04 Protection
Required barrier transmission
B from weekly dose limit, workload, use and occupancy factors.
Listen
Listen · English
Simulation
Required barrier transmission — Change the numbers; the scene follows.
Where it works
Treatment vault
Primary barrier
In the bunker maze and barriers — time, distance, TVL, WUT, and weekly controlled-area dose.
Open this machineFormula
Variables
Results
B
Required transmission
3.6000e-7
1/B
Required attenuation
2.78e+6
Explanation
What it means
NCRP 151 primary-barrier transmission: B = P d² / (W U T). P is the weekly design goal at the point of interest, W the workload (Gy/week at 1 m), U the use factor (fraction of beam pointing that way), T occupancy. This is a working relation in Radiation protection.
Where it is used
Clinically it sits on the Treatment vault — Primary barrier. In the bunker maze and barriers — time, distance, TVL, WUT, and weekly controlled-area dose. 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
Public uncontrolled P ≈ 0.02 mSv/week = 2×10⁻⁵ Gy/week. Controlled design is often 0.1 mSv/week. Then take B to the thickness calculator with the right TVL. Secondary barriers use scatter/leakage workloads instead of W. Change one input and watch the curve and the simulation follow.
Symbols
- PWeekly dose limit2.0000e-5 Gy/week
- dDistance3 m
- WWorkload500 Gy/week
- UUse factor1
- TOccupancy factor1
Worked example
A typical case from the default values: P = 2.0000e-5 Gy/week (Weekly dose limit); d = 3 m (Distance); W = 500 Gy/week (Workload); U = 1 (Use factor); T = 1 (Occupancy factor). Substituting into the relation gives B = 3.6000e-7; 1/B = 2.78e+6. These are teaching numbers — align them with your machine.
Typical values give
- B = 3.6000e-7
- 1/B = 2.78e+6
Where it comes from
The displayed formula is the working relation. B from weekly dose limit, workload, use and occupancy factors. Usual reference: NCRP 151. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: NCRP 151
Assumptions & limits
Primary barrier, inverse-square in air, no patient attenuation. Workload W must match how you tabulated it (isocentre vs 1 m). IMRT factor increases W.
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. Primary barrier, inverse-square in air, no patient attenuation. Workload W must match how you tabulated it (isocentre vs 1 m). IMRT factor increases W.
Keep this
W in Gy/week at 1 m, P is weekly dose limit at the point, d in metres.
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