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. 


    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.

Atoms are very special: they like certain particular partners, certain particular directions, and so on. It is the job of physics to analyze why each one wants what it wants.

– Richard P. Feynman -