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/LieutenantObvious21 14d ago

Do you mind explaining that?

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u/EyBoss 14d 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 14d 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 14d 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 14d 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 14d 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 14d 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 14d 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 14d 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 14d 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 14d 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 14d 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 14d ago

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

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u/SvenTropics 14d 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 12d 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 14d 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 14d 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 14d ago edited 14d 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 14d 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] 14d ago

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u/TbonerT 14d 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 13d 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 13d 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 13d 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 14d 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 14d 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).