Which materials provide thick shielding for neutrons?

• A. Lead
• B. Water, plastic, concrete
• C. Vacuum
• D. Gold

Answer: (B)

Neutron shielding relies on slowing and absorbing neutrons using hydrogen-rich materials. Fast neutrons lose energy most effectively when they collide with light nuclei, especially hydrogen, so materials like water, plastics (such as polyethylene), and concrete are excellent at reducing neutron flux when used in thick layers. Once slowed, neutrons are more easily captured by absorbers, and these hydrogenous shields can be made even more effective with additives that capture neutrons (like boron or cadmium). Heavy metals such as lead or gold are not efficient at attenuating fast neutrons on their own and are mainly used for gamma shielding; vacuum offers no material interaction at all, so it provides no shielding. Therefore, the thick shielding that best protects against neutrons comes from hydrogen-rich materials like water, plastic, and concrete.