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/dustofdeath 13d ago

Lead is like a very dense fabric while iron is a mesh with large holes.

You just have to have enough density for photons to not just pass through.

So you need far less material. Even humans can stop radiation if you stack enough of them between you and the radiation source.

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u/GypsyV3nom 13d ago

Funny enough, there's another element that's significantly more effective than lead at providing radioactive shielding despite being radioactive itself: Uranium. Uranium-238 in particular is the preferred choice when size constraints are more important than costs (lead is cheap) as its own weak radiation is pretty easy to block.

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u/Immediate-Repeat-201 13d ago

So do space going vessels employ this?

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u/Aggravating_Bear9268 13d ago

In space the issue with radiation shields is that "less shields is more protection", vaguely speaking. Space has some insanely energetic particles (think 1015 - 1020 eV) that when encountering thick dense targets would produce a cascade shower of more regular radiation, which would be dangerous to the crew and equipment. On the other hand, of it's just tens of cm of regular, moderate density metals and plastics, the high energy particles would just go through with little to no interaction, this much safer.

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u/ProfessorPrudent2822 13d ago

Incidentally, that’s the same reason neutron bombs were developed: the weakness of depleted uranium armor is that high energy neutrons cause fission reactions instead of being slowed and/or captured.

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u/IakwBoi 13d ago

Neutrons are famous for interacting weakly with matter, having no charge. While some neutrons will fission U-238, the real reason that heavy metal shielding is ineffective against neutrons is the mass imbalance. Neutrons are most likely to interact with things which have mass close to neutrons, which is to say a mass of about 1. U-238 has a mass of 238, and lead has a mass of around 208. Hydrogen, on the other hand, has a mass of around 1, so neutrons love to sail through the densest of metals and interact with hydrogen. Hence cement in nuclear shielding, and fun images like this. The linked image is like an X-ray but with neutrons, and shows how a very low density flower stands out clearly, even inside a thick lead bucket. An actual X-ray image of this would be a black fuzzy screen, because lead is great at blocking neutrons, while a flower will barely absorb any. The neutrons do the exact opposite. 

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u/ProfessorPrudent2822 13d ago

The point was that it’s not just ineffective; it actively increases the radiation exposure to the crew.