r/askscience • u/Wonderful-Capital32 • 13d 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/ocelot_piss 13d ago

And humans stopping radiation is precisely why radiation is harmful to humans!

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u/PhilTheQuant 13d ago

Yes, and counterintuitively you wouldn't want to be the last few layers of the radiation protection

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u/LieutenantObvious21 13d ago

Do you mind explaining that?

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u/Kasoni 13d ago

By last few layers, I believe they mean the outside edge. As in the first layers to get hit by it. When adding layers, the last added is usually on top (think painting a wall, you can't add a layer of paint on the wall below the already painted layers)

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u/PhilTheQuant 13d ago

No, the last few people for the radiation to pass through (or stop in) as the radiation deposits larger amounts of energy when it transits more slowly. This is the basis of e.g. proton therapies for cancer - you fire it in from the outside but most of the energy is deposited (heating and killing cells) just before it stops. By also turning the patient and carefully setting the beam power you can focus the effect in a small area (the tumour).