Which materials are used for neutron shielding?

• A. Lead or tungsten
• B. Hydrogen-rich materials like polyethylene or water, often with boron or cadmium
• C. Concrete with high iron content
• D. Aluminum alloys

Answer: (B)

Neutron shielding works best when fast neutrons are slowed down by collisions with light nuclei and then captured by materials with high neutron absorption. Hydrogen-rich substances, like polyethylene or water, are especially effective moderators because protons are similar in mass to neutrons, so energy transfer during collisions is efficient. Adding neutron-absorbing elements such as boron or cadmium gives a strong capture mechanism for the slowed (thermal) neutrons, greatly reducing the neutron flux. This combination—hydrogenous material for moderation plus boron or cadmium for absorption—provides the most effective neutron shielding.

Lead or tungsten are excellent gamma shields but don’t moderate neutrons well, and can even interact in ways that aren’t helpful for neutron protection. Concrete with high iron content offers some attenuation but is not as effective for slowing and capturing neutrons as hydrogen-rich materials. Aluminum alloys lack enough hydrogen to serve as a good neutron moderator, so they’re not ideal for shielding neutrons.