Alpha and beta particles have a finite range and are stopped completely by a thin barrier, but gamma rays are only attenuated: each half-value layer of material removes half of the radiation passing through it, and no thickness removes all of it. The quantity that governs the result is mass per unit area between the source and the person, not the name of the material.
Part of Radiation Safety
Penetration is how far a kind of radiation travels through matter; shielding is placing matter in its path. The two kinds of behaviour, a fixed range for particles and exponential thinning for gamma rays, need different thinking.
Particles Stop, Gamma Rays Only Thin
Alpha and beta particles lose energy continuously and come to rest. The parent article, Nuclear Safety, gives their stopping materials: alpha is stopped by a sheet of paper, a few centimetres of air or the dead outer layer of skin; beta is stopped by a few millimetres of aluminium, thick wood or heavy clothing. Neutrons are stopped best by hydrogen-rich materials such as water, concrete and polyethylene.
Gamma rays behave differently. A photon either passes through a layer untouched or is absorbed or scattered in it, so each layer removes a fixed fraction. That fraction defines the half-value layer (HVL), the thickness that halves the intensity. After n half-value layers the fraction remaining is 2 to the power minus n.
Worked Example: How Many Layers Make 1 Percent
- Inputs: remaining fraction after n layers = (1/2)^n.
- Seven layers: 2^7 = 128, so the fraction is 1/128 = 0.78 percent.
- Exactly 1 percent needs n = log2(100) = 6.64 layers.
The parent article rounds this to “7 half-value layers for 1 percent”. With the parent’s HVL for concrete (6 cm) that is 7 x 6 = 42 cm; 6.64 x 6 = 39.9 cm. See Half-Value Layers for the full table and derived mass per square metre.
Areal Density Is The Real Variable
For gamma rays at the energies that matter in fallout, attenuation depends mainly on how much mass the photon crosses, not on which material it is. That is why the parent article’s HVLs differ by a factor of more than 20 in centimetres (1.3 cm for lead, 29 cm for wood) while the mass per square metre of each HVL lies in a narrow band. Shielding Materials derives that band, 135-232 kg/m² per half-value layer, from the parent’s HVLs and densities. A heavy dense material is compact; a light one needs the same mass spread over a greater thickness. This is the reason earth, water and rubble can stand in for concrete if you pile enough of them.
Beta Shielding Has A Catch
Stopping beta particles in a high-atomic-number metal such as lead can generate secondary X-rays (bremsstrahlung). The usual advice in radiation-protection practice is to stop beta with a light material such as plastic or aluminium first and put heavy material behind it only if gamma is also present. This is recalled from general radiation-protection teaching rather than from the parent article, and for a settlement it matters mainly for an unknown strong beta source, which should not be handled at all.
Shielding Does Not Fix The Wrong Problem
Shielding addresses external radiation. It does nothing about contamination, which is material on a surface or inside a body; see Contamination vs Irradiation. A shelter wall that cuts gamma by a factor of 100 gives no protection from dust drawn in through an unfiltered vent.
Common Mistakes
- Treating “concrete stops gamma” as a verdict. It thins gamma by a fixed fraction per layer. How thick depends on how much reduction you need.
- Counting only the walls. Radiation arrives through the roof and by scatter around openings; a shelter with thick walls and a thin roof is shielded to the level of its roof.
- Using one HVL table for every source. HVLs depend on the gamma energy of the source; the parent article does not state which energy its table assumes.
Safety
A shield reduces a dose rate, it does not remove it
Seven half-value layers leave about 0.78 percent of the original rate. If the original rate was very high, that remainder can still matter over days. Without an instrument you cannot know the original rate, so use the thickest practical shield and still limit time inside.
What’s Next
- Learn next: Half-Value Layers — the arithmetic of thickness
- Combine with: Shielding Materials — what to build with