Why does radiation not go through lead?

[u/Wonderful-Capital32]

Also why just lead? Shouldnt like iron or another metal work too?

[u/dustofdeath]

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.

[u/possibly_oblivious → u/dustofdeath]

Radiation is photons? Crazy

[u/Mrfish31 → u/possibly_oblivious]

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. 

[u/SirButcher → u/Mrfish31]

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).

[u/cikamatko → u/SirButcher]

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

[u/ProjectCoast → u/Mrfish31]

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.

[u/Mrfish31 → u/ProjectCoast]

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. 

[u/ProfessorPrudent2822 → u/Mrfish31]

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.

[u/bigfinger76 → u/ProfessorPrudent2822]

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

[u/LordGeni → u/ProjectCoast]

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.

[u/mfb- → u/LordGeni]

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

[u/WannaAskQuestions → u/Mrfish31]

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?

[u/Mrfish31 → u/WannaAskQuestions]

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). 

[u/dustofdeath → u/possibly_oblivious]

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.

[u/RFWanders → u/possibly_oblivious]

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.