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.

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    Safety Case

    Safety Cases are used with in the nuclear industry,in other high risk industries, and where the public are exposed to systematic risks on a daily basis (e.g. railway transport systems). A safety case will set out how operators manage and control the health and safety of employees, members of the public and the environment. The safety case will also detail Contingency plans for dealing with incidents and accidents by assessing all Reasonably Foreseeable adverse events. In essence, the safety case provides a 'safe operating envelope' which considers things like safety policy, Risk Assessment, safety management systems, risk control measures, reliability assessments and contingency.


    With respect to Radiation Protection, scintillation is the process where by a material will emit light Photons when exposed to Ionising Radiation. The light photons can be measured with a photo-multiplier tube which will multiply the events to produce an electrical signal or pulse. The pulse can be counted to give an indication of the magnitude of the incoming radiation. Materials that have scintillation properties include Zinc Sulphide (used in many Alpha emission detectors), Sodium Iodide (used in many X-Rays / Gamma emitter surveying probes), and scintillant cocktails (used in Liquid Scintillation Counting).

    Sealed Source

    With respect to Radiation Protection, a sealed source is a source of Ionising Radiation in the form of Radioactive material which is encapsulated. Sealed radioactive material can not escape and will not cause a Contamination hazard. Sealed sources are used in irradiators (food, products, blood), and thickness & level gauges. The activity of the sealed source can vary from a few Bq to many 10's of TBq. See Closed Source for additional definitions and Unsealed Source for the alternative type of source. Also see HASS sources.

    Shielding (Radiation)

    Shielding is a major protection principle for reducing exposure to Ionising Radiation (the two other related principles being Time and Distance). See Lead for a shielding example.


    The Sievert (Sv) is the SI unit of Equivalent Dose & Effective dose. The equivalent older unit is the Rem where 1Sv = 100 rem.

    Specific Activity

    Specific activity is taken to be the Activity (Bq) per unit mass (grams),normally expressed as Bq/g. Specific activity is related to Half-Life such that the shorter the half life the higher the specific activity. For example, the specific activity of natural Uranium is 25.4 kBq/g and has a half life of 4.51 E9 years (4510 million years). Compare this to Polonium - 210 which has a specific activity of 166500 GBq/g and a half life of 138 days.

    Specified practice

    A specified practice is a term defined in Regulation 7 of the Ionising Radiations Regulations 2017 (IRR17). A specified practice is work with ionising radiation in the UK, which is placed into the highest risk category, and therefore requires a 'consent' from the Health and Safety Executive (HSE). A list of specified practices will include the following:

    (a) the deliberate administration of radioactive substances to persons and, in so far as the radiation protection of persons is concerned, animals for the purpose of medical or veterinary diagnosis, treatment or research

    (b) the exploitation and closure of uranium mines

    (c) the deliberate addition of radioactive substances in the production or manufacture of consumer products or other products, including medicinal products

    (d) the operation of an accelerator (except when operated as part of a practice within sub-paragraph (e) or (f) below and except an electron microscope);

    (e) industrial radiography

    (f) industrial irradiation

    (g) any practice involving a high-activity sealed HASS source (other than one within sub-paragraph (e) or (f) above);

    (h) the operation, decommissioning or closure of any facility for the long-term storage or disposal of radioactive waste (including facilities managing radioactive waste for this purpose) but not any such facility situated on a site licensed under section 1 of the Nuclear Installations Act 1965

    (i) practices discharging significant amounts of radioactive material with airborne or liquid effluent into the environment.


    With respect to Radiation Protection, stochastic effects (also referred to as Probabilistic) represent radiation harm for which there is no threshold (see Linear Dose Response). Even the smallest quantity of Ionising Radiation exposure can be said to have a finite probability of causing an effect, and this effect being either cancer in the individual or genetic damage. Dose Limits are set to ensure that these effects are minimised to broadly acceptable levels. Also see Deterministic Effect.

If you start from nothing, it is very difficult to get anywhere

– Gerhard Herzberg -