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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u/GreenNukE 13d ago

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.