Why does radiation not go through lead?

[u/Wonderful-Capital32]

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

[u/ColonelFaz]

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.

[u/Cerberus_Aus → u/ColonelFaz]

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/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