In nuclear reactors, neutron moderators are used to slow down fast neutrons produced by nuclear fission reactions. The main reason for using moderators with low atomic mass, such as light water (H₂O) or graphite, is to enhance the probability of neutron-moderator collisions and maximize the efficiency of the moderation process. Here's why:
Neutron moderation: Fast neutrons have high kinetic energy and are less likely to cause additional fission reactions. To sustain a nuclear chain reaction, it is essential to slow down these fast neutrons to a lower energy level where they are more likely to induce fission. Moderators serve this purpose by colliding with fast neutrons, transferring some of their kinetic energy to the moderator material, and reducing the neutron speed.
Elastic scattering: The process by which neutrons lose energy in a moderator is typically through elastic scattering. When a neutron collides with an atomic nucleus, it bounces off in a different direction, effectively losing some kinetic energy. The probability of an elastic collision is higher for neutrons with lower masses, as they experience less momentum transfer during the collision. Therefore, lighter nuclei in the moderator material are more effective at slowing down neutrons.
Energy transfer: In elastic scattering, the energy transferred to the moderator material is inversely proportional to the mass of the target nucleus. Lighter nuclei, such as hydrogen in water or carbon in graphite, can effectively absorb a greater amount of energy per collision with a neutron. This energy transfer allows the moderator to slow down the neutrons efficiently.
Neutron capture: Another factor to consider is the likelihood of neutron capture, which occurs when a neutron is absorbed by a nucleus. Materials with heavy nuclei, such as those with high atomic masses, have a greater chance of absorbing neutrons, reducing their availability for sustaining the chain reaction. Therefore, using materials with low atomic mass as moderators minimizes neutron capture and maximizes neutron moderation.
By utilizing moderators with low atomic mass, nuclear reactors can effectively slow down fast neutrons, increasing their chances of inducing fission and sustaining a self-sustained chain reaction.