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    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

    The 'High Activity Sealed Sources Directive' (HASS) is designed to provide greater control of high activity sealed sources. Only significant activity sources are included, for example, Cobalt-60 sources of more than 4 GBq will be included. 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 can not 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.

    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).

Reality is merely an illusion, albeit a very persistent one

– Albert Einstein