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.

Search the Glossary by either clicking on a letter or typing a Keyword into the search box.

T

    10th Value Thickness (TVT)

    10th value thickness (TVT), sometimes known as 10th value layer (TVL) is used in simple radiation shielding calculations. Sometimes TVT is all you need, whereas more complex shielding problems may need computer codes (e.g. MCNP) to optimise the shielding.


    Simply stated, the TVT is the thickness of a radiation shield that will reduce radiation dose rate to 1/10 of the original non shielded value. There are a number of factors that will potentially interfere with this approach, but TVT is still a good approximation in many cases. In order to use a TVT you need to know the following:

    • For x-ray (photon) beams you need to know the energy (e.g. kV / MV) and the shielding material of choice (e.g. lead, concrete of specified density, steel etc).

    • For radioactive materials you need to know the radioactive material for consideration (e.g. Cs-137, Co-60, F-18) and the shielding material of choice (as above).


    For either of the above you need a reliable data source - we will not reference them here, but some are available elsewhere on our site. Ionactive also has a soon to be released radiation protection calculator which will feature TVT values we have quality assured. Note that the TVT does NOT apply to alpha or beta radiation.


    Examples of TVT are as follows:

    • TVT for lead with Cs-137 is 22 mm
    • TVT for lead for a positron emitter (e.g. F-18) is 17 mm
    • TVT for 100 kV x-rays could be 5.1 cm of 2.35 density concrete or 0.8 mm of lead


    TVT works in the following way. Add the TVT thickness, whilst multiplying the attenuation. Let's use the Cs-137 data as an example.


    Cs-137 and lead example for TVT


    You have a Cs-137 source with a dose rate of 1000 micro Sv/h at 1m. Reduce the dose rate at 1m to 1 micro Sv/h using lead.


    In order to reduce dose rate from 1000 micro Sv/h to 1 micro Sv/h requires 3 TVT (TVT+TVT+TVT) = 1/10*1/10*1/10 = 1/1000.


    So this requires 22mm lead (TVT) + 22mm lead (TVT) + 22mm Lead (TVT) = 66mm lead.


    This is somewhat simplified but provides the general idea.


    Another concept is HVT (half value thickness) which is self explanatory. Note that 3.32 HVT = 1 TVT.


    We have not explained absolute attenuation or the means of calculating fractions of a TVT or HVT. This will feature in a future Ionactive article.

    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).

Physics is really nothing more than a search for ultimate simplicity, but so far all we have is a kind of elegant messiness

– Bill Bryson -