Effective neutron moderators are materials that slow down fast neutrons (high-energy neutrons) to thermal (low-energy) neutrons, which are more likely to induce fission in nuclear fuel. Neutron moderation is a critical process in nuclear reactors. The materials used for this purpose are typically light elements that can efficiently transfer energy to the neutrons.

The most effective neutron moderators include:

1. Light Water (H₂O):
   • Composition: Water (H₂O) contains hydrogen (protons), which is highly effective in slowing down neutrons due to its similar mass to a neutron.
   • Use: It is the most commonly used moderator in nuclear reactors, especially in pressurized water reactors (PWRs) and boiling water reactors (BWRs).
2. Heavy Water (D₂O):
   • Composition: Heavy water contains deuterium (hydrogen isotope with one proton and one neutron) instead of regular hydrogen.
   • Use: Heavy water is used in reactors like the CANDU reactor, as it is more effective than light water for neutron moderation. This is due to the lower neutron absorption cross-section of deuterium compared to regular hydrogen.
3. Graphite (Carbon):
   • Composition: Graphite is a form of carbon, typically used in the form of graphite blocks or rods.
   • Use: It was used as a moderator in reactors like the early British gas-cooled reactors and the RBMK reactors (such as Chernobyl). Graphite moderates neutrons effectively because carbon has a good balance between neutron scattering and low absorption.
4. Beryllium:
   • Composition: Beryllium is a light metal that is effective in scattering neutrons.
   • Use: It is used in certain experimental reactors or in combinations with other materials. Beryllium’s advantage is its low neutron absorption and relatively high neutron scattering ability.
5. Liquid Hydrogen:
   • Composition: Hydrogen in its liquid state is also an excellent moderator.
   • Use: While not as commonly used in large-scale reactors, liquid hydrogen could be useful in specialized reactor designs or research applications due to its very low mass and high neutron scattering efficiency.

The effectiveness of a moderator depends on several factors, including its ability to scatter neutrons, its density, and the neutron absorption cross-section. The ideal moderator should have a low neutron absorption cross-section, a mass similar to that of a neutron (for efficient energy transfer), and a high scattering cross-section.