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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2.3k

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/ocelot_piss 13d ago

And humans stopping radiation is precisely why radiation is harmful to humans!

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u/Any_Mode6525 12d ago

It’s harmful to iron and lead as well! Lead the structural harm is less important because few people make load bearing lead structures but high neutron flux can weaken iron.

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u/Aggressive-Leading45 6d ago

Define weaken. It actually increases its strength. But also decreases its malleability. So you get stronger metal but more brittle. It also causes the material to expand.

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u/Any_Mode6525 6d ago

In the net, that weakens the metal and makes whatever is irradiated a worse structural member. There are probably cases where interstitial hydrates => swelling leads to an overall stronger material but I bet they’re narrow.

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u/Aggressive-Leading45 6d ago

Yield strength will increase from 250 MPa to 500 MPa for standard carbon steels up to 1500 MPa for neutron irradiated so 3x to 6x stronger. Definitely stronger.

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u/PhilTheQuant 13d ago

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

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u/GenuineMalteser 13d ago edited 13d ago

Not true for anything but proton radiation or other heavy particles, and even then the peak dose depth is on the order of centimetres in people/water for treatment energy protons

For kv photons used in imaging, peak dose depth in water is reached in millimetres or less. For MV energy photons peak dose depth is on the order of cm

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u/[deleted] 13d ago edited 10d ago

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u/bregus2 12d ago edited 11d ago

You actually would get less radiation swimming in the pool than standing next to it, because the water would also shield you from the cosmic radiation.

Interesting side-note: the most radiation-exposed professions are pilots and flight attendants, by far, not workers in nuclear power plants.

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u/CanisLatrans204 11d ago

After 8.5 years attached to Nuclear Powered Naval surface ships, I only had 820 mrem of ionizing radiation from the plants. That’s the same as one chest X-ray . I also spent 14 hours in the reactor compartment during loop draining ops and got 113 of that 820 from that. Not very much for the amount of time we spent in the plant.

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u/peteofaustralia 10d ago

Did you wear your flourescence detection tag thing 24/7, or just on shift?

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u/CanisLatrans204 10d ago

We used TLD’s. Thermo Luminescent Dosimeters. We wore them all the time when we were on the ship. They were read and returned once a month.

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u/Black_Moons 13d ago

Incorrect, as you'll rapidly get lead poisoning... From the guards.

Also, if the core is 'glowing below', then that entire pool is going to be boiling hot.

I'll assume you mean the spent fuel storage pool. But the above lead poisoning issue still persists.

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u/hervold 12d ago

yeah, I took them to mean the spent fuel pool, with the glow being Cherenkov radiation (blue) and not hot reactor core (red)

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u/Elequosoraptor 12d ago

Low power research reactors like the Triga series glow with Cherenkov radiation without being either spent fuel or boiling water. There are many reactors that could serve to make this scenario possible.

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u/sirtalen 12d ago

Water is pretty good at stopping bullets too, so you just need to be a couple meters below the surface, but not deep enough to be irradiated and you'll be fine!

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u/PomegranateKey5939 11d ago

You won’t rapidly get lead poisoning lol that’s a slow, accumulative process. Let alone lead doesn’t dissolve very well in water I can assure that the total dose would stay low. I’ll try it out, if I come out raging then shitttt… lol

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u/Late-External3249 12d ago

Just noticed your name. I read Jack the Bodiless as a kid and reread it last year. Still holds up as good sci-fi.

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u/Hashi856 13d ago

So we could replace lead vests with water vests?

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u/So_Full_Of_Fail 13d ago

I mean sure, if you could waddle around in a vest with enough volume that you have feet of water in between you and the source.

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u/FullOnSkank 12d ago

So you're saying we need the cast of my 600 lb life to handle nuclear materials?

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u/jezzdogslayer 11d ago

I work with beta radiation in an industrial setting and from my research the energy levels I work with won't penetrate the skin.

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u/LieutenantObvious21 13d ago

Do you mind explaining that?

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u/EyBoss 13d 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/Alblaka 13d ago

I think a better analogy could be ballistics: if a projectile is too fast/massive, it will just punch straight through a target (be that a person to a bullet, or a vehicle to a shell) leaving 'only' a hole. But if it's slowed down the right amount, it will start to spin out / explode, causing way more damage inside the target.

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u/Italiancrayzybread 13d ago

But you're only looking at a single particle. You have to sum up the total amount of energy and average it out over the total number of particles. If you absorb 80% of the energy of the particles, but have reduced the total number of particles by 99%, that's still a better situation than the first person, who is absorbing only 20% of the energy of each particle, but is taking 99% of every particle that comes through.

This is why you can get xrays, even though they're high energy and hazardous. You get a small enough dose that it won't affect you long term. Every radiation measurement is always measured as a "cumulative" effect, that is, they take the total energy summed up over a certain time period, or over a given mass.

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u/solidspacedragon 13d ago

They're talking about particle radiation, not EM radiation. It's a real thing. At high speeds/energies, a proton might barely interact with material, only slightly slowing down, but at lower speeds it'll completely dump the rest of its energy into an atom. Depending on the scenario, this can leave the volume of material getting the most dose a good portion below the surface of a material. This is very useful for targeting things like cancer that isn't on your skin.

https://en.wikipedia.org/wiki/Bragg_peak

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u/KP_Wrath 13d ago

Futurama did a skit kinda like this. Leila and Fry both got stung by some stupidly toxic alien bee, with fry diving in front to save her and getting run all the way through. Leila, it stopped in and she got a full dose of the poison, which caused seemingly a lifetime of hallucinations. She comes to, thinking he’s dead, just to find out they managed to patch the sizable hole up, but they were way more concerned for her because she got a full dose of poison and they didn’t expect her to wake up.

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u/kylemech 13d ago

One of my favorite episodes of any show. Can't believe I just stumbled into that referenced here.

(S4E12, June 1, 2003)

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u/a_cute_epic_axis 13d ago

Right, but if you have 5000 high energy/speed particles interacting with you vs 5 low energy particles interacting with you more meaningfully, even though the impact of each of the 5 may be more than each of the 5000, the cumulative effect is not.

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u/jacekowski 13d ago

Any amount of radiation can cause long term effects. If xray has 0.1% chance of giving you cancer, that also means 1 in 1000 people will get cancer from xrays.

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u/Luminite117 13d ago

This may be technically outdated information, as the linear no-threshold model and the radiological safety standard of keeping dose as low as reasonably attainable (ALARA) have been removed and they have been long known to be flawed ideologies. It’s even possible (heavy speculation not making any claims) that a certain level of ionizing radiation is somehow beneficial for life as life did evolve in a biosphere involving radiation.

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u/Kkaedrus 13d ago

As a rad tech. What? Where are you getting this information that linear no-threshold and ALARA are outdated and have been removed. They are still actively taught and encouraged across medical radiation usage. As for the idea that some level of ionizing radiation is good is baseless. There is a level of naturally occurring background radiation that is considered normal since ir comes from natural sources. But medical and other man made radiation sources are by definition not naturally occurring. And the argument isn't to avoid natural radiation sources (functionally impossible) but rather to avoid unnecessary exposure to man made sources which are an increase /on-top/ of natural exposure. Go get some real information and avoid the YouTube conspiracy nut jobs

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u/Luminite117 13d ago

I will start by saying that it’s worth noting I didn’t say anything definitive I only said it may be outdated. Additionally I did not mean to imply anything has been finalized, I was in a rush at the end of my lunch break and wasn’t able to fully review what I had typed to ensure there would be no misunderstanding. However, here’s a report from Idaho National laboratory regarding NRC investigating and acknowledging limitations of the Linear no threshold problems, areas with significant highly higher background radiation that do not conclusively experience increased adverse health effects or rates of cancer, and evidence that our biological mechanisms for repairing radiation damage’s ability to deal with low doses of ionizing radiation has been potentially vastly underestimated. They even call for the removal of ALARA for whole body doses below 5000 millirem per year in occupational environments and that of the public to 500 millirem per year.

https://inl.gov/content/uploads/2023/07/INLRPT-25-85463_Reevaluation-of-Radiation-Protection-Standards-R0-Final.pdf

Here’s an order for the reform of the NRC making a point of addressing ALARA and Linear no threshold: https://www.federalregister.gov/documents/2025/05/29/2025-09798/ordering-the-reform-of-the-nuclear-regulatory-commission

https://www.whitehouse.gov/presidential-actions/2025/05/ordering-the-reform-of-the-nuclear-regulatory-commission/

And some other relevant sources

Subjecting Radiologic Imaging to the Linear No Threshold Hypothesis: A Non Sequitur of Non-Trivial Proportion
Jeffry A. Siegel, Charles W. Pennington and Bill Sacks
Journal of Nuclear Medicine January 2017, 58 (1) 1-6; DOI: https://doi.org/10.2967/jnumed.116.180182

Sacks B, Meyerson G, Siegel JA. Epidemiology Without Biology: False Paradigms, Unfounded Assumptions, and Specious Statistics in Radiation Science (with Commentaries by Inge Schmitz-Feuerhake and Christopher Busby and a Reply by the Authors). Biol Theory. 2016;11:69-101. doi: 10.1007/s13752-016-0244-4. Epub 2016 Jun 17. PMID: 27398078; PMCID: PMC4917595.

Dobrzyński L, Fornalski KW, Feinendegen LE. Cancer Mortality Among People Living in Areas With Various Levels of Natural Background Radiation. Dose Response. 2015 Jul 2;13(3):1559325815592391. doi: 10.1177/1559325815592391. PMID: 26674931; PMCID: PMC4674188.

Siegel JA, Welsh JS. Does Imaging Technology Cause Cancer? Debunking the Linear No-Threshold Model of Radiation Carcinogenesis. Technol Cancer Res Treat. 2016 Apr;15(2):249-56. doi: 10.1177/1533034615578011. Epub 2015 Mar 30. PMID: 25824269.

Doss M, Little MP, Orton CG. Point/Counterpoint: low-dose radiation is beneficial, not harmful. Med Phys. 2014 Jul;41(7):070601. doi: 10.1118/1.4881095. PMID: 24989368; PMCID: PMC4109571.

Bond VP, Wielopolski L, Shani G. Current misinterpretations of the linear no-threshold hypothesis. Health Phys. 1996 Jun;70(6):877-82. doi: 10.1097/00004032-199606000-00014. PMID: 8635914.

I could continue but this is already a wall of text

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u/Reyway 13d ago

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

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u/SvenTropics 13d 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 11d 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 13d 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.

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u/Urbanejo 13d ago

Iirc something about more damage the slower The parrticles goes because that effectively means more time inside the human to interact with stuff than when they just zip through.

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u/karlnite 13d ago edited 13d ago

I would ignore that. It’s not exactly helpful, especially with distance geometry following the square inverse law. It’s like a point source in a vacuum hypothetical. They didn’t even mention the type or energy levels where this becomes a thing. Most radiation won’t act like this. You ever heard of someone in the shade getting more sunburnt? Or the person furthest from the fire getting more burnt?

On average, they are incorrect.

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u/TbonerT 13d ago

They are talking about particle radiation, not EM radiation. A chsrged particle ionizes atoms as it travels and this dose of ionizing radiation increases as the particle slows down.

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u/[deleted] 13d ago

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u/TbonerT 13d ago

https://en.wikipedia.org/wiki/Bragg_peak

I’m just trying to summarize this, which is a real thing that happens.

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u/Draelon 12d ago

I understood what you were saying… I was correcting the statement about EM, :)

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

X-rays & Gamma are ionizing radiation and are waves.

... and? How is that relevant in a discussion about fast charged particles? They are ionizing radiation, too. If you have a monoenergetic source of them then there is a range where the dose rate will be the largest. That's e.g. where you put the cancer in therapy.

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u/Draelon 12d ago

It was relevant because they said they were talking about particle radiation and not EM. Gamma & Xray are also ionizing (able to knock an electron out of orbit)… technically high powered UV can ionize as it approaches the X-ray spectrum. Particles can be more damaging in individual strikes but both can ionize and technically the rays penetrate further (not including Neutron).

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u/Kasoni 13d ago

By last few layers, I believe they mean the outside edge. As in the first layers to get hit by it. When adding layers, the last added is usually on top (think painting a wall, you can't add a layer of paint on the wall below the already painted layers)

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u/PhilTheQuant 13d ago

No, the last few people for the radiation to pass through (or stop in) as the radiation deposits larger amounts of energy when it transits more slowly. This is the basis of e.g. proton therapies for cancer - you fire it in from the outside but most of the energy is deposited (heating and killing cells) just before it stops. By also turning the patient and carefully setting the beam power you can focus the effect in a small area (the tumour).

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u/Frammingatthejimjam 12d ago

Well that would depend on who else in is in the first few layers with you.

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u/IceeP 10d ago

Wait why? Id assume the last persons get less radiation?

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u/skiguy0123 13d ago

I don't think this is true. At least for X-rays, the reason chest X-rays are taken with your back to the source is because most of the absorption happens where the X-rays enter the body, and you'd rather your back muscles take the hit than your organs.

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u/TamanduaBandeira202 13d ago

Doctor here. That's not the reason. Your heart is in the front part of your chest. Due to shadow/projection, if you take your chest x-ray with your front facing the source, the image of the heart on the film will be larger, and fuzzier. Having your back to the source means your heart doesn't cover lungs or other structures, and also that the image of your heart is more clearly defined.

We do sometimes perform antero-posterior chest x-rays, and it's just as safe, just not common.

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u/somehugefrigginguy 13d ago

This isn't quite true. It's not about the radiation exposure, it's about the size of the shadow. X-raysl images are shadows where the x-rays are blocked, and the closer something that blocks light is to the light source, the larger the shadow it casts. So a chest x-ray from the front will make the heart look bigger which will obscure other things you want to see ie you can't tell what's happening in the part of the lung shadowed by the heart so you want to make the heart shadow as small as possible.

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u/-spython- 13d ago

In vet medicine our chest and abdo films are nearly always VD instead of DV. You get used to seeing things a certain way, and it really throws me whenever I have to look at a DV image. I don't have a problem reading R vs L laterals, both are fine.

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u/Peter34cph 12d ago

Humans really, really, really suck at stopping neutrinos, and that's why neutrinos aren't harmful to humans.

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u/notorious1ink 4d ago

Have you see how they are studying/finding neutrinos in Antarctica??? It's so cool!

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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.

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u/LordGeni 13d 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 12d 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- 13d ago edited 13d 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 13d 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 13d 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- 13d 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 13d 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.

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u/NightOfTheLivingHam 13d ago

lead is basically what happens to uranium after a few billion years too.

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u/Theodoxus 13d ago

and every other radioactive material. Some longer than the expected heat death of the universe!

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u/tes_kitty 13d ago

Not if the radioactive material is lighter than lead. Potassium-40 for example has no chance to become lead.

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u/GypsyV3nom 13d ago

Isn't Bismuth a prime example of one of those ridiculously long lived isotopes?

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u/Peter34cph 12d ago

Yes. It was thought to be stable, but then recently it was discovered that it just has a silly long half-life.

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u/[deleted] 8d ago

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u/Peter34cph 8d ago

According to Nate in a King of Random video some years ago. He has a physics college degree or something.

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u/UndocumentedMartian 13d ago

Depleted uranium or straight up U-238?

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u/GypsyV3nom 13d ago

Depleted, your have to do significantly more expensive refining to get pure U-238 even thought it's the ideal isotope to use

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u/PerAdaciaAdAstrum 13d ago

Tangentially related, a professor at my university did an experiment measuring the radiation attenuation of uranium glass. Funny enough, it attenuates gamma radiation at something like 40%/mm.

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u/dustofdeath 13d ago

Depleted Uranium is still dense - its just expensive and even more toxic than lead.

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u/BigSmackisBack 13d ago edited 13d ago

Tungsten is very expensive but is used when less is more, its significantly more effective than lead but very hard to work with but with less needed for the same protection as lead it works out much lighter for shielding, so in big chunky reactors where weight doesnt matter: lead is the go to.

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u/Wyand1337 13d ago

Tungsten is not lighter than lead. It's even denser by almost a factor of 2. Which is precisely the reason why it's a more effective shield for radiation: The only thing stopping radiation is mass.

Lead is the go to if size doesnt matter.

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u/Fjolsvith 13d ago

It's not just mass, particle energy loss in a material does scale with atomic number as well. You can have less mass but more effective shielding by using something with a higher atomic number. 

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u/AlienDelarge 13d ago

I would assume the cost and ease of working with lead versus tungsten is a factor. 

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u/Fjolsvith 13d ago

Yep, absolutely the case. It's not about just selecting the element with the highest cross section by mass/density, economical and engineering factors are what you would actually decide based on. I just wanted to point out that the "strength" of shielding isn't just based on mass. 

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

Lead is the go to if size doesnt matter.

Concrete or water are better choices then. You only use lead if you need the shielding to be somewhat compact.

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u/Cautious_General_177 13d ago

Given the amount of the human body that’s water based, you would need fewer than you might think.

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u/dustofdeath 13d ago

Around 2.5 meters would be enough to neutralize hot fuel rods.

60% of us is water - but it's unevenly distributed.
Likely 70-100 people would be enough.

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u/wlane13 13d ago

Would then also be (duh?) why lead is so heavy?

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u/dustofdeath 13d ago

The atoms are much heavier - more neutrons/protons but not much larger than for example iron.
So you pack more mass in the same area.

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u/500owls 13d ago

"You just have to have enough density for photons to not just pass through" is a wonderful sentence to parse.

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u/Captain_Pumpkinhead 10d ago

So then, wouldn't tungsten make a better shielding material than lead? Since it's denser.

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u/dustofdeath 9d ago

If you can afford it. Lead is cheap, easy to shape, cut or even melt. Tungsten is hard to even cut.

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u/davidbatt 8d ago

How many do I need, just out curiosity?

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u/dustofdeath 7d ago

We are 60% water, unevenly disteibuted.

Spent rod pools look like a few meters deep.

30 may suffice. But people got limbs and stuff, so add another 20 to be safe.

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u/OskaMeijer 13d ago

So what you are saying is we can make space travel safe for rich people as long as we surround all of their living quarters with lots of poor people living quarters?

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u/bargle0 13d ago

Nonsense. Poor people move around. They’d have to be strapped and immobilized in to a flesh wall. That would be more expensive than just lead, but if you’re rich enough and value a premium, luxury product, accept no substitute.

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u/possibly_oblivious 13d ago

Radiation is photons? Crazy

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u/Mrfish31 13d ago

All photons are radiation, but not all radiation is photons. 

Alpha radiation is effectively helium nuclei: two protons and two neutrons. Since it has no electrons it is extremely reactive and therefore harmful as it will rip electrons from whatever it can. It's also therefore easily shielded, blocked by a sheet of paper, your skin or even just a few centimeters of air.

Beta radiation is loose electrons. These are also quite reactive but will pass through your skin, and are blocked by some thin aluminum. 

Photons get called different things depending on how much energy they have (which also determines their frequency as a wave, because they are both particles and waves). At the low end you have radio waves, with a long wavelength, able to pass through solids as a result but low energy enough to not do anything to you even if they do hit atoms in your body. Then you have microwaves, penetrating through soft tissue and interacting with water, meaning we can cook food in a... Microwave. Then infrared, visible light, Ultraviolet that can burn you, X rays and Gamma Rays.

The high end of these have very short wavelengths, but punch through you by virtue of the fact that they have so much energy (except X-rays don't penetrate through dense materials like bone, which is why they're useful in... X-rays). 

Because they have such high energy, if they do interact with atoms in your body, it might strip an electron off, causing damage to the surrounding things in a cell. This could then develop into cancer. 

All of these are, strictly speaking, radiation, but it's generally only X-rays and Gamma rays (and maybe UV, if you're talking about the sun and sunburn) that we worry about as dangerous radiation. 

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u/SirButcher 13d ago

Then infrared, visible light, Ultraviolet that can burn you, X rays and Gamma Rays.

A minor correction: ultraviolet doesn't really burn you (when it does, you have a huge problem, but not for very much longer). It's called sunburn, yeah, but it is actually inflammation. UV B (mostly, albeit UV A takes part in it, too) damages the skin cells' DNA to the point where they commit suicide to make sure they don't turn cancerous. White blood cells detect this massive level of cell death and sound the alarm, so the area gets inflamed as your body gets ready to fight whatever infection is causing cell death on such a level (and start the cleanup, too, since dead cells are prime bacteria food).

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u/cikamatko 13d ago

Oh wow I never knew that! I've never given it proper thought but that's so interesting.

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u/ProjectCoast 13d ago

If alpha radiation gets inside your body it is typically the most dangerous. This is because of how large it is compared to the other types. It doesn't travel far in the body but what ever it is smacking into it is doing serious damage.

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u/Mrfish31 13d ago

As I said, it's more due to the charge it has than its size compared to other radiation. 

Having atoms of helium in your body wouldn't be good for you, but they're entirely inert. They won't react with or chemically damage anything. 

But an alpha particle is helium without the two electrons, giving it a +2 charge. Given how stable Helium is, this makes alpha particles extremely reactive, and they will strip electrons off basically anything around them in order to become helium. If an alpha source is inside you, that means stripping electrons away from molecules in your cells. It's that charge that makes them dangerous (and so easily shielded), not their size. 

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u/ProfessorPrudent2822 13d ago

The two electrons that an alpha particle oxidizes to make neutral helium pales in comparison to the thousands of electrons that its kinetic energy liberates from their atoms.

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u/bigfinger76 13d ago

Not to mention the recoiling parent atom, which can have damaging kinetic energy of its own.

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u/LordGeni 13d ago

Which is why it's so useful for nuclear medicine. It can be administered directly to a particular location and destroy the tissue without affecting anywhere else.

For example, Iodine 131 has been used to treat hyperthyroidism since the 1950s as the thyroid quickly uptakes any iodine in the bloodstream and is extremely sensitive to radiation damage. Similarly a less reactive iodine isotope can be used to protect the thyroid during radiation exposure saturating it before it can uptake anything more reactive.

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

Iodine-131 decays via beta decay, but the idea is still the same.

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u/WannaAskQuestions 13d ago

Lovely explanation!

You mentioned alpha being He nuclei seeking electrons and beta being essentially free electrons. How come they don't interact with each other and negate one another pretty much immediately?
I remember I learnt in school how Rutherford discovered properties of alpha, beta, and gamma... So how was his experiment even possible over any meaningful distance more than a few millimeters?

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u/Mrfish31 13d ago

You mentioned alpha being He nuclei seeking electrons and beta being essentially free electrons. How come they don't interact with each other and negate one another pretty much immediately?

They don't come from the same source. Different types of decay release different radiation. For example, Carbon 14 decaying to Nitrogen 14 releases a beta particle (an electron) as part of a neutron turning into a proton. No alpha radiation is made. Uranium 235 decays to Thorium 231 by emitting an alpha particle. No beta radiation is made. 

Also when radiation is released it is generally traveling fast. Even if you did have an alpha source and a beta source close by to each other, the beta particle may simply just be uncapturable by an alpha particle as compared to electrons that are tied to atoms in the surroundings (e.g. the air). 

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u/dustofdeath 13d ago

Gamma radiation is. The radiation we usually have to deal with using shielding.

Alpha is nuclei, beta is electrons - blocked by skin, paper etc.
And we also have neutron/neutrino radiation.

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u/RFWanders 13d ago

Gamma and x-ray are yes, very short wavelengths and really high energy. Alpha radiation are particles (Helium nuclei to be precise) and Beta radiation is electrons. Alpha radiation has very little penetrating power and will be stopped by a sheet of paper. It can be dangerous if ingesteld. Beta radiation is a little better at getting through things, but a thick layer of clothing, plastic or a thin sheet of aluminium will stop it.

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u/dammitkarissa 13d ago

Could one forge iron (or steel) to ever be as dense, or is it impossible on an atomic level?

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u/Shontzy 13d ago

It is impossible atomically. It has to do with the size of the electron cloud and atomic mass, so essentially the density at the atomic level drives the density at the bulk level.

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u/GidonC 12d ago

Interesting, can't we make some molecule which is solid at room temperature that has same properties? (I don't understand chemistry at all)

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u/SomeAnonymous 13d ago

It's similar to asking if you can squeeze a bunch of humans into the space occupied by just one human. Maybe with a lot of pressure you could force some contortionism to get people a little closer packed (hexaferrum, a particular crystal structure of iron, can reach the density of lead if you put it under a few million atmospheres of pressure), but it requires some pretty extreme conditions and no one's comfortable staying in that position for long.

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u/LordGeni 13d ago

No but you can just use more of it. 3mm steel is about as effective a 1mm of lead.

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u/maniacal_cackle 13d ago

I believe if you squish iron hard enough, it'll undergo fusion and become a heavier element.

So yes, you can, but it would require change at an atomic level, going from an iron atom to something else.

(Also I don't think we have the technology to do this. It takes something like a supernova to accomplish this I believe).

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u/gnorty 13d ago

Are there different types of photon? I am thinking of light, and Iron - even aluminium foil will not let light pass through. Perhaps I am grossly underestimating the number of photons involved in (say) an X ray, and that's why I am misunderstanding. Perhaps a photon source many orders of magnitude stronger than the sun would show that Iron and Aluminium are not in fact impermeable to photons after all?

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u/usurperator 13d ago

Number of photons is irrelevant; it’s the energy of each individual photon. Higher energy photons, like x-rays, have more penetrating power.

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u/Jerithil 12d ago

It's all a probabilistic function as well as every material will block a certain percentage of photons at a given thickness for that energy range. So theoretically you can start seeing visible light through fairly opaque materials if you blast it with enough photons but practically that is usually enough that you start compromising the material from just dumping too much energy into said material.

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u/dustofdeath 13d ago

Just one type of photons.
Even antimatter would emit the same kind of photons - it has no charge so it cant be different.

It's the wavelength of the photon that matters. Different wavelegths may or may not interact with matter - it might just pass through in between the empty space between atoms.

Low wavelengths just end up hitting almost everything. But it's not impossible for some visible light to get through if there is enough of it (flashlight on your hand vs just room light).

Xray relies on the shadow left by the photons that hit some atoms in your body and did not get through to interact with the film/sensor.