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. 


    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 gamma / x-ray dose rate (or dose) to 1/10 of the of the pre-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.

    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 or neutrons (although a similar process can be used with neutrons in certain cases).

    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). This means an attenuation of:


    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.

    More TVT information resource is available elsewhere on our site. For example: How do I convert TVT (10 value thickness) values to attenuation for Gamma or X-ray sources of radiation?

    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).


    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, for example I-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.

    Whilst an intake of radioactive iodine into the thyroid occupationally is undesirable, it is also used in medicine. For example, I-131 can be used in radioiodine therapy for thyroid cancer where somewhere between 1-5 GBq will be used. It can also be used in thyroid diagnostic scans where much less activity is used. In medical use the radiation exposure is justified so that the risk of the exposure is outweighed by the benefits (of treatment and diagnostic information).


    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).

    Transport Index (TI)

    The transport index (TI) is a special number applied to the labels of Type A and Type B radioactive packages for transport (as specified by the ADR legislation and IAEA - "Regulations for the Safe Transport of Radioactive Material", 2018, SSR-6). The formal way of determining the TI is to measure the dose rate at 1m from the package, (from all sides that are reasonably accessible to obtain the highest value), in mSv/h. This value is then multiplied by 100 and rounded up to the first decimal place. Example: The maximum dose rate at 1m from a package is found to be 0.0127 mSv/h. This is multiplied by 100 to give 1.27, and this is then rounded up to give a TI of 1.3.

    An alternative method is to measure the dose rate in micro Sv/h and then divide this value by 10 and then round up as before. Example. The maximum dose rate at 1m from a package is found to be 12.7 micro Sv/h. This value is then divided by10 to give 1.27, and this is then rounded up to give a TI of 1.3 (as above).

    The TI should be made up of the sum of all radiations present, which in the main will be gamma radiation but could also include neutrons (in the case of an AmBe source or a radionuclide such as Cf-252). Where possible the neutron dose rate should be measured with a neutron monitor, a TI calculated, and then summed with the gamma TI to provide the full transport TI. However, it is often common practice to infer the neutron dose rate from a known ratio of neutron to gamma dose rates (this having being determined by the source supplier or by the user where both gamma and neutron monitors are available periodically).

Physics is, hopefully, simple. Physicists are not

– Edward Teller -