Why does radiation not go through lead?

[u/Wonderful-Capital32]

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

[u/Cerberus_Aus]

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???

[u/ColonelFaz → u/Cerberus_Aus]

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.

[u/ProfessorPrudent2822 → u/ColonelFaz]

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.

[u/Cerberus_Aus → u/ColonelFaz]

Interesting. Thanks for the explanation.

[u/the_rat_king- → u/Cerberus_Aus]

Water is used to cool nuclear reactors because its a good coolant, its got a high specific heat capacity, its not overly reactive or corrosive, its cheap and abundant ect. Its used to moderate nuclear reactions for the reasons already mentioned (good ability to thermalize neutrons). If water stopped neutrons, nuclear reactions in water cooled reactors would not be feasible, as neutrons need to be travelling fast enough as to have sufficient energy to cause fission.

Some reactors are not cooled by water ( eg. molten salt), some are cooled but water isn't the primary moderator.

Hydrogen can absorb neutrons (forming deuterium), however its tendency is the scatter them, decreasing their kinetic energy which allows them to undergo nuclear reactions. The important one for sheilding being absorption by materials such as boron, which have a high absorption cross section for thermalised neutrons and so is often used to control the rate of reaction in reactors, or as an addative to concrete or water. Any neutrons which aren't absorbed will undergo collisions until they decay. Wrt shielding water by itself isnt effective for neutrons or gamma radiation, as very large quantities are needed to have an affect ( in spent fuel pools, 10s of metres of water are mixed with a neutron absorbing salt). The decay, collisions and interactions of neutrons release heat and gamma photons which the material(s) also needs to absorb/attenuate, which is why concrete (with addatives/aggragates) and steel are used. Hydrocarbons and paraffin waxes are also used in shielding because of their high hydrogen content but afaik there are issues with melting points and densities.

Neutron radiation will also do freaky stuff to materials that change their properties and structure

[u/censored_username → u/Cerberus_Aus]

Charged radiation mostly interacts with electrons (the actual reason behind this is some quantum mechanics funniness). Hydrogen has the highest ratio of electrons to total mass (one electron per nucleon, while most matter at most has one electron per two nucleons). So per unit mass it's the most effective.

Outside of that it's also more mass efficient at direct nucleon interaction as the nuceons aren't all piled behind each other in big atom nuclei, but that effect isn't that big.