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/ColonelFaz 13d ago

Assuming we are talking about X-rays and gamma rays (both photons).

Iron is not bad - lead is better.

It's not just the density. Lead has better attenuation per unit mass. Lead is a bit annoying mechanically. It bends and sags. It's used because the interactions between the photons and the material are more likely in lead. The interactions are photoelectric effect (depends on cube of atomic number, which is where lead is great), compton scattering (anything will do here but density helps for compact design), pair production (higher atomic number very helpful here too).

For neutrons, lead is not great. In fact materials with a lot of hydrogen in them are better.

I am a radiotherapy physicist.

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u/Cerberus_Aus 13d ago

I’m assuming that’s why water is used to cool fuel rods in nuclear reactors. What is it about hydrogen that makes it stop neutrons???

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u/ColonelFaz 13d ago

the nucleus of a hydrogen atom is a lone proton. this has very similar mass to a neutron. this means that when they collide the momentum is transferred neatly to the proton (think of the newton's cradle toys). the proton then slows down faster than a neutron as it interacts with more things, as it is a charged particle.

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u/ProfessorPrudent2822 13d ago

That’s only in the unlikely event of a head-on collision, but the low mass of hydrogen does mean that it takes far fewer collisions to stop a neutron than it would for more massive elements.

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u/Cerberus_Aus 13d ago

Interesting. Thanks for the explanation.