10th Value Thickness (TVT)
Published: Aug 15, 2021
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.