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. 



    In general terms, LD-50 is used as an index to describe effectiveness of some entity producing a response in a subject, where that response occurs in 50% of the subjects. In Radiation Protection it is commonly used to describe the value of Absorbed Dose in humans which would lead to Deterministic Effects and therefore radiation sickness (leading to death without medical intervention). The quantity varies depending on the literature but is likely to be between 3 and 5 Gray (Gy) (with some variance related to medical treatments).


    With respect to Radiation Protection, lead together with perhaps concrete, is the most likely shielding material for attenuating X-Rays and Gamma Rays. It has a density approximately 11 times greater than that of water and is easily formed into sheets and interlocking bricks. For example, 4 cm lead will attenuate Co-60 gamma rays to 1/10th of the unshielded value (ignoring geometric and beam effects).


    LET is short for Linear Energy Transfer. This quantity actually reflects the linear rate of energy absorption, by the absorbing medium, as the Ionisation event traverses the medium. In simple terms low LET radiation (e.g. Beta Particles) transfer less energy per unit path length than high LET radiation (e.g. Alpha Particles) to absorbing mediums. LET has most use in radiobiology and has the units of KeV/micron.


    It is generally thought that Leukaemia is a likely form of malignancy which can result from whole body exposure to Ionising Radiation, the likelihood being Probabilistic in nature and increasing linearly with dose without threshold (although there does remain a controversy around the non-threshold hypothesis).


    Limitation is the last of the three principles of the ICRP system of protection. Limitation, together with Justification and Optimisation, are used as the basis of Radiation Protection internationally. Limitation is defined in ICRP 103 as '..effective dose to individuals shall not exceed the dose limits recommended..'. ICRP requires that Deterministic Effects are avoided and that Probabilistic / Stochastic effects are as low as reasonably achievable (ALARA).

    Line Source

    With respect to Ionising Radiation, a line source describes a Radioactive source which can be represented by a line between points A-B in any chosen plane. Calculations of Dose Rates from line sources are more complicated than those from Point Sources and in many cases, direct measurements are going to always be preferable. Such calculations usually involve knowing (or estimating) the Activity per unit length of the source, then calculating the dose rate at a chosen point by integrating contributions from integral points along the line. Commercial programs such as Microshield can undertake this process automatically.

    Linear Attenuation Coefficient

    The Linear Attenuation Coefficient is used in the description and calculation of exponential absorption of Gamma Rays. The quantity normally has the dimensions cm-1 and describes the fraction of gamma rays that are attenuated per unit thickness of absorber.

    Linear Dose Response

    Linear Dose Response in Radiation Protection relates to the zero-threshold model which predicts that every small addition of radiation exposure above background contributes to an increment in the probability of a Probabilistic / Stochastic effect (excess cancer). The response relies on the assumption that even one Photon has the ability to cause an Ionisation event in DNA which may initiate cancer (or other genetic effects). In adopting this model one has to remember that there is no certainty that an ionising radiation event actually leads to biological damage (and thus cancer etc), rather, it it the likelihood of induction which increases. LNT is the basis for Dose Limits, but many argue it exaggerates the risks from ionising radiation.

    Local Rules

    Local Rules are a requirement of the UK Ionising Radiations Regulations 2017 (IRR17) , particularly where you one or more designated areas (Controlled Area or Supervised Areas). Local Rules are the written instructions in order to comply with IRR17 and work safely with ionising radiation. They are closely related to the Radiation Protection Supervisor (RPS) who is a person appointed under IRR17 to ensure the Local Rules are implemented. They will detail RPS contact details, a description of the designated area, the dose investigation level, details of dosimetry or monitoring requirements and other written arrangements to restrict exposure (i.e. ALARP).

    Low Level Waste (LLW)

    Low-Level Waste (LLW) consists mainly of items such as protective clothing, laboratory equipment, packing materials, containers and site equipment which have come into contact with Radioactive material and are no longer useful. In the UK the definition of LLW is waste with a radioactive content not exceeding 4 GBq/tonne of Alpha activity or 12 GBq/tonne of Beta / Gamma activity. See Radioactive Waste for more information.

Be less curious about people and more curious about ideas

– Marie Curie -