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. 

    10th Value Thickness (TVT)

    10th value thickness (TVT), sometimes known as 10th value layer (TVL) is used in simple radiation shielding calculations. Sometimes TVT is all you need, whereas more complex shielding problems may need computer codes (e.g. MCNP) to optimise the shielding.

    Simply stated, the TVT is the thickness of a radiation shield that will reduce radiation gamma / x-ray dose rate (or dose) to 1/10 of the of the pre-shielded value. There are a number of factors that will potentially interfere with this approach, but TVT is still a good approximation in many cases. In order to use a TVT you need to know the following:

    • For x-ray (photon) beams you need to know the energy (e.g. kV / MV) and the shielding material of choice (e.g. lead, concrete of specified density, steel etc).
    • For radioactive materials you need to know the radioactive material for consideration (e.g. Cs-137, Co-60, F-18) and the shielding material of choice.

    For either of the above you need a reliable data source - we will not reference them here, but some are available elsewhere on our site. Ionactive also has a soon to be released radiation protection calculator which will feature TVT values we have quality assured. Note that the TVT does NOT apply to alpha or beta radiation or neutrons (although a similar process can be used with neutrons in certain cases).

    Examples of TVT are as follows:

    • TVT for lead with Cs-137 is 22 mm
    • TVT for lead for a positron emitter (e.g. F-18) is 17 mm
    • TVT for 100 kV x-rays could be 5.1 cm of 2.35 density concrete or 0.8 mm of lead

    TVT works in the following way. Add the TVT thickness, whilst multiplying the attenuation. Let's use the Cs-137 data as an example.

    Cs-137 and lead example for TVT

    You have a Cs-137 source with a dose rate of 1000 micro Sv/h at 1m. Reduce the dose rate at 1m to 1 micro Sv/h using lead.

    In order to reduce dose rate from 1000 micro Sv/h to 1 micro Sv/h requires 3 TVT (TVT+TVT+TVT). This means an attenuation of:


    So this requires 22mm lead (TVT) + 22mm lead (TVT) + 22mm Lead (TVT) = 66mm lead.

    This is somewhat simplified but provides the general idea. Another concept is HVT (half value thickness) which is self explanatory. Note that 3.32 HVT = 1 TVT.

    We have not explained absolute attenuation or the means of calculating fractions of a TVT or HVT. This will feature in a future Ionactive article.

    More TVT information resource is available elsewhere on our site. For example: How do I convert TVT (10 value thickness) values to attenuation for Gamma or X-ray sources of radiation?

    Absorbed Dose

    The quantity of energy imparted to unit mass of matter (such as tissue) by Ionising Radiation. Unit Gray (Gy). {1Gy = 1 joule per kilogram}. In the older (non SI) units it can be shown that 1Gy = 100 RADS.


    With respect to Radiation Protection, absorption describes a mode by which Radioactive materials may enter the body leading to an Internal Radiation hazard. For example, it is well known that H-3 contamination on the skin will be readily absorbed and taken up by the body.

    Accumulated dose (cumulative dose)

    Accumulated dose (cumulative dose) is the total radiation dose accumulated over a defined period such as minute, hour or year. Its value can be used operationally (i.e. for a particular task working with ionising radiation), or may be defined in terms of a dose constraint or annual legal dose limit. Generally accumulated dose requires a dose rate (exposure per unit time) and an exposure duration (time) such that:

    \[ Accumulated\;Dose=Dose\; rate(\frac{Dose}{Time})\times Duration(Time)\]

    Often this is easy to understand and measure such as:

    • Measuring the accumulated dose from a static radioactive source with a fixed dose rate and distance (over a certain period of time.
    • Measuring the accumulated dose from a static x-ray generator with a fixed dose rate and distance (over a certain period of time).

    The above are quite easy to calculated with simple maths and using simple concepts such as the inverse square law.

    If the dose rate and / or distance is changing (per unit time) than the accumulation can still be measured simply with a suitable integrating radiation measurement device. If no such monitor is available and you wish to calculate the dose (with a changing dose rate) you have to use more in-depth mathematical analysis and perhaps a computer model. However, sometimes complicated maths can be distilled down into quite simple expressions, an example of this is found here: The accumulated radiation dose when moving up to a source.

    Accumulated dose to the whole body and / or extremities can be measured with a passive dosimeter or active dosimeter).

    Accumulated dose from the intake of radioactive material (via inhalation or ingestion etc) is more difficult to determine and will usually require a combination of biological dosimetry (e.g. urine / blood), perhaps combined with environmental or personal air sampling, and the use of computer based dosimetric models. In this case accumulated dose is stated as the total dose accumulated in the year of intake, even though the total does uptake might be over a longer period (this depends on the radioactive half life and biological half-life of the material taken into the body).


    With respect to Radiation Protection, accumulation describes the process and location where Radioactive materials preferably accumulate in body organs. For example, when I-125 enters the body it will accumulate in the Thyroid, whereas Ca-45 will accumulate in the bone. When used within the context of radioactive waste, accumulation can describe the process of storing waste prior to assessment and / or Disposal.


    Actinides are a transition group between actinium and lawrencium (inclusive). The group includes actinium, thorium , protactinium , uranium , neptunium , plutonium , americium , curium , californium etc. The members of the group are called actinides after the first in the series and they are all radioactive. Of the group, only thorium, uranium, plutonium, americium and californium are significant as the others are only produced artificially and have a relatively short Half-Life. The radiologically significant members all have much longer half-life's (e.g. plutonium-239 is in excess of 24,000 years) and are all Alpha Emitters. This means they have the potential to cause a significant Internal Radiation hazard through Inhalation and Ingestion.

    Active Dosimeter

    An active dosimeter is used in Dosimetry to measure Radiation exposure, usually to individuals. Being active, the dosimeter can provide real-time instant information about radiation Dose and Dose Rate. See EPD for more specific information on devices, and Passive Dosimeter for information on alternative passive detection methods.

    Active Radiation Detector

    An active radiation detector can describe either Dosimetry or Radiation measurement equipment which gives instantaneous real time information, rather than an accumulative count over a time period. The RAM GENE-1 dose rate/contamination monitor is an example of an active radiation detector. The Thermo Fisher EPD is an example of active dosimetry. See Passive Radiation Detector for an alternative measurement methodology.


    In effect means:- 'how much Radioactive material' - in terms of rate of transformations where 1 Becquerel (Bq) =1 transformation per second. Therefore, for 1MBq of activity the disintegration rate will be 1,000,000 transformations per second. The non-SI unit of activity is the Curie (Ci) where 1Ci = 37 GBq (i.e. 37,000,000,000 transformations per second). Note that the rate does not represent the number of particles emitted per second - take Cobalt-60 (Co-60) for example, each decay produces two gamma ray photons per disintegration.


    Advanced gas cooled reactor of a MAGNOX design using enriched Uranium oxide fuel.

    Air Sampling

    With respect to Radiation Protection, air sampling involves the collection of samples of air in order to measure and detect the presence of airborne Radioactive material. This information can be used to determine the likely Inhalation risk and associated Internal Radiation hazard. Normally the radioactive material will either be trapped on a filter paper or in a liquid bubbler.


    ALARA: As Low as Reasonably Achievable (social and economic factors being taken into account). This term was introduced by the ICRP and requires that all be reasonably done to lower Radiation exposures below Dose Limits.


    ALARP: As Low as Reasonably Practicable. In essence, ALARP is the UK definition of ALARA, although they are not the same since ALARP suggests a balance between Risk and benefit (UK Case Law), where as ALARA takes social and economic factors into account. ALARP is key to UK Radiation Protection and introduces a test of reasonableness ensuring that workers work down from Dose Limits rather than up to them.

    Alpha Particle

    A positively charged particle consisting of two Neutrons and two protons which is emitted from Atoms undergoing Alpha Decay. The range of the alpha particle is short (< 1cm in air) and they are easily shielded (stopped by a single sheet of paper). They only present a significant hazard where they enter the body - even the most energetic alpha particles are not able to penetrate the dead outer layers of skin.

    AmBe Americium-Beryllium neutron source

    An AmBe (Americium-Beryllium) source is a special type of industrial neutron source. It is made in the form of a capsule which contains a finely blended powder of radioactive Am-241 (Americium) and Beryllium (Be). The AmBe source works on the interaction of the alpha particles from the Am-241 with Beryllium:

    \[\alpha+Be^{9}\rightarrow C^{12}+n\]
    The neutrons formed are emitted in the energy range of 0.1 - 11 MeV, with an average energy of 4.2 MeV.

    Typical neutron output can be specified as 2.2 x 106neutrons/s per 37 GBq (1 Ci). The half-life of the source is 432 years (i.e. based on Am-241); the source usually has a recommended working life of 15 years.

    Typical dose rates will be in the order of 0.7 micro Sv/hr at 1 m per GBq (gamma) and 0.6-0.7 micro Sv/hr at 1 m per GBq (neutron). Knowing the neutron and gamma dose rates per known activity is useful when determining the transport index (TI) - this is required when transporting the packaged source around.

    Typical applications include:

    • Well logging
    • Industrial gauges
    • Porosity / moisture measurements
    • Prompt Gamma Neutron Activation Analysis (PGNAA) - security applications

    Typical source activities available range from around 37 MBq-740 GBq (1 mCi to 20 Ci). Note that where the Am-214 activity exceeds 60 GBq then the source becomes a High Activity Sealed Source (HASS). More Ionactive resource on HASS sources can be found here: 5. High Activity Sealed Sources (HASS).

    Annihilation (Positron - Electron)

    Annihilation (Positron - Electron) radiation occurs when an electron (negatively charged) collides with a Positron (positively changed & the electron 'anti-particle'). The usual result is the emission of two gamma ray photons , each of 511 KeV and travelling away from each other. The example in this glossary shows the decay of F-18 by positron emission followed by the resulting annihilation radiation. This process has application in Positron Emission Tomography (PET imaging).


    The constituent of elements - the smallest building block which can combine chemically with other atoms and therefore form compounds.. The atom consists of an electron cloud surrounding a nucleus. The nucleus contains positively charged protons and neutral neutrons, whereas the electron cloud is made up of negatively charged electrons. The number of protons (Z) determines the chemical element and the number of neutrons determines the isotope of the element.

    Atomic Mass

    The atomic mass represents the total number of Protons & Neutrons in an atom. For example, in a Carbon 14 atom there are 6 Protons and 8 neutrons packed into the nucleus. Therefore the atomic mass is 14 (and hence we write C-14).

    Atomic Number

    The number of Protons within an Atom. E.g. Uranium-238 (92) contains 92 protons. Also gives the number of Electrons.


    The term 'Background' can be applied to either natural 'Background Radiation', or anywhere where a measurement of Ionising Radiation is required. In analytical Contamination and Radiation measurements it is usual to subtract the background count from the source counts of interest.

    Background Radiation

    Ionising Radiation in our environment which we are all exposed to, the exact magnitude depending on our location in the world. Examples include Radon Gas, Cosmic Rays and K-40.


    The SI unit for Activity. The Becquerel (Bq) is equivalent to 1 disintegration per second (dps) . Also see the older unit of activity, the Curie.

    Beta Particle

    The beta particle has the form of a high speed negatively charged electron (or a positively charged electron in the case of the positron). In beta decay (electron emission) a neutron in the nucleus is converted to a proton with the release of a high speed electron and an anti neutrino. For example, C-14 decays to N-14 and the atomic number has increased by one whilst the mass number at 14 is unchanged. The beta particle is more penetrating than alpha particles but still much less so than gamma rays or x-rays. For every beta emitter there is a unique energy spectrum characterised by average and maximum beta energy. For Tritium (H-3) this is around 18.5 KeV, for C-14 its 156 KeV and for P-32 it is about 1.7 MeV.

    Biological Dosimetry

    Biological dosimetry is a branch of the field of Dosimetry which uses biological samples, usually taken from individuals who have been exposed to radioactive materials, as a means to assess intakes by Inhalation and Ingestion. For example analysis of urine can be used to assess Tritium uptake whilst analysis of faecal matter can be used to determine Actinide uptake. Biological sampling can also be used to assess direct body Irradiation from External Radiation hazards.

    Biological Half-Life

    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.


    Brachytherapy is a type of cancer treatment where Radioactive seeds (Sealed Sources) are placed in or near a tumour, therefore giving a high Radiation Dose to the tumour while minimising the radiation exposure in the surrounding healthy tissues. Years ago the sources would be manual placed into the tumour site by using tweezers or a needle, where as modern techniques place the source remotely using an afterloader unit.

    Braking Radiation

    See Bremsstrahlung radiation.


    Bremsstrahlung, also known as Braking Radiation occurs when ever a charged particle undergoes a change of velocity as it interacts with an absorber. Electromagnetic Radiation (X-Rays) are the result. For Radiation Protection purposes, Bremsstrahlung radiation resulting from the interaction of fast moving Electrons (Beta Particles) with shielding materials, are of particular concern. Here, the degree of conversion to Bremsstrahlung radiation and the magnitude of its energy is proportional to the incident electron energy and the atomic number of the absorber. Hence shielding for high energy Beta emitters like P-32, needs low atomic number material such as Perspex.

    Cherenkov Radiation

    Cherenkov radiation is Electromagnetic (non-ionising) radiation emitted when a charged particle (e.g. an Electron) passes through a medium (e.g. water) at a speed greater than that of light in the medium. This can be seen as the characteristic "blue glow" where sources of large activity (e.g. irradiator sources or spent nuclear fuel) are stored under water.

    Chronic Exposure

    Exposure to sources of Ionising Radiation over a long period of time, possibly resulting in adverse health effects such as cancer or genetic disorders in offspring of exposed parents. Likely result of a Probabilistic / Stochastic effect of ionising radiation.

    Classified person

    A classified person is a designation given in the UK Ionising Radiations Regulations 2017 (IRR17). A classified person is someone who could be exposed to ionising radiation, through occupational exposure including reasonably foreseeable incidents, who could receive more than the following exposures: 6mSv/year whole body effective dose, 150 mSv/year equivalent dose to the extremities, and 15mSv/year to the lens of the eye. It also specifically applies to an employee who works with a source of ionising radiation where a dose rate could deliver a whole body exposure of 20mSv (or 500 mSv to the extremities or 20 mSv to the lens of the eye) within a few minutes from a reasonably foreseeable event.

    Closed Source

    With respect to Radiation Protection, a closed source is is a source of Ionising Radiation in the form of Radioactive material which is encapsulated or otherwise contained. The aim is that closed radioactive material can not escape and will not cause a Contamination hazard. Closed sources have many applications including use in irradiators (food and products), medical blood irradiators and density gauges. Whilst the term 'Closed Source' is comparable with 'Sealed Source' , it is defined in a particular way in some of the UK legislation.

    Collective Dose

    More accurately known as Collective Effective dose. This quantity is derived from summing the individual effective doses within an exposed population (or workforce). One type of unit to express this quantity is the man Sv. This quantity has been used to assess overall detriment and therefore as an aid to decision making techniques in optimising radiation protection (e.g. Risk Assessment). It is less well used nowadays where dose constraints are preferred instead .

    Consumer Products

    Any household product that contains a quantity of radioactive material yielding Ionising Radiation for reasons of functionality of that item. Examples include ionisation smoke detectors (e.g. Am-241) and luminising items such as watches and clocks which may contain, for example, radium. Consumer products from by-gone-days are a potentially significant radiation hazard if not carefully stored and protected from damage.


    Usually an undesirable situation where radioactive material in an Unsealed Source (open source) state is present in the working environment, or otherwise non-contained. Contamination can either be loose (easily removed) or fixed. Loose contamination is usually of more concern since intakes of radioactive material through Inhalation, Ingestion and Injection may occur.


    With respect to Radiation Protection, contingency means preparing for, and taking action, in the event of an unplanned release of Radioactive material or other unplanned Radiation incident which could lead to radiation exposure to the individual, the population or environment. Contingency may be determined by simple risk assessment or by a more comprehensive Probabilistic Safety Assessment (PSA) as part of a Safety Case. Usually contingency arrangements deal with Reasonably Foreseeable (credible) events, although for some industries (e.g. nuclear), the contingency plans have to be extendable.

    Controlled Area

    Controlled Area is defined in the Ionising Radiations Regulations 2017 (IRR17). A Controlled Area is an area where any person is likely to receive more than 6 mSv Effective dose (greater than 15mSv to the lens of the eye, or 3/10 of any other UK Dose Limit) and / or an area where specific and detailed procedures need to be followed in order to restrict exposure from Ionising Radiation and ensure that doses are ALARP. The area can also be designated on the basis of dose rate, such that a Controlled Area will generally be required where the dose rate exceeds 7.5 micro Sv/h when averaged over a working day. Conversely, an area may not need to be designated as Controlled where the dose rate is less than 7.5 micro Sv/h averaged over the working day, and where the instantaneous dose rate (IDR) does not exceed 100 micro Sv/h. However, where high IDR dose rates arise careful consideration of the ALARP concept is required.

    The above concise definition avoids over complicating this glossary entry, a more detailed explanation can be found here: IRR17 (17) - Designation of controlled or supervised areas . There are a number of important caveats and conditions and the reader is encouraged to visit this additional resource.

    Cosmic Rays

    Radiation originating from outside the Earth's atmosphere. The term 'cosmic ray' can actually include a number of classes of high energy radiation including Gamma Rays, Electrons and Ions.


    The term 'credible' is used in a number of areas of Radiation Protection, including Risk Assessments, Safety Cases and Probabilistic Safety Assessments. Credible can be taken to mean an incident or accident which is thought to be Reasonably Foreseeable. Credible 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 Curie (Ci) is the traditional unit of Activity (where its SI equivalent in the Becquerel). 1 Ci is equivalent to 3.7 E10 disintegrations per second (dps) and since 1Bq=1dps it follows that 1Ci = 3.7 E10 Bq or 37GBq (approximately). The Ci was based on the activity found in 1 g of Radium.

    Dead Time

    Dead Time is used in counting / detector systems to describe the time between two recorded events (e.g. where an Ionisation event has been recorded), where either the detector, or its electronics, are unable to detect other incoming events. A counting system which has a large dead time will more than likely miss true events that occur at intervals less than the dead time.


    Also known as Radioactive Decay. Radioactive substances undergo radioactive decay, the rate of which is determined by the properties of the radionuclide. As decay proceeds the resulting activity of the parent Nuclide reduces and will eventually disappear. The daughter product may be stable (inactive) or may itself be Radioactive and undergo further decay (i.e. as part of a decay series). The rate of decay can be expressed in terms of its Half-Life (the time taken for the activity to reduce by half).

    Decay Constant

    The decay constant represents the probability of a Decay of a Radioactive material per unit time.

    Decay Product

    A radionuclide produced as a result of a parent radionuclide's decay. The decay products may be derived from their immediate predecessor, of through several other decays in a decay chain series. One example would be Radon Decay products where Ra-222 will decay to products including Polonium 218 (via alpha decay) and then Lead-214 (also alpha decay).


    A general term applied to situations were nuclear plant (or any other plant containing sources of Ionising Radiations) comes to the end of its useful life. Decommissioning is then required in order to dismantle the plant and recover Radioactive materials for either reuse, recycling or disposal. Decommissioning uses an extremely controlled approach and on larger plants (e.g. nuclear power stations) will require full Safety Cases and regulator attention.


    A general expression applied to situations where undesirable Contamination is removed from plant, fixtures, fittings or people. This process may simply be wiping up the contamination with a tissue, although could require applying an industrial striping technique. Two more specific subsets of this term are Environmental Decontamination and Personal Decontamination.

    Dental x-ray

    A diagnostic procedure undertaken by a Dental Surgeon / Nurse, usually involving patient exposure to x-rays where by the required image is captured on a film, which is then developed and used for assessment and treatment planning. The level of x-ray's are carefully controlled and usually amount to a small fraction of normal annual background radiation.

    Depleted Uranium

    A by-product of Enriched uranium production. Depleted Uranium can be used as Radiation Shielding) (approximately twice as effective as lead for gamma rays), battle armour and in conventional weapons (its high density and self-sharpening properties being more important than its modest Ionising Radiation proprieties).

    Deterministic Effect

    A deterministic effect describes Ionising Radiation induced damage where a Dose threshold exists, and for which the severity of damage increases with increasing Dose above that threshold. Examples will include radiation burns (skin reddening), hair loss, cataracts and radiation sickness (nausea, vomiting and diarrhoea). All of these effects results from acute high doses of radiation to either a part of the body or the whole body. For whole body exposure it is generally thought that an absorbed dose of between 3-5 Gy will cause 50% of those exposed to die within 30 days if medical intervention is not given. This is known as the LD-50 dose.

    Detriment (Radiation)

    With respect to Radiation Protection, detriment is a term used to describe the 'total harm' experienced by exposing a population (and their descendants) to Internal Radiation. ICRP uses detriment to effectively sum all the Risks (probabilities) that exposure to ionising radiations might produce. For example it will include probability of fatal cancer induction, non-fatal cancer induction (and therefore years of life lost). It therefore as the dimensions of probability and thus can be expressed as a risk. In ICRP publication 60, radiation detriment is developed and used to derived Dose Limits.

Radiation is one of the important factors in evolution. It causes mutation, and some level of mutation is actually good for evolution

– David Grinspoon -