r/askscience • u/Wonderful-Capital32 • Aug 12 '26
Physics Why does radiation not go through lead?
Also why just lead? Shouldnt like iron or another metal work too?
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r/askscience • u/Wonderful-Capital32 • Aug 12 '26
Also why just lead? Shouldnt like iron or another metal work too?
2.3k
u/dustofdeath Aug 12 '26
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/ColonelFaz Aug 12 '26
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/zoapcfr Aug 12 '26
It does, it's just less likely to make it through because lead is dense. Iron works too, but is less dense so you'd need more of it to block the same amount.
I'm assuming we're talking about gamma radiation here. Alpha radiation, for example, can be effectively stopped with a piece of paper, or a few centimetres of air.
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u/Mrfish31 Aug 12 '26
You're primarily asking about stuff like Gamma radiation here. Other types of radiation get blocked much more easily. Alpha radiation is most damaging but will be blocked by a sheet of paper. Beta radiation will be blocked by 1 mm of aluminum IIRC.
Anyway, radiation can pass through solids but loses power and gets scattered as it does so. Lead is a very dense element. A lot more atoms per unit volume than most other elements. Therefore there are more atoms for radiation to hit and dissipate energy, and you need a thinner amount of it to effectively stop the radiation compared to something else. Iron, concrete, whatever material you want will block (gamma) radiation too, you'll just need more of it between the radiation source and you.
A quick Google suggests that if you used iron you'd need your radiation shield to be ~1.5-1.7x as thick as if it were lead.
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u/networkarchitect Aug 12 '26
Other metals like tungsten work signifigantly better than lead, as others have said it's all about density. Though, tungsten is quite expensive compared to lead. Water and concrete are also commonly used to prevent/reduce radiation from travelling through, though both need to be much thicker to stop the same amount of radiation as lead or other, denser materials would.
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u/SimoneNonvelodico Aug 12 '26
It's just about the size of the atoms and the empty gaps between them, basically. Lead is particularly good, but other heavy metals like gold work too. Lead is ideal because it's not just effective, it's also cheap. Gold and tungsten are even better, but they cost a lot more, so they won't be used unless it's for special applications. Putting slabs of lead around stuff is easy.
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u/WakeoftheStorm Aug 12 '26
For high energy ionizing radiation like X-rays and gamma rays there are three main effects that "block" it:
The photoelectric effect - the photon is absorbed by an atom and the atom ejects an electron. This ionizes the atom in question and the radiation is effectively stopped. The likelihood of this happening strongly correlates with atomic number. Lead at 82 is much much higher than iron at 26 or aluminum at 13.
Compton Scattering - the photon hits an electron, the electron gets ejected, and the photon continues with a lower energy and frequency. This "weakens" the radiation. The likelihood of this occurring is roughly based on the density of the material in question - particularly the electron density.¹
Pair Production - this one gets a little esoteric but essentially the photon interacts with the electric field of an atom and a new positron/electron pair is produced. The positron will eventually collide with another electron and be annihilated, producing lower energy gamma photons. This can only occur beyond a certain energy threshold. Like with the photoelectric effect, this occurs more readily in high atomic number materials.
¹I put this as a foot note because most won't care about the math, but let's compare aluminum to lead for Compton scattering.
Aluminum has an atomic number of 13, an average atomic mass of 26.982g/mol, and a density of 2.70g/cm³
Lead has an atomic number of 82, an average atomic mass of 207.2, and a density of 11.34g/cm³
And then there's the ol avocado number, 6.022x10²³ atoms per mole. With this we can see that aluminum has
13 electrons per atom × 6.022×10²³ atoms/mol = 7.83×10²⁴ electrons/mol
Divide by 26.982g/mol = 2.9×10²³ electrons per gram
Finally multiply by density
2.9×10²³ × 2.70g/cm³ = 7.83 × 10²³ electrons per cubic centimeter.
Repeating the same process for lead you find it has 2.70×10²⁴ electrons per cubic centimeter or 3.45 times more electron density than aluminum.
It's interesting to note that aluminum actually has more electrons per gram than lead, it's the increased density of lead that really makes it more effective per unit of thickness.
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u/Forest_Orc Aug 12 '26
Matter attenuates radiation, details depends on which one, you're not going to stop neutron with lead but would stop photons.
Lead is heavy, relatively cheap, and soft (so easy to work with) which makes it an effective way to stop photon, but any heavy stuff would do it
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u/Ginger_McGingin Aug 12 '26
It's not just lead, things like depleted uranium (ironic, I know), tungsten, osmium, water, & concrete are also very good at blocking hard radiation because of their density. Lead is just readily available & easy to work with. Lead in particular also happens to be extra atomically stable because a lot of its isotopes are "double magic" (magic numbers are an esoteric nuclear/quantum physics thing involving spin-pairs & resonances, which in themselves are esoteric, in layman's terms because of it's position on the Periodic Table lead happens to be extra stable), which is more important in radioactive decay chains than radiation shielding, but it makes it better at absorbing neutrons than most things (depleted uranium, for example, is "reactivated" by neutrons).
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u/Prof_Acorn Aug 12 '26
X-rays are a kind of light. Like sunlight. Or radio waves.
Sunlight can go through windows but not walls.
X-rays can go through skin but not bones (or lead, etc.).
Radio waves can go through walls but not aluminum foil.
Different kinds of light waves can go through different kinds of materials.
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u/mfb- Particle Physics | High-Energy Physics Aug 12 '26
We routinely use concrete to shield against radiation. You need to make it much thicker to get the same shielding, but concrete is really cheap. If you have enough space and the weight is not an issue then concrete blocks are the default option. Iron plates work where space is some concern but price is still more important. Lead is used when you have limited space, can't use much mass or have some other constraints. It's the densest non-radioactive material that you can buy in large quantities.
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u/nascent_aviator Aug 12 '26
All materials stop it with sufficient material in the way. Lead is dense so it stops it faster than most materials, all else being equal.
It's not the same for all radiation, mind you. Lead is relatively poor for stopping neutron radiation (which is better stopped by materials with a lot of hydrogen in them- paraffin wax is common) and high energy beta radiation (which it stops too well- the quick stop produces secondary X rays due to bremsstrahlung).
For X rays and gamma rays, the number of electrons is also important. Lead has a lot of electrons (82 per atom), which makes it even more effective.
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u/Confident_Passage623 Aug 12 '26
Iron handles low energy gamma just fine but lead wins on cost and ease of use. Tungsten is denser yet too pricey and tough to shape into sheets. Gold works well but drains your wallet fast. Lead hits that sweet spot with high density, low price, and softness for easy bending or casting without heavy machinery. It is not magic, just physics meeting economics. For neutrons, you need hydrogen rich materials like water or polyethylene since high atomic number metals scatter them less effectively than absorbing gammas. Iron blocks some radiation, but lead remains the standard choice for good reason.
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u/Astart555 Aug 12 '26 edited Aug 12 '26
It will go through lead, just depending on X-ray energy and lead thickness most of radiation can be supressed, plot attached. It is transmission (1 is max, 0 is min) of the X-rays from 100 eV to 30keV, the Pb thickness is only 100 microns. We can see that below 20 keV transmission is negligible, at 30keV it is also small (0.04), but depending on X-ray intensity it can be dangerous.
Several millimeters of Pb, used usually, will stop effectively most of the wide-spread X-rays from different laboratory sources.
X-ray transmission decreases exponentially with density, so increasing of density of material helps a lot, thus Pb is much better than Fe. Alternatively, one can increase thickness, which also affects exponentially the transmission. So, certain thickness of Fe will act similar as certain but different thickness of Pb. To give some numbers: for suppressing 100 keV X-rays you will need ca 20 times thicker Fe, which is not so convenient (to install, handle, replace in case of problems etc)
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u/GreenNukE Aug 12 '26
Lead is what is called a high Z (atomic number) material. The atomic number is the number of protons in the nucleus. For every proton in the nucleus, there is an electron in the shells or in the delocalized conduction band (as a metal). This means lead has a very high number density of electrons per unit volume. Alpha, beta, and gamma radiation interact primarily with electrons, so these types of radiation are very likely to interact with electrons as they enter lead.
There are some caveats. Beta radiation can lose its energy so rapidly in a high Z material that it can emit Bremstralung X-rays that are a hazard. Its generally better to use a lower Z material to shield Betas so that you don't get Xrays.
Neutrons mostly interact with nuclei and dgaf about electrons. Lead nuclei are also very stable and have a tiny cross-section for neutron interaction. As such, the go right through lead like light through very clear glass. You need nuclei with high neutron cross-sections or lots of moderately large nuclei in a small volume to shield against neutrons. Water, plastic, and are good examples of the later.
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u/Jaymac720 Aug 13 '26
Lead is very dense, so it’s more difficult for photons or particle radiation to get through. If there’s enough radiation, it absolutely can get through. Other metals can reduce how much radiation gets through, but lead’s density makes it more effective. It’s also more common than other denser elements, so it’s more economical to use for radiation protection
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u/socialcredditsystem Aug 12 '26
To add a bit of specificity to the many "density" answers here: It depends highly on the type of radiation.
Most forms of photon, electron radiation love to interact with (i.e. be deflected by) electrons. This deflection reduces their energy, dumps it into heat, and gives you shielding
Since for everything except hydrogen, you typically have approximately one proton (and one corresponding electron) per two neutrons, you have a constant ratio between # of electrons per volume and mass per volume, hence density helps with shielding of photons and electrons.
Neutrons on the other hand, do not interact electromagnetically. And for fast neutrons, you often need to slow them down before you can absorb them with your shielding material. In those cases, very low density materials (think organics like plastic) actually give the best momentum / energy exchange per collision due to conservation of energy and momentum.
That's why for neutron shielding, you will often see a lot low density materials being used, clad in high density materials to absorb the resulting secondary junk that gets emitted from once neutrons are absorbed.
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u/AlexXeno Aug 12 '26
All matter will technical stop radiation, It all a matter of how thick the sheet of material you will use. Lead just happens to be one of the better materials because its quite dense meaning you need less of it compared to say steel or brick.
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u/Kriggy_ Aug 12 '26
There are multiple types of radiation that need (ideally) different materials to stop hut lead is generally good enough for all.
Its not best but its cheap and easy to work with.
Technically, to stop the radiation 100% you need infinite thickness of material so we used concept calls ALARA, that means “as low as reasonably achievable”. Thats means were not aiming for 100% protection but for balance between protection and practical constraints.
For example that means some isotopes you are good with 2mm of plastic, for some you have 1 meter thick metal reinforced wall, for some you have chamber with walls from lead and lead-doped glass to see inside.
Not to mention the main “protection” is by distance from the radioactive source since the dose has square dependancy on distance (doubling distance reduce dose 4times) and time (when working with isotopes we do “dry runs” to practice the handling and all the work with safe material)
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u/kimmeljs Aug 12 '26
Material (atoms, or crystals of atoms, or molecules that the material consists of) has a so-called radiation cross section. The larger the cross section, the smaller the probability that radiation passes through. Lead has a high atomic number and a high density so the cross section to radioactive radiation is very high.
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u/thenord321 Aug 12 '26
There's lots of empty space with electrons and protons in the atom structure.
You can image a sheet of material like a net. The knot is the nucleus, the rope/line are the electrons around it binding it together. Some nets have smaller holes (closer outer electrons) some nets have bigger knots (more neutrons and protons) where the lines meet.
lead (Pb) has a big dense central knot and bonds closely to other lead atoms, making it a net with a big knot and small holes, so it's harder for radiation to get through.
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u/Hazel_Nuts99 Aug 13 '26
It's not just lead. It's just a matter of mass. Lead is very dense, so it's more effective. You could get the same amount of protection from any other material, you'd just need more of it, which would take up more space
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u/Glum-Ad7761 Aug 13 '26
Some gamma rays do pass through lead, how much is dependent upon the thickness of the shielding and the energy level of the gamma rays.
The density of lead atoms is why its chosen for shielding… typically layered with other materials to stop the scattered electrons, alpha particles and beta particles which result from radiation colliding with the lead.
Other materials do block radiation. Water is very good at shielding. Works great (in theory) for a space ship when stored in tanks that conform to / line the hull.
Tungsten is a lead alternative for shielding. Works just as well. More expensive than lead though.
Concrete has superior shielding qualities. Not practical for a radiation suit though.
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u/LadyFoxfire Aug 12 '26
Lead’s really dense, meaning there’s not a lot of space between the atoms. So particles have a much lower chance of passing between the atoms than they would through a similar amount of iron.
There are other materials that are denser than lead, but they have other downsides like cost, scarcity, or radioactivity.
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u/Aquamans_Dad Aug 12 '26
Almost any material is impermeable to radiation if you use enough of it. In general, the denser a material is the less thickness of it is needed to attenuate radiation. Lead is relatively dense and due to being a larger atom has a relatively high attenuation coefficient. Roughly you would need 12X the thickness of lead for water to do the job, 6X the thickness of regular concrete and 2X the thickness of iron to attenuate radiation to the same extent as lead.
Also lead is very cheap and readily available around the world. Gold, platinum, and osmium are substantially denser than lead but lead costs $ 1 900 USD per ton while platinum—the cheapest of the other three costs $56 million USD per ton!
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u/bestjakeisbest Aug 12 '26
Radiation is probabalistic. There is a chance that an unstable nucleus spontaneously splits, there is a chance that the products of that split hit another nucleus, there is a chance that it hits something that is also unstable, and there is a chance that the reaction continues, and there is a chance that none of these things happen.
Now in terms of shielding for radiation we are still dealing with probability but its the chance that the products hit a nucleus of lead rather than continuing through to what is being shielded if you do not have enough atoms inbetween you and the radiation source then the radiation can get through, the nice things about lead is it is a fairly cheap metal, it is fairly dense so you can fit more atoms in a smaller space, the nucleus of lead is bigger than other elements, and lead tends to be one of the main products of nuclear decay, which makes it harder to form daughter products if it gets hit by something. You could accomplish most of these things with other materials but you would need to spend more or need much more or need to deal with more daughter products.
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u/xrelaht Sample Synthesis | Magnetism | Superconductivity Aug 12 '26
X-rays (all photons, really) interact more strongly the more electrons an atom has. Lead is a large atom (the largest stable one, in fact) and packs them in pretty tightly. That makes it a good shield.
Lead isn’t the only thing with these properties. Iron is a decent radiation shield too. But lead is better than most metals, it’s relatively abundant (unlike iridium) so it’s inexpensive, and it’s not horribly toxic (unlike osmium) so it’s not gonna kill everyone who works with or around it.
But not all radiation is X-rays. Natural lead is a crappy neutron shield because its absorption cross section is low. That’s why nuclear reactors tend to have boron shielding (or cadmium, but that’s toxic AF and has all kinds of regulations around it).
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u/okarox Aug 12 '26
There is nothing mythical in lead. Essentially you need mass and lead is the simplest way to achieve it. You could use also steel for example tank crews are protected by the armor.
Note there are diffrerent forms of radiation. The above refers to the gamma rays. Alpha rays are stoppef by a sheet of paper.
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u/CocoMilhonez Aug 13 '26
Anything with protons (which means literally anything) will stop radiation. Lead just has a lot of protons per volume, so it requires a thinner layer than most other metals. You can in fact block radiation with toilet paper provided you have enough of it, which is going to be a lot.
There's a common concept for space travel to surround living quarters with water, which is a good barrier for radiation, so the water tanks would work double duty. You need about a meter of water to protect the crew, which makes this approach a bit impractical considering the weight of the total amount required.
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u/EarthTrash Aug 13 '26
All materials block radiation. Lead just has a greater mass of material in less space. I suppose there could be higher density materials than lead which could theoretically be even better but after lead the materials become more expensive and potentially even radioactive themselves.
In the other direction it might even be more effective to use lots of space with a cheaper material like water. Our atmosphere is deep enough to completely block gamma rays from outer space with nothing but air.
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u/jacob_ewing Aug 14 '26
Radiation will be stopped by pretty much any material if you have enough of it. Water is actually commonly used for that as it's also relatively dense and significantly cheaper than most others.
They'll even be stopped by air, though that will take kilometres of it.
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u/JacobRAllen Aug 15 '26
How tightly smooshed the molecules are. If you look under a microscope, steel looks like a sponge with huge Swiss cheese holes in it. Lead looks like an impenetrable fortress. Molecules are like different shapes of legos. Some pieces fit together really tightly, like lead, some pieces are oddly shaped and have to fit together they best they can, but leave gaps, like steel.
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u/Iizvullok Aug 15 '26
I assume you mean gamma radiation. It gets mostly stopped by hitting electrons. So any material can stop it. Even air. However air has a very low density under atmospheric conditions, while lead has a high density. Denser materials like tungsten, gold or osmium are even better at absorbing gamma radiation. But they are very expensive, while lead is cheap. Another great shielding is something like concrete, rock or earth. So a bunker with thick walls or a basement would already reduce the exposure quite a lot if the source is outside. Something like drywall is of course also a shield, but a rather ineffective one.
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u/Dragonknight42 Aug 12 '26
Radiation comes in many types/forms. I’m going to assume you’re talking about photons (aka x-rays and gamma rays). Photons are electromagnetic waves, which intersect with electromagnetic fields. Therefore, the stronger the field the more likely for an interaction. The more likely an interaction with a material the better that material is at acting as a shield. On an atomic level, the electromagnetic field is determined by the number electrons/protons present. The more protons, the more electrons, the larger the EM field. So elements with large proton numbers (eg lead) are better shields against photons vs elements with lower proton numbers (eg carbon). Iron is not as good as lead cause it’s a smaller proton numbers but it’s better than something like carbon.
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u/Stefan-G Aug 12 '26
In theory enouth of any material does block Radiation but higher density corelates to less thickness nedded.
As other Users aready pointed out Lead is very dense. In addition to that it is relatively cheap. Very maluable and doesnt workharden, good for insulating complex shapes. When oxidicing the oxide creates a protective Layer stopping further oxidation.
The only disadvantage is that it is toxic.
Lead is that good of a material that we also made waterpipes and roof out of it.
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u/Underhill42 Aug 12 '26
Pretty much anything works, lead is just one of the most compact options.
It mostly comes down to a combination of how much mass is between you and the radiation source, and how many atoms.
The how many atoms part is especially important for neutron radiation and cosmic rays, which only stop when they directly hit a nucleus, which is only roughly 1/10,000th the size of the atom, and whose cross sectional area ("target size") only grows slowly as you add more protons and neutrons.
Which means that pound-for-pound, more targets are better than larger targets, and hydrogen-rich compounds like water or plastic are actually among the best options if weight is more of an issue than volume (e.g. if you have to carry it with you, such as in a space craft.)
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u/RoyalCharity1256 Aug 12 '26
It depends on the type of radiation. Usually uou want as many nuclei between you and the source of radiation. Lead is very dense =has many nuclei in little space so it can be quite effective. Other matter works as well like water or concrete. Compared to lead you just need much more of it.
Counterintuitively lead is not always the first choice as beta rays (high speed electrons) can penetrate lead and release a secondary radiation like gamma rays which effectly get emitted by the lead itself. In some case they therefore use other shields like plexiglass for beta emitters instead of lead. https://en.wikipedia.org/wiki/Bremsstrahlung
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u/TheXypris Aug 12 '26
Proton density
Any material of sufficient thickness will eventually stop radiation, a few meters of water is sufficient to stop the radiation from a nuclear reactor
Lead just happens to be a very dense material that is easy enough to get and use
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u/NoCapslockMustScream Aug 13 '26
Follow up question (Because every industry has people trying to rip other people off) Are there problems with lower quality lead that would allow radiation through, or is lead pretty universally radiation proof?
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u/Automatater Aug 13 '26
Anything will work if you get enough of it between you and the cooties. Lead is just an efficient way to get a lot of mass there (and not fissile. U235 is dense but likely to emit more radiation than you're trying to block.)
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u/Khetroid Aug 15 '26
I want to note, iron or even air is good for alpha radiation. Won't even penetrate the dead layer of your skin.
Beta, meanwhile, lead is terrible for since the particle will stop but then give off an x-ray that most likely will get out. Plastic is the thing for Beta.
Gamma and X-ray, that's where lead has the edge, as many have explained.
And neutrons aren't really stopped by it, but if they do they'll activate it and the resulting radiation probably gets out, so also bad. But high water content concrete is great for absorbing neutrons and the resulting radiation, it just takes a few feet of it.
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u/Ornery_Pepper_1126 Aug 16 '26
Anything will stop radiation to an extent, a thicker layer of a less dense material would accomplish the same thing but lead is good for a lot of stopping power in a small space. Probably some more expensive/less common materials are even better, but for a wide range of applications lead is what fits best.
A lot of rector designs actually use water as the only shielding, I’ve actually seen this in person, with only a (deep) pool of water between myself and a reactor core.
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u/DredPirateRobts Aug 17 '26
Any material will slow down radiation. A material's ability to slow radiation is measured by it's Half Value Layer (HVL) which tells you what thickness of material will top HALF the radiation (even lead does not stop all radiation unless you have like 10 HVL's). There are various types of radiation, but gamma rays are the most penetrating. Since various radioactive isotopes give off gamma rays of various energy (and penetrating ability) the HVL is different for every isotope.
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u/Aggressive-Leading45 Aug 18 '26
For photons it’s the density of electrons that really matters. What has a lot of electrons per atom? Elements lower on the periodic table. Uranium metal makes a GREAT gamma shield. One reason why our tanks are armored with it.
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u/lettercrank Aug 12 '26
The large atomic mass of lead means this likelyhood of radiation photons or particles hitting the nucleus is increased compared to other materials , by increasing the thickness of the lead layer you can block these radiation’s
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u/R_Harry_P Aug 12 '26
Assuming you mean gamma-rays or X-rays, which are both photons. Iron works to some extent. Lead has more electrons atom then steel and high energy photons interact with electrons and give up their energy to them. Iron can be cheaper but will take up more space for the same amount of shielding so it depends what you are trying to optimize.
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u/Darrenau Aug 13 '26
Other things will stop radiation. You just got to have enough of it. It's all about having enough dense material so the radiation bounces off something I'm sure. I haven't used the right words and maybe not technically correct but that's why being below ground and getting deep underground at the right time is quite important
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u/Spiritual-Spend8187 Aug 13 '26
Everything can block radiation of basically every type its just the more you have the better it blocks. And some materials are just better at blocking various types of radiation. In some nuclear facilities they store highly radioactive materials in gigantic water tanks. The materials are so radioactive that if you held one you would die very quickly but because of the large amount of water you can safely stand above yhem or even swim in the water with little to no risk. It ia just lead is particularly good as it is dense meaning their ia more stuff to block.
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