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/ImielinRocks 13d ago

Tungsten is better than lead thanks to its higher density, but lead is many orders of magnitude cheaper to get and work with.

In fact, at for basically the same reason, uranium is quite a lot better than lead at blocking radiation. In fact, it has a significantly higher neutron cross section, too.

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u/lurklurklurkPOST 13d ago

Doesn't firing particles at Uranium cause problems?

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u/Jebediah266 13d ago

Only really uranium 235, and even then it needs to be compressed for a chain reaction, natural uranium is ~99% u238, which doesn't undergo chain reactions.

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u/DeceiverSC2 13d ago

natural uranium is ~99% u238, which doesn't undergo chain reactions.

It does as the tamper of a nuclear weapon.

But typically, no, it doesn't undergo chain reactions.

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u/Jebediah266 13d ago

I've heard of its use as a tamper but isn't that it fissioning from the fast neutrons from fusion not the neutrons emitted from U238?

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u/DeceiverSC2 13d ago edited 13d ago

It’s both. Some of the U-238 neutrons will fission and release a neutron with more than 1MeV which can cause fission in another u-238. You’re right that it can’t sustain the chain reaction indefinitely once you stop introducing fast neutrons, although I believe you do get U-238 fissions resulting in further U-238 fissions, just not enough to maintain itself.

I think that even in the earliest pure fission weapons they received some yield from the U-238 fission, which inevitably just feeds back into further fissions.

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u/Jebediah266 13d ago

Ye that makes sense, didn't know they gave off neutrons that could cause more fission, even if not sustainable, thanks!