Osłony RTG / Knowledge / Substitute materials
Replacing lead: concrete, barite concrete, steel, brick
Lead is rarely the only shielding material — more often the question is how much concrete or masonry is already standing and what is missing. Below are the conversions from tables 4–9 of PN-86/J-80001 for 200 kV.
Conversion for 200 kV
| Material | Density [g/cm³] | Thickness instead of 4.00 mm Pb |
|---|---|---|
| Steel | 7.90 | 60 mm |
| Barite concrete | 3.20 | 78 mm |
| Ordinary concrete | 2.15 | 300 mm |
| Solid brick masonry | 1.90 | 360 mm |
The difference between ordinary and barite concrete — 300 mm against 78 mm — comes from the barium content and its high atomic number. Where wall thickness and floor area matter, barite concrete often works out cheaper in its consequences than ordinary concrete, despite the higher price per cubic metre.
Density correction
The tables give values for one specific density. If the concrete on site is 2.35 g/cm³ instead of the tabulated 2.15, it shields better and the thickness can be reduced; if it is aerated or has a density of 2.0, thickness has to be added. Take the density from the as-built documentation or from a test, not from the manufacturer's catalogue.
Layered barriers
A room wall is usually several materials: plaster, masonry, possibly a lead or steel sheet. The layers are computed in the order in which the radiation meets them, with the build-up of scattered radiation in the thicker layers taken into account. Adding lead equivalents together without that correction underestimates the required thickness — and always in the dangerous direction.
What must not be left out
- Doors and the viewing window need a lead equivalent no lower than the wall they sit in — including the frame, the threshold and the gaps.
- Service penetrations: a ventilation duct or a cable tray crossing the shielding is a hole in it; route it at an angle or as a labyrinth.
- The range of the tables: above it the result is an extrapolation of the curve and the design has to state that clearly.
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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
How much concrete replaces 4 mm of lead?
At 200 kV, 4.00 mm Pb corresponds to roughly 300 mm of ordinary concrete of density 2.15 g/cm³, 78 mm of barite concrete 3.2 g/cm³, 60 mm of steel or 360 mm of solid brick masonry 1.9 g/cm³. The values come from tables 4–9 of PN-86/J-80001 for that voltage.
Does the lead equivalent depend on the tube voltage?
Yes, strongly. The same thickness of concrete gives a different equivalent at 100 kV and at 200 kV, because the spectrum is harder. The conversion is always made for the working voltage of the tube, never for an average.
What if the concrete on site has a different density than the table?
The actual density has to be corrected for. The tables give values for one density (ordinary concrete 2.15 g/cm³, for instance); concrete of 2.35 g/cm³ shields better, aerated concrete worse. Take the density from the as-built documentation, not from a catalogue.
Can materials be combined in one barrier?
Yes — a typical wall is plaster, brick and a lead sheet. The layers are computed in the order in which the beam meets them, with the build-up of scattered radiation taken into account. Adding lead equivalents together without that correction underestimates the result.