Which additive is commonly used with hydrogen-rich shielding to capture neutrons?

• A. Aluminum
• B. Lead
• C. Cadmium or boron
• D. Mercury

Answer: (C)

Neutron shielding works best when you slow fast neutrons down and then trap them. Hydrogen-rich materials like water or polyethylene are great moderators because hydrogen nuclei are about the same mass as neutrons, so neutrons lose energy efficiently on collisions. Once the neutrons are slowed to thermal energies, they are much more likely to be captured by certain nuclei. Materials with high neutron capture cross sections—boron-10 and cadmium are classic examples—are used as additives to the hydrogenous shield to soak up these thermal neutrons. That combination, a moderator plus a strong absorber, dramatically reduces the number of neutrons that can escape the shielding. Aluminum is mostly structural and doesn’t provide strong neutron capture, lead mainly shields gamma rays, and mercury isn’t used for neutron absorption in this context.