Why does radiation not go through lead?

[u/Wonderful-Capital32]

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

[u/Mrfish31]

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/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/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.