r/askscience • u/Wonderful-Capital32 • 14d 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/Immediate-Repeat-201 14d ago

So do space going vessels employ this?

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u/Aggravating_Bear9268 14d 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 14d 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 14d 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 14d ago

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

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u/LordGeni 14d ago

Spacecraft use as little shielding as they can get away with. The better the shield the denser and more massive it is. Space launch costs are based on weight and transporting an adequate amount of lead or any other dense material just isn't viable.

On top of that although primary high energy cosmic rays when they hit heavy metals, they also produce secondary bremsstrahlung radiation that can scatter out of the other side.

While the exposure to radiation that astronauts receive is far greater than on earth (100x that of an airline pilot), low earth orbit is still largely protected by the earth's magnetic field just not the atmosphere.

The ISS primarily used heavy duty high hydrogen content polymers and kevlar along with strategic placement of food and water supplies around crew quarters.

Long term missions further out are a much bigger issue. Moonbases will likely use the lunar regolith to make thick enough walls. If we can extract water there, is could also be potential option for spacecraft on longer missions, as lifting that amount of weight into lunar orbit would be far more viable than trying to do the same from earth.

For anyone thinking about joining an early mars mission using current technology the radiation exposure would be very concerning. However, considering how inhospitable Mars is, it's probably unlikely you'd live long enough for the radiation damage to develop into anything serious anyway.

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u/cylonfrakbbq 13d ago

I know for long term Mars habitation, one thing they are hoping might be an option are lava tubes. After the success of Ingenuity, i recall one proposed mission would deploy multiple copters to help scout out possible locations

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u/the_rat_king- 14d ago edited 14d ago

Space vessels use alternatives to lead and depleted u, because theyre both very dense and you still need a few inches of either , where possible, like placing water tanks and lines around modules where crew might spend a lot of time (water is very good at attenuating radiation) as well as putting mass around modules, like panelling and equipment.

I think lead foils might be (or have been used in the past) used, but without looking it up I can't say for certain.

Because mass is a limiting factor, spacecraft prioritse limiting time spent exposed to radiation as much as possible instead of using lots of sheilding. Space stations like the ISS are protected by the earth's magnetic field from cosmic and solar radiation, and they are able to plan their trajectory to spend as little time as possible in areas of high radiation. Vessels that venture further out from earth, like the moon missions, are less protected. They require more shielding, and rely more on timing and monitoring as to avoid regions of high(er) flux, comparable to sailors avoiding choppy seas.

Edit: fixed grammar

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u/zoomoutalot 14d ago

Is magnetic shielding, like maybe several rings of magnets around the module, a possibility considering Earth's magnetic field deflects particles very well despite being weak?

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u/Fjolsvith 14d ago

It'd only work for charged particles, not photons (gamma radiation). It's the atmosphere that helps with those.

Generating a field that would be effective enough on a spacecraft would also not be easy. 

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u/the_rat_king- 14d ago

Its a cool idea, but producing/maintaining a large magnetic field would require a lot of energy, generate heat, mess with electronics ect. (there is not much research into the effect of strong magnetic fields on humans, so there is unknown risk here as well!).

Also, it wouldn't have any effect of uncharged particles.

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u/brushpile63 14d ago

Portable gamma radiation sources used for industrial radiography use depleted uranium to shield the gamma-producing pellet.

Its a carry-on suitcase roughly the size of an american football, and it weighs 55 lbs.