In shielding design for mixed neutron-gamma fields, which statement is most accurate?

• A. For mixed neutron-gamma shielding, a combination of hydrogenous materials for moderation and high-Z materials for gamma attenuation, often with boron additives for neutron capture, is used.
• B. Hydrogenous materials alone provide best gamma attenuation.
• C. Lead provides both gamma and neutron shielding equally.
• D. A single material is always sufficient for shielding both gamma and neutrons.

Answer: (B)

In shielding mixed neutron-gamma fields, you have to address two very different types of radiation. Neutrons are slowed most effectively in light, hydrogen-rich materials (moderation), and their thermalized forms are often captured more efficiently when a neutron-absorbing additive like boron is present. Gamma rays, on the other hand, interact with matter mainly through processes that are more probable in heavy elements (high-Z), so high-Z materials such as lead provide much better attenuation for gammas. Because of these different requirements, the most effective design uses a layered or composite approach: a hydrogenous layer to moderate neutrons, boron to capture slowed neutrons, and a high-Z layer to attenuate gamma radiation. Hydrogenous materials alone do not provide the best gamma attenuation, since their low atomic number offers relatively poor gamma shielding. Lead and other high-Z materials are excellent for gamma attenuation but do not effectively shield neutrons, and a single material cannot optimally protect against both types.