Which of the following is also used for neutron shielding?

• A. Graphite
• B. Water
• C. Steel
• D. Plastic

Answer: (B)

Neutron shielding works best when the material has a lot of hydrogen because fast neutrons lose most of their energy in collisions with nuclei that have a similar mass. Water fits this role perfectly since it contains a large amount of hydrogen atoms in every molecule. Those hydrogen nuclei are very effective at slowing down fast neutrons through elastic scattering, turning them into lower-energy, easier-to-absorb neutrons. Water’s practicality—easy to obtain, pump, and circulate as a shield or coolant—also makes it a common and versatile choice in shielding design.

Graphite can slow neutrons due to its light carbon nuclei, but it’s not as effective per thickness as hydrogen-rich material for broad shielding needs. Steel attenuates some neutrons, but its lack of hydrogen means it doesn’t slow fast neutrons as efficiently. Plastic can also be hydrogen-rich and useful for shielding, especially as a lightweight option, but water's combination of high hydrogen content, volume shielding, and ease of implementation often makes it the go-to example in neutron shielding applications.