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Radiation Protection Glossary

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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 , where as 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 distinct 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 (a few cm's 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.

Ionactive Training Resource - Alpha Radiation
Annihilation (Positron - Electron)
Annihilation (Positron - Electron) radiation occurs when an electron (negatively charged) collides with a Positron (positively changed & the electron’s anti-particle). The usual result is the emission of two gamma ray photons – each of 511 KeV 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 the Carbon 14 atom shown above there are 6 (red) Protons and 8 (blue) 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 .