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. 



    Half-life is closely related to the property of Radioactive Decay and represents the time taken for half the Atoms in a Radioactive substances to undergo decay and change into another nuclear form (either a radioactive daughter product or a stable form). It is therefore the time taken for the Activity of a radioactive sample to decay by half and is commonly given the symbol t1/2.

    Half-Life (Biological)

    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. For example, Tritium contaminated ingested water will tend to clear quickly (quicker still if the individual drinks plenty of water), whereas Ca-45 will tend to bind to bone. The solubility of the radioactive substance will also effect retention time in the body.

    HASS Sources

    HASS sources (High Activity Sealed source) require significant control measures due to the significant radiation hazards that may exist should the source become uncontrolled. The aim is to ensure that proper controls are in place throughout the entire life cycle of the source from production, purchase, use, storage and eventual disposal. These controls are designed to ensure that sources cannot be lost, that the threat of theft is minimised, and that sources can not be purchased unless there is a disposal route and resources available to affect that disposal when required.

    Note that a HASS source ceases to become a HASS source when the radioactivity is below the HASS threshold. Examples of HASS source activity are as follows:

    • Am-241 (60 GBq)
    • Cs-137 (100 GBq)
    • Cf-252 (20 GBq)
    • Co-60 (30 GBq)
    • Ir-192 (80 GBq)
    • Ra-226 (40 GBq)
    • Se-75 (200 GBq)

    The units above are in becquerels (i.e. 10^9 Bq).

    Health Physics

    A term used for the practice of Radiological Protection. A practitioner may be known as a Health Physicist or perhaps in the UK a Radiation Protection Adviser (RPA).

    Hereditary Effects

    Hereditary effects (with respect to Radiation Protection) are those effects present in the offspring of those exposed to Probabilistic / Stochastic levels of ionising radiation. See Genetic Effects.

    Hot (Source)

    A common 'slang' term used in Health Physics and Radiation Protection to mean something with high levels of radioactivity or Dose Rate. It has no technically recognised meaning and should be used with care since the word can be used subjectively. UK universities commonly use the term 'Hot Lab' to mean a laboratory which contains higher than normal levels of radiation or radioactive material (these labs generally have low levels of activity compared to industry).

    HVT - Half Value Thickness

    Simply stated, the HVT is the thickness of a radiation shield that will reduce radiation gamma / x-ray dose rate (or dose) to 1/2 of the of the pre-shielded value. There are a number of factors that will potentially interfere with this approach, but HVT is still a good approximation in many cases.

    HVT is related to TVT by the expression 3.32 HVT = 1 TVT. Head over to the TVT resource for a fuller explanation of this concept.

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