Which material is particularly effective for shielding neutrons due to its hydrogen content?

Which material is particularly effective for shielding neutrons due to its hydrogen content?

• A. Water
• B. Concrete
• C. Polyethylene
• D. Tungsten

Answer: (C)

Hydrogen-rich materials are best for slowing fast neutrons because the neutron transfers energy most efficiently when colliding with nuclei that have a mass similar to the neutron. Hydrogen has nearly the same mass as a neutron, so elastic collisions with hydrogen atoms remove a large portion of the neutron’s energy in each interaction, effectively moderating the neutrons to thermal energies within a relatively short distance. Polyethylene is essentially a hydrocarbon polymer with a very high density of hydrogen atoms, so it provides a lot of targets for those energy-sapping collisions in a compact, solid form. This makes it particularly effective as a neutron shield and moderator, easy to shape into blocks or layers for shielding around radiological sources or facilities. Water also contains hydrogen and moderates neutrons well, but it’s a liquid requiring containment and can be impractical for fixed shielding. Concrete has some hydrogen content but includes heavier elements, so its moderation per thickness is not as efficient as a hydrogen-rich plastic. Tungsten is a dense high-Z material that’s good for gamma shielding, not for slowing neutrons, so it doesn’t provide the same neutron-moderating benefit as hydrogenous materials.

Hydrogen-rich materials are best for slowing fast neutrons because the neutron transfers energy most efficiently when colliding with nuclei that have a mass similar to the neutron. Hydrogen has nearly the same mass as a neutron, so elastic collisions with hydrogen atoms remove a large portion of the neutron’s energy in each interaction, effectively moderating the neutrons to thermal energies within a relatively short distance.

Polyethylene is essentially a hydrocarbon polymer with a very high density of hydrogen atoms, so it provides a lot of targets for those energy-sapping collisions in a compact, solid form. This makes it particularly effective as a neutron shield and moderator, easy to shape into blocks or layers for shielding around radiological sources or facilities.

Water also contains hydrogen and moderates neutrons well, but it’s a liquid requiring containment and can be impractical for fixed shielding. Concrete has some hydrogen content but includes heavier elements, so its moderation per thickness is not as efficient as a hydrogen-rich plastic. Tungsten is a dense high-Z material that’s good for gamma shielding, not for slowing neutrons, so it doesn’t provide the same neutron-moderating benefit as hydrogenous materials.