Gamma shielding is governed by mass per square metre, not by the name of the material: at the parent article’s half-value layers, a layer that halves a gamma beam holds roughly 135 to 232 kg per square metre for concrete, earth, water, steel, lead and wood alike. Lead is compact, and earth, water and rubble are bulky, but any of them gives the same protection if enough mass is piled between the source and people.
Part of Radiation Safety
A shielding material is any bulk substance placed between a gamma source and people; its usefulness is the mass per unit area it presents, which is thickness multiplied by bulk density.
Mass Per Square Metre Is The Quantity That Matters
Mass per unit area (areal density) is thickness times bulk density. It is the correct way to compare materials because gamma attenuation at these energies depends mainly on how much matter the photons cross. The parent article, Nuclear Safety, gives half-value layers (HVL). Multiplying each by a density gives the mass per half-value layer:
| Material | HVL (parent) | Density, kg/m³ | Mass per HVL, kg/m² | Mass for 7 HVL, kg/m² |
|---|---|---|---|---|
| Lead | 1.3 cm | 11,340 | 147 | 1,032 |
| Steel | 2.5 cm | 7,850 | 196 | 1,374 |
| Concrete | 6 cm | 2,400 | 144 | 1,008 |
| Packed earth | 9 cm | 1,500-2,000 | 135-180 | 945-1,260 |
| Water | 18 cm | 1,000 | 180 | 1,260 |
| Wood | 29 cm | 500-800 | 145-232 | 1,015-1,624 |
Arithmetic, for concrete: 0.06 m x 2,400 kg/m³ = 144 kg/m²; for seven layers 7 x 144 = 1,008. For earth: 0.09 x 1,500 = 135 and 0.09 x 2,000 = 180; seven layers 945 and 1,260. All values from the parent’s HVLs and these densities, shown to the nearest kilogram.
Where the densities come from: earth 1,500-2,000 kg/m³ is the range used in Earth Load Calculations; concrete about 2,400 and steel 7,850 are used in the corpus (concrete as a recalled typical value); water is 1,000; lead at 11,340 and the wood range are recalled typical values, not from the corpus. Weigh a sample of your own material if you can.
Reading The Table: Equivalence Is The Decision
The whole table sits between 135 and 232 kg/m² per layer. So thicknesses are equivalent when masses per square metre are: 42 cm of concrete is 1,008 kg/m², and 63 cm of earth is 945-1,260 kg/m², overlapping. 126 cm of water is 1,260 kg/m². A pile of earth, a row of full water barrels or a rubble bank can stand in for concrete if you give it the mass. Looser or wetter fill changes the density, so the thickness must change to keep the mass.
Choosing A Material
- Earth and rubble. Cheapest and everywhere; thick; needs structure to hold it. See Underground & Earth-Sheltered Buildings.
- Water. Needs strong containers and thickness; the parent article’s example is a filled bathtub between you and the source. Water is also best against neutrons, the parent notes.
- Concrete and masonry. Compact and durable; good where space matters.
- Steel. Compact but heavy to move.
- Lead. The most compact. It is toxic, so avoid cutting, melting or sanding it; see Hazardous Materials and Battery & Electrical Safety for the acid in lead batteries.
- Wood. Weak: the parent’s 29 cm HVL means impractical thickness; do not rely on a timber wall.
Worked Example: Is My Wall Enough
Hypothetical: a wall of 360 kg/m². Dividing by the extremes of the table, 360 / 232 = 1.55 and 360 / 135 = 2.67 half-value layers. Each is a factor 2^-n: 0.34 and 0.16. So the wall passes roughly 16 to 34 percent of a gamma beam, far from 1 percent. The wall’s mass is invented; the method is to compute mass per square metre, divide, and reach the number of layers.
Limits On These Numbers
The parent does not state the gamma energy its HVLs assume, and HVLs change with energy. Fallout contains many energies. Thick shields also pass scattered radiation, so real thickness needed is often larger than the simple arithmetic. Treat all values here as minimums; see Half-Value Layers.
Common Mistakes
- Choosing by name. “Steel is better than earth” is meaningless without mass per square metre.
- Loose, hollow or wet-and-drying fill. Mass per area changes with the fill.
- Gaps. Radiation passes through any thin place.
- Using lead carelessly. The toxicity can injure more people than the shielding protects.
Safety
Heavy loads and toxic metal
A metre of earth on a roof is a large load; check the structure first with Roof Structures for Earth Loading. Do not heat, cut or file lead without proper protection. Stacks of barrels or blocks can fall; build them stable.
What’s Next
- Learn next: Half-Value Layers — the arithmetic of thickness
- Combine with: Shelter Design — placing mass where it counts