What property allows some materials to be good thermalizers of neutrons?

The correct answer is linked to the presence of large amounts of hydrogen in a material, which significantly enhances its ability to thermalize neutrons. Hydrogen is particularly effective in this process because its nucleus, consisting of just one proton, can effectively slow down fast neutrons through elastic scattering interactions. When fast neutrons collide with hydrogen nuclei, they lose energy and can be reduced to thermal or slow neutrons, which are more likely to be captured in various nuclear reactions.

This property makes hydrogen-rich materials, such as water or paraffin, excellent moderators in nuclear reactors and radiation shielding applications. Their ability to efficiently slow down neutrons is crucial for maintaining the right conditions for fission reactions and for controlling neutron flux levels in nuclear systems.

The other options do not provide the same effectiveness in thermalizing neutrons. High density might suggest a better stopping power in certain contexts, but it doesn’t specifically relate to neutron moderation. Low atomic weight is relevant, but it does not directly highlight the unique role that hydrogen plays. Electrical conductivity is not directly associated with neutron moderation properties.