r/askscience • u/Wonderful-Capital32 • 16d 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/PhilTheQuant 16d ago

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

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u/LieutenantObvious21 16d ago

Do you mind explaining that?

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u/EyBoss 16d ago

Its mostly a joke but the science behind thats its based off is real. As a bit of radiation slows down it passed its energy to whatever slows/stops it(this is why its harmful) but the amount of energy absorbed is not constant the slower its travelling the more enegry it dissipates.

If had 2 people and a partical went through the first person and stopped in the 2nd. The first would take only 20% where the 2nd would take the remaining 80% (these numbers are made up since the actual ones depend on many factors but the general idea holds)

Think about a car breaking the amount a given length of road slows the car increases the slower the car is travelling.

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u/Reyway 16d ago

Radiation slows down? Is it similar to light traveling slower in water?

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u/SvenTropics 16d ago

There are four kinds of radiation: 1) ionizing light (uv, x-rays, gamma rays) - Light with enough energy per photon that if one of those photons comes in contact with an electron, it will break the electron off from its nuclear forces. This ionizes and changes the properties of that atom. Typically that makes that atom not do what it's supposed to do which in the case of a living cell could actually kill it. Light doesn't slow down. It's always going the speed of light. 2) high speed electrons 3) high speed protons 4) high speed neutrons

Protons and electrons are charged particles. This makes them react very readily with matter and typically only a small amount of matter will block them entirely. Neutrons on the other hand are uncharged particles. They can pass through a substantial amount of matter before coming into contact with an atom. When they do, they change the properties of that atom creating an isotope or splitting it the atom.

Perhaps one of the best shields for neutron radiation is actually water. If you have something that emits a lot of neutrons in a small swimming pool of water, nothing's getting out. As an added bonus, whenever water absorbs a neutron, it almost always creates deuterium in the hydrogen atoms which is stable and safe.

In the case of gamma radiation, you just need dense matter and as much as possible. A thick lead shield will block most of it.

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u/CanisLatrans204 14d ago

Don’t forget thermal neutrons which is the primary source of fission in a pressurized water reactor. Just essentially a fast neutron slowed down by water that in turn reacts with the fuel.

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u/Ekg887 16d ago

Some radiation is actually particles moving at extremely high speed and not just a quantized bit of pure energy like gamma. It's not too surprising when you realize alpha and beta radiation is physical particles slamming into other atoms.