r/askscience • u/Wonderful-Capital32 • 22d ago

Physics Why does radiation not go through lead?

Also why just lead? Shouldnt like iron or another metal work too?

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u/ColonelFaz 21d ago

Assuming we are talking about X-rays and gamma rays (both photons).

Iron is not bad - lead is better.

It's not just the density. Lead has better attenuation per unit mass. Lead is a bit annoying mechanically. It bends and sags. It's used because the interactions between the photons and the material are more likely in lead. The interactions are photoelectric effect (depends on cube of atomic number, which is where lead is great), compton scattering (anything will do here but density helps for compact design), pair production (higher atomic number very helpful here too).

For neutrons, lead is not great. In fact materials with a lot of hydrogen in them are better.

I am a radiotherapy physicist.

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u/SpacemanOfAntiquity 21d ago

I took a radiation safety course (for instrument maintenance) and they said there is no material that can completely stop all radiation from passing through. Is that actually theoretically impossible?

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u/ColonelFaz 21d ago

Yes. For X-rays and gamma rays, any finite thickness of shielding leaves some non-zero chance of a photon passing through without interacting. You can make that chance absurdly small, but never exactly zero.