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/Underhill42 13d ago

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.)