    A general term applied to Radioactive Wastes which require disposal, for which there is no intention of recovery. In reality there may be circumstances where some capability for recovery is maintained (e.g. ILW wastes at UK nuclear power stations).


    Distance, in the context of Radiation Protection, relates to one of the three key principles of protection against External Radiation hazards (i.e. Time, Distance & Shielding). In simple terms, increasing the distance between a static source of Ionising Radiation and the absorbing medium (e.g. a person) will reduce the exposure to that person. For certain defined geometries, (e.g. Point Source), the Inverse Square law can be applied which can be stated as: 'double the distance, quarter the dose'.


    The full name for DNA is Deoxyribonucleic Acid. DNA is a substance found in the nucleus of living cells and is used to encode genetic information. Its use is to determine the structure, function and behaviour of all cells in a living entity. It is relevant to Radiation Protection since it is at the DNA level that concern is raised by the effects of Ionising Radiation, which may lead to cancer induction or genetic damage.


    'Dose' is a general term applied to the quantity of Ionising Radiation received by an exposed body (person, part of a person or object). Some degree of care is required when using the word 'dose' since it can mean a number of different quantities. Furthermore it is more often useful to express the dose in units which give the magnitude of damage sustained or perhaps the risk of cancer induction at some time after exposure. It is usually more useful to prefix (or postfix) terms which identify the quantity under consideration, e.g. Absorbed Dose, Effective dose, Dose Equivalent. At a fundamental level, 'Dose' may be best described as 'the amount of radiation absorbed per unit mass of material with which it interacts with'.

    Dose Equivalent

    Dose Equivalent (some refer to this as Equivalent Dose) is a quantity which takes into account 'radiation quality' which relates to the degree in which a type of Ionising Radiation will produce biological damage. The Dose Equivalent is obtained by multiplying the Absorbed Dose by a Quality Factor. The resulting quantity can then be expressed numerically in Rem (old units) or more commonly Sieverts (Sv). It is worth emphasising that the quantity is independent of the absorbing material (i.e. tissue, water, air).

    Dose Limits

    With respect to Radiation Protection, dose limits (an accumulated dose) are recommended by international bodies such as the ICRP, and then set as legal limits within individual counties legislation. For example, in the UK Dose Limits to employees, members of the public, pregnant employees etc are set out in the Ionising Radiations Regulations 2017 (takes you to our guide). Whilst dose limits set a legal maximum, the practice of radiation protection requires that all doses are kept as low as reasonably practical (ALARP) as required by the ICRP concepts of Optimisation and Limitation.

    Dose Rate

    Dose rate is a term applied for the rate at which Ionising Radiation is being absorbed by a medium (e.g. tissue). Whilst it is accurate to apply a prefix or postfix (e.g. 'absorbed dose rate'), the term is probably most often applied to Effective dose, which is a useful quality in expressing harm or overall risk of harm from whole body irradiation.


    A general term applied to devices designed to record Personal Exposure, as apposed to Environmental Exposure. The devices may be in the form of a Passive Dosimeter or an Active Dosimeter. Passive dosimeters will include Film Badges and TLD, whilst active dosimeters will include EPDs. The dosimeter is the standard way of measuring personal exposure for the purposes of complying with statutory Dose Limits.


    Dosimetry is a general term applied to the practice of measuring radiation exposure. Dosimetry has a wide scope in Radiation Protection, see Dosimeter, Passive Dosimetry, Active Dosimetry and Biological Dosimetry for more specific information.

    Effective dose

    The Effective Dose is obtained by taking the Equivalent Dose (Dose Equivalent) and multiplying by a Tissue Weighting Factor which relates to the organs / tissues under consideration. Effective Dose can therefore also be considered a doubly weighted Absorbed Dose since it takes into account the type of radiation (radiation weighting factor) and the target organ / tissue. The quantity can be used to express Detriment to the whole body as a summation of several different doses of radiation with varying radiation weighting factors (radiation type) and targets.

    Electromagnetic spectrum

    The electromagnetic spectrum covers a wide range of wavelengths and Photon energies. It ranges from Gamma Rays at one end (High Frequency, High Energy and Low Wave Length) to radio waves at the other (Low Frequency, Low Energy and Long Wave Length). For Ionising Radiation protection purposes we are concerned with X-Rays and Gamma Rays.


    The electron is a low mass particle (1/1836 that of a Proton) with a unit negative electric charge. In simple terms the electrons are said to orbit around the Nucleus of Atoms. Positively charged electrons can also exist, these being known as Positrons. The electron is closely related (identical in fact) to the Beta Particle.

    Electron volt

    The electron volt (eV) is a unit used in Radiation Protection / Health Physics to describe the energy of Ionising Radiation. The value of the eV is derived from the energy required to accelerate an electron through a potential of 1 volt. In more familiar units the eV is approximately equivalent to 1.6 E-19 joules. In everyday use the units of KeV (or MeV) are used as the eV is obviously an extremely small quantity. In training we often say the eV represents the 'punch' that the ionising radiation has to do work (damage) and so can be related to hazard potential.


    An element represents the simplest form of a chemical where all the Atoms share the same Atomic Number. This will include, for example, Hydrogen, Oxygen and Carbon.

    Enriched uranium

    Uranium where the content U-235 is increased above its natural value of around 0.7% by weight. The enrichment process will also yield Depleted Uranium. Enriched Uranium is Fissile and can undergo nuclear fission under certain conditions, it's therefore used in Nuclear Power production and Nuclear Weapons.

    Environmental Decontamination

    With respect to Radiation Protection, environmental decontamination refers to the systematic clean-up of Radioactive Contamination within the workplace or wider environment. Also see Personal Decontamination.

    Environmental Exposure

    With respect to Radiation Protection, environmental exposure refers to Ionising Radiation exposure within the workplace or wider environment. Also see Personal Exposure.


    With respect to Radiation Protection, EPD stands for 'Electronic Personal Dosimeter'. A number of types of device can function as an EPD which is a type of Active Dosimeter. It is designed to provide real time information on Dose and Dose Rate.

    Equivalent Dose

    Equivalent Dose (can be referred to as Dose Equivalent) is a quantity which takes into effect 'radiation quality', which relates to the degree to which a type of Ionising Radiation will produce biological damage. Equivalent Dose is obtained by multiplying the Absorbed Dose by a Radiation Weighting Factor or Quality Factor if Dose Equivalent is used . The resulting quantity can then be expressed numerically in Sieverts (Sv) or in the old units of Rem. The quantity is independent of the absorbing material (i.e. tissue).


    Erythema presents itself as a reddening of the skin which is caused by blood vessel dilation. It is a common sign of Deterministic Radiation Effects, particularly from high energy Beta emitters or X-Rays.

    External Radiation

    The external radiation (hazard) exists where an absorber (typically a person) is being exposed to a source of Ionising Radiation external to the body. Examples would include exposures to Sealed Sources, dental X-Rays and Cosmic Rays. One feature of this hazard is that moving the absorber away from the source (usually) results in a reduction in the Dose of ionising radiation to that absorber, which is not the case for Internal Radiation hazards.


    Fallout is a general term applied to Radioactive materials, produced by detonation of a nuclear device in the atmosphere, which fall back to earth. Fallout can sometimes be thought of as a secondary (radiation / contamination) hazard from a nuclear weapon once the explosion, prompt Gamma and Neutron radiation has passed. Where as the initial radiation doses delivered will be fixed by the physical distance between the explosion and those exposed, fallout can be carried great distances by atmospheric conditions and thus deliver significant radiation doses to those not effected by the initial explosion. Recently, and particularly in the light of the Chernobyl accident, fallout can also be applied to any nuclear accident (event) which results in atmospheric dispersion of radioactive material.

    Fast Neutrons

    Neutrons which have been ejected from a fissioning Nucleus. (Also see Thermal Neutrons).

    Film Badge

    The Film Badge is a type of Passive Dosimeter. It consists of a photographic emulsion film, which responds to Ionising Radiation by changing optical density, in a light tight wrapping (contained in a holder). The holder, which also contains various filters (for measuring tissue equivalent quantities), is usually worn on the trunk of the body.


    (Nuclear) Fission is the process where a heavy nuclei (e.g. U-235) decays by splitting into two equal fragments (fission fragments). This process proceeds with the emission of Neutrons and Gamma Rays, the neutrons being available to initiated further fissions and thus a nuclear chain reaction. Some nuclides such as Cf-252 can undergo spontaneous fission, although fission is induced in other nuclides such as U-235 by incoming neutrons. The fission fragments can undergo further Radioactive Decay producing a host of fission products.

    Fluence Rate

    Fluence can be defined as the total number of particles (typically Gamma Ray Photons) crossing over a sphere of unit cross section which surrounds a Point Source of Ionising Radiation. The Fluence rate is the number of particles crossing per unit time (which is numerically equal to the product of number of particles and their average speed). This is a useful quantity in Radiation Protection when calculating Dose Rates from point sources or other geometries (e.g. Line Sources, plane source or Volume Sources). The dose delivered by a fluence at a point in space will be related to the energy of the photons.

    Free Radical

    Free radicals can be formed in biological materials (e.g. DNA) when they undergo Ionisation (by interaction with Ionising Radiation). The free radical can be thought of as a reactive charged molecule which will readily combine with other cell constituents. A typical example is the ionisation of water which will produce H+ and OH- ions which can further react to produce Hydrogen Peroxide, which is highly oxidising and potentially very damaging to DNA.


    A process in which two or more light nuclei are formed into a heavier Nucleus releasing large amounts of energy. This process can be achieved by using extremely high temperature plasma and some believe this will eventually lead to a new readily available source of energy.

    Gamma Rays

    Gamma Rays are a type of high energy radiation the form of Photons which have no mass. They are part of the electromagnetic spectrum. In Radioactive Decay they originate from changes in the structure and energy levels of the Atomic Nucleus, or, through electron-positron annihilation or by nuclear fission. Gamma rays travel greater distances than either Alpha Particles or Beta Particles and are much more difficult to shield. Whilst their mode of formation is different, they are identical to X-rays.

    Gamma-ray Constant (Specific)

    The (specific) Gamma-ray Constant is a useful numerical quantity which can be used to predict exposure in terms of Equivalent Dose per unit activity per unit distance for gamma emitters. As the term is a constant, exposure (in terms of dose rate) will vary proportionally with distance and activity values, making approximate calculations (especially from Point Sources) quite easy.

    For example, the gamma-ray constant for Caesium-137 is 76 micro Sv/h per GBq at 1m from an unshielded point source. From this we can see that:

    • the dose rate will be 152 micro Sv/h at 1m from a 2GBq Cs-137 source.
    • the dose rate will be 19 micro Sv/h at 2m from a 1GBq Cs-137 source (using the inverse square law).

    It is important to note that the gamma-ray constant is specific to a particular radionuclide.

    Geiger counter

    A Geiger Counter (G-M Counter) is a type of detector used to measure levels of Radiation or Contamination. The counter is relatively simple to make and is quite robust so it is used regularly in the field to take quick measurements. Due to its mode of operation, the output of the Geiger Tube is independent of the incident energy or the incoming Ionisation event. Thus it is strictly a 'counter' rather than an energy spectrometer (but within specific circumstances energy compensation can be implemented). Whilst the geiger counter will respond to Gamma Rays they are particularly suited to medium and high energy Beta Particles (e.g. C-14, S-35, P-32) and alpha emitters if fitted with a think window (e.g. Po-210, Am-241). For more information check out this Ionactive resource: The Geiger-Muller tube - radiation detector (video).

    Genetic effects

    Genetics effects (with respect to Radiation Protection) are those effects present in the offspring of those exposed to Probabilistic / Stochastic levels of Ionising Radiation.

    Glove Box

    The glove box is a type of enclosure used to completely contain Radioactive materials, thus separating them from the operator who needs to manipulate them. They are normally constructed from either a fibre glass material or stainless steel and incorporate extraction systems to maintain a negative pressure relative to the general working environment. They are particularly useful for Alpha emitting radioactive materials which can otherwise present a significant internal hazard if inhaled. If high energy Gamma emitters are also present the box may be shielded by lead sheet or bricks (with leaded glass viewing windows).


    The Gray (Gy) is the SI unit of Absorbed Dose. 1Gy is equivalent to an energy of 1 Joule / Kg of absorbing medium. 1 Gy is also equal to 100 Rads (the rad being the older unit of absorbed dose, still used in the US).


    Half-life is closely related to the property of Radioactive Decay and represents the time taken for half the Atoms in a Radioactive substances to undergo decay and change into another nuclear form (either a radioactive daughter product or a stable form). It is therefore the time taken for the Activity of a radioactive sample to decay by half and is commonly given the symbol t1/2.

    Half-Life (Biological)

    The biological half-life is the time taken for half of a radioactive material, (present in a body as a result of inhalation, ingestion, injection or absorption), to be eliminated by the biological processes in that body. For example, Tritium contaminated ingested water will tend to clear quickly (quicker still if the individual drinks plenty of water), whereas Ca-45 will tend to bind to bone. The solubility of the radioactive substance will also effect retention time in the body.

    HASS Sources

    HASS sources (High Activity Sealed source) require significant control measures due to the significant radiation hazards that may exist should the source become uncontrolled. The aim is to ensure that proper controls are in place throughout the entire life cycle of the source from production, purchase, use, storage and eventual disposal. These controls are designed to ensure that sources cannot be lost, that the threat of theft is minimised, and that sources can not be purchased unless there is a disposal route and resources available to affect that disposal when required.

    Note that a HASS source ceases to become a HASS source when the radioactivity is below the HASS threshold. Examples of HASS source activity are as follows:

    • Am-241 (60 GBq)
    • Cs-137 (100 GBq)
    • Cf-252 (20 GBq)
    • Co-60 (30 GBq)
    • Ir-192 (80 GBq)
    • Ra-226 (40 GBq)
    • Se-75 (200 GBq)

    The units above are in becquerels (i.e. 10^9 Bq).

    Health Physics

    A term used for the practice of Radiological Protection. A practitioner may be known as a Health Physicist or perhaps in the UK a Radiation Protection Adviser (RPA).

    Hereditary Effects

    Hereditary effects (with respect to Radiation Protection) are those effects present in the offspring of those exposed to Probabilistic / Stochastic levels of ionising radiation. See Genetic Effects.

    Hot (Source)

    A common 'slang' term used in Health Physics and Radiation Protection to mean something with high levels of radioactivity or Dose Rate. It has no technically recognised meaning and should be used with care since the word can be used subjectively. UK universities commonly use the term 'Hot Lab' to mean a laboratory which contains higher than normal levels of radiation or radioactive material (these labs generally have low levels of activity compared to industry).

    HVT - Half Value Thickness

    Simply stated, the HVT is the thickness of a radiation shield that will reduce radiation gamma / x-ray dose rate (or dose) to 1/2 of the of the pre-shielded value. There are a number of factors that will potentially interfere with this approach, but HVT is still a good approximation in many cases.

    HVT is related to TVT by the expression 3.32 HVT = 1 TVT. Head over to the TVT resource for a fuller explanation of this concept.


    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+).

Be less curious about people and more curious about ideas

– Marie Curie -