04 Protection
Required thickness
Barrier thickness that yields transmission B.
Listen
Listen · English
Simulation
Required thickness — Change the numbers; the scene follows.
Where it works
Treatment vault
Shielded door
At the shielded door — transmission, required thickness, stacked TVLs.
Open this machineFormula
Variables
Results
x
Thickness
16.6096mm
TVL
Tenth-value layer
8.3048mm
n
Number of TVLs
2
Explanation
What it means
Invert transmission to get thickness: x = HVL × log₂(1/B) = TVL × log₁₀(1/B). A required B of 0.01 is two TVLs or about 6.6 HVLs. This is a working relation in Radiation protection.
Where it is used
Clinically it sits on the Treatment vault — Shielded door. At the shielded door — transmission, required thickness, stacked TVLs. Protection equations are the wall, the occupancy factor, and the badge: time, distance, shielding, and WUT. They turn a room into a legal design.
Treatment vault · Open this machineHow to use it
First compute B from the NCRP barrier formula, then convert to millimetres of the chosen material using the matching HVL/TVL at that energy. Change one input and watch the curve and the simulation follow.
Symbols
- BRequired transmission0.01
- HVLHalf-value layer2.5 mm
Worked example
A typical case from the default values: B = 0.01 (Required transmission); HVL = 2.5 mm (Half-value layer). Substituting into the relation gives x = 16.6096 mm; TVL = 8.3048 mm; n = 2. These are teaching numbers — align them with your machine.
Typical values give
- x = 16.6096mm
- TVL = 8.3048mm
- n = 2
Where it comes from
The displayed formula is the working relation. Barrier thickness that yields transmission B. Usual reference: NCRP 147 / 151. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: NCRP 147 / 151
Assumptions & limits
Same monoenergetic narrow-beam caveat. Concrete, lead and steel have very different TVLs — never mix materials without a dedicated multilayer method.
Pitfalls
Tenth-value layers are for the broad beam in that material and that energy — not a photocopy from another bunker. Occupancy T is not a guess; it is a use pattern. Inverse-square fails against a large scatter source. Same monoenergetic narrow-beam caveat. Concrete, lead and steel have very different TVLs — never mix materials without a dedicated multilayer method.
Keep this
Time, distance, shielding — in that order — then calculate the wall. Same monoenergetic narrow-beam caveat. Concrete, lead and steel have very different TVLs — never mix materials without a dedicated multilayer method.
In this specialty