Radiation Protection Glossary

A radiation protection glossary for Radiation Protection Supervisors (RPS) and anyone interesting in radiation safety terminology. The glossary is a mixture of health physics terms, and phrases related to radiation legislation, transport, practical safety and similar.

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T

    Thermal Neutrons

    Thermal neutrons are a class of Neutron which are said to be in 'thermodynamic equilibrium' which means they are moving with the same kinetic energy as their surroundings. At room temperature the average energy of a thermal neutron is around 0.025 eV. Their significance in Radiation Protection is quite different to that of Fast Neutrons.

    Thermo Luminescent Dosimeter (TLD)

    The thermo Luminescent Dosimeter (TLD) is a type of Passive Dosimeter which is used to measure exposure from Ionising Radiation. It is one of a number of methods used in the area of Dosimetry. The TLD consists of a crystal (e.g. CaSO4) which gives off light when heated, the light being proportional to the degree of exposure seen by the TLD. The crystal is usually placed in a holder which contains filters which can be used to differentiate between skin doses and penetrating doses of ionising radiation. The TLD is normally worn on the trunk of the body but can also be worn on the extremities (e.g. for measuring doses to the fingers).

    Thyroid

    The thyroid is a butterfly-shaped gland in the neck that secretes thyroid hormones which regulate a number of physiologic processes, including growth, development and metabolism. With respect to Ionising Radiation, the thyroid is vulnerable to intakes of radioactive Iodine -125 by either Inhalation or Ingestion. In the event that an intake of I-125 occurs the thyroid will be the organ or accumulation and will preferentially uptake the iodine from the systemic system. This creates an Internal Radiation hazard.

    Time

    In Radiation Protection 'time' is still considered one of the key principles for protection from External Radiation sources of Ionising Radiation. For a given Dose Rate, the exposure from the source can be minimised by minimising the time spent near the source. Whilst this concept is still valid, and in some circumstances its vital, it is usually easier to comply with the principles of ALARP by using the other related methods such as maximising Distance, using Shielding or using an alternative to ionising radiation.

    Tissue Weighting Factor

    The tissue weighing factor is an ICRP multiplier use to determine the Effective dose from the Equivalent Dose in one or more organs or tissues. The factor takes account of the different sensitivities of different organs and tissues for induction of Probabilistic Effects from exposure to Ionising Radiation (principally induction of cancer).

Reality is merely an illusion, albeit a very persistent one

– Albert Einstein