Radiation Protection Glossary

A radiation protection glossary for Radiation Protection Supervisors (RPS) and anyone interesting in radiation safety terminology. The glossary is a mixture of health physics terms, and phrases related to radiation legislation, transport, practical safety and similar.

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    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.

    Absorption

    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.

    Accumulation

    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

    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.

    Activity

    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.

    AGR

    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

    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

    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.

    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).

    Atom

    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 aCarbon 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.

    Background

    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.

    Becquerel

    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

    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

    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.

    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 and luminising items such as watches and clocks which contain radium. Consumer products from by-gone-days are a potentially significant radiation hazard.

    Contamination

    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.

    Contingency

    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.

    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.

    Credible

    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).

    Curie

    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.

    Decay

    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).

    Decommissioning

    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.

    Decontamination

    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.

    Disposal

    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

    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'.

    DNA

    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

    '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'.

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