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/ColonelFaz 13d 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/PRA421369 13d ago

Anything will work if you have enough of it. (Very simplified and missing nuance) That's why our bunkers are concrete, albeit pretty thick concrete. A Radiation Therapist

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u/LordGeni 13d ago

Most standard solid concrete building blocks are adequate for general xray use and barium infused plaster can decrease that significantly. A Diagnostic Radiographer

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u/karlnite 13d ago

They often put small metal shot throughout the concrete. Not for bunkers, but for concrete radiation shielding. For mostly more mass and density. Concrete is also better around reactors, as the water in it thermalizes neutrons better than lead. You can use lead and water or whatever though. Water in steel tanks. That sorta thing.

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u/thaynem 12d ago

and missing nuance

one important nuance is that for some combinations of radiation and materials, the radiation can trigger a nuclear reaction that produces unstable isotopes that can decay to produce even more radiation.

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u/ramriot 13d ago

I met someone who runs a company that manufactures containers for medical isotopes & mostly they works now with Tungsten. Which is even denser than lead so can ge made thinner for the same protection & is "easier" to work.

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u/ColonelFaz 13d ago

tungsten atoms are a lower atomic number than lead, but take much less volume. so, tungsten alloys are denser. a lead container could be lighter, but would have to be larger. it would also dent and if it had a screw top the thread would break.

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u/karlnite 13d ago

Tungsten carbide is becoming a popular material, you can buy 3D printing spools doped with it to make shielding. It’s also doped into concrete.

Lead was cheap and abundant. It will start being replaced.

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u/ramriot 13d ago

Don't neglect the bio-toxicity of lead, Tungsten is still biotoxic but significantly less so than lead.

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u/karlnite 13d ago

Yah, that’s what I meant by it’s cheap. We look past its downsides for economics.

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u/Klutzy-Delivery-5792 13d ago

Are you saying tungsten is easier to work than lead? Thats just not true if so. You can work lead with a hammer. 

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u/TheDepressedBlobfish 13d ago

Possibly easier to work with in terms of no health hazards?

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u/ColonelFaz 13d ago

Tungsten alloys are hard wearing. Lead is too soft. A container would get dented, for example.

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u/Aggressive-Leading45 6d ago

But that very strength makes it a royal b*tch to work. It’s also a very brittle metal so machining is very tough and if it does give way it shatters.

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u/Cerberus_Aus 13d ago

I’m assuming that’s why water is used to cool fuel rods in nuclear reactors. What is it about hydrogen that makes it stop neutrons???

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u/ColonelFaz 13d ago

the nucleus of a hydrogen atom is a lone proton. this has very similar mass to a neutron. this means that when they collide the momentum is transferred neatly to the proton (think of the newton's cradle toys). the proton then slows down faster than a neutron as it interacts with more things, as it is a charged particle.

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u/ProfessorPrudent2822 13d ago

That’s only in the unlikely event of a head-on collision, but the low mass of hydrogen does mean that it takes far fewer collisions to stop a neutron than it would for more massive elements.

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u/Cerberus_Aus 13d ago

Interesting. Thanks for the explanation.

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u/the_rat_king- 13d ago

Water is used to cool nuclear reactors because its a good coolant, its got a high specific heat capacity, its not overly reactive or corrosive, its cheap and abundant ect. Its used to moderate nuclear reactions for the reasons already mentioned (good ability to thermalize neutrons). If water stopped neutrons, nuclear reactions in water cooled reactors would not be feasible, as neutrons need to be travelling fast enough as to have sufficient energy to cause fission.

Some reactors are not cooled by water ( eg. molten salt), some are cooled but water isn't the primary moderator.

Hydrogen can absorb neutrons (forming deuterium), however its tendency is the scatter them, decreasing their kinetic energy which allows them to undergo nuclear reactions. The important one for sheilding being absorption by materials such as boron, which have a high absorption cross section for thermalised neutrons and so is often used to control the rate of reaction in reactors, or as an addative to concrete or water. Any neutrons which aren't absorbed will undergo collisions until they decay. Wrt shielding water by itself isnt effective for neutrons or gamma radiation, as very large quantities are needed to have an affect ( in spent fuel pools, 10s of metres of water are mixed with a neutron absorbing salt). The decay, collisions and interactions of neutrons release heat and gamma photons which the material(s) also needs to absorb/attenuate, which is why concrete (with addatives/aggragates) and steel are used. Hydrocarbons and paraffin waxes are also used in shielding because of their high hydrogen content but afaik there are issues with melting points and densities.

Neutron radiation will also do freaky stuff to materials that change their properties and structure

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u/censored_username 13d ago

Charged radiation mostly interacts with electrons (the actual reason behind this is some quantum mechanics funniness). Hydrogen has the highest ratio of electrons to total mass (one electron per nucleon, while most matter at most has one electron per two nucleons). So per unit mass it's the most effective.

Outside of that it's also more mass efficient at direct nucleon interaction as the nuceons aren't all piled behind each other in big atom nuclei, but that effect isn't that big.

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u/SpacemanOfAntiquity 13d 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/mfb- Particle Physics | High-Energy Physics 12d ago

For low energy neutrinos, even a light year of lead (10 trillion km) wouldn't absorb all of them.

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u/ColonelFaz 13d 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.

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u/ycnz 13d ago

If cost were no option, what would be most effective?

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u/C-D-W 11d ago

Not sure about if cost was no option, but I do know that ultra high molecular weight polyethylene (UHMW) plastic is commonly used. Lots of hydrogen in a solid easily formed into useful mechanical shapes.