Osłony RTG / Knowledge / Zones and areas
Prohibited, controlled and supervised area — how to set the distances
Zones follow from the dose rate as a function of the distance from the focus and from the dose limits. Below: the boundaries, the thresholds and example distances for a 200 kV tube.
Thresholds and legal basis
| Area | Criterion | Who may stay there |
|---|---|---|
| Prohibited | above the worker dose limit (20 mSv/year) | nobody during exposure |
| Controlled | 6 mSv/year may be exceeded | category A workers |
| Supervised | 1 mSv/year may be exceeded | category B workers |
| Free | below 1 mSv/year | members of the public |
Dose limits: 20 mSv per year for a worker and 1 mSv per year for a member of the public (Polish Atomic Law, art. 14). The split into categories A and B comes from art. 17: category A means the dose may exceed 6 mSv per year.
Where the distances come from
The dose rate falls with the square of the distance, so the boundary of each zone is the distance at which the annual dose drops to the threshold. Three sources enter the calculation and the largest is taken: radiation scattered by the object, albedo radiation reflected from walls and ceiling, and leakage through the tube housing.
For zoning, at least 100 hours of tube operation per year are assumed. The verification against DIN 54113-3 adds its own dose-rate thresholds for the Sperrbereich and Kontrollbereich, and the calculator takes whichever criterion is stricter.
Example: 200 kV tube without fixed shielding
200 kV tube, 3.7 mA, work in the open, 100 h per year:
| Boundary | Distance from the focus |
|---|---|
| Prohibited area | 31.0 m |
| Controlled area | 69.3 m |
| Supervised area | 138.7 m |
| Free area | beyond 253.2 m |
These numbers explain why site radiography needs barriers and warning signals tens of metres out, and why a room needs fixed shielding. Every barrier in the path shortens the distance by as much as its attenuation factor.
What to check besides the distance
- Light and sound signals switched on automatically with the exposure.
- A door interlock that terminates the exposure when the door opens.
- An emergency stop inside the room — somebody may be left in there.
- Individual dosimetry for category A and environmental monitoring of the room.
- Measurement points behind every barrier, including the door and the viewing window.
Calculate your own room in the calculator See the demo on prepared rooms
These calculations support the shielding design. The final assessment, the classification of workers and the approval of the design are made by the head of the organisation in consultation with the radiation protection officer.
Frequently asked questions
What are the dose limits?
20 mSv per calendar year for a worker and 1 mSv per year for a member of the public (Polish Atomic Law, art. 14, following Directive 2013/59/Euratom). A worker is assigned to category A if the dose may exceed 6 mSv per year, below that to category B (art. 17).
What is the difference between a controlled and a supervised area?
A controlled area is set where 6 mSv per year may be exceeded; special protection rules apply there and category A workers work in it. A supervised area is where the dose may exceed 1 mSv per year but stays below the controlled-area threshold.
How far does the zone reach for a 200 kV tube?
For a 200 kV / 3.7 mA tube at the assumed minimum of 100 operating hours per year the distances from the focus are: prohibited area up to 31.0 m, controlled area up to 69.3 m, supervised area up to 138.7 m, free area beyond 253.2 m. These are values for work without fixed shielding — barriers shorten the distances.
How many operating hours per year should be assumed?
Not less than 100 hours of tube operation per year, even if the actual time is shorter. The zone then does not depend on the current workload and does not have to be redrawn every time the schedule changes.