Are good moderators also good reflectors?

Are good moderators also good reflectors?

• A. NO, they are unrelated.
• B. NOT generally; only specific materials exhibit both properties equally.
• C. YES, because of their high scattering cross sections which help both slowing neutrons and back-scattering neutrons.
• D. YES, but only if they have high absorption cross sections.

Answer: (C)

Neutron interactions rely on scattering; how a material slows or redirects neutrons comes from its scattering and absorption properties. A substance that has a high elastic scattering cross section provides many collisions, which both reduce neutron energy (slowing) and change the neutron’s direction. When such material is arranged so that scattered neutrons are likely to be redirected back toward the reactor core or surrounding regions, it acts effectively as a reflector as well. This is why a good moderator often serves as a good reflector: the same strong scattering that slows neutrons also increases the chances that neutrons will be scattered back toward where they came from, rather than being absorbed. However, if the material has a high absorption cross section, neutrons are more likely to be captured than reflected, which is not desirable for reflection. So, the best choice states that yes, because their high scattering cross sections help both slowing neutrons and back-scattering neutrons.

Neutron interactions rely on scattering; how a material slows or redirects neutrons comes from its scattering and absorption properties. A substance that has a high elastic scattering cross section provides many collisions, which both reduce neutron energy (slowing) and change the neutron’s direction. When such material is arranged so that scattered neutrons are likely to be redirected back toward the reactor core or surrounding regions, it acts effectively as a reflector as well.

This is why a good moderator often serves as a good reflector: the same strong scattering that slows neutrons also increases the chances that neutrons will be scattered back toward where they came from, rather than being absorbed. However, if the material has a high absorption cross section, neutrons are more likely to be captured than reflected, which is not desirable for reflection.

So, the best choice states that yes, because their high scattering cross sections help both slowing neutrons and back-scattering neutrons.