In fission reactions, neutrons are often slowed down, or moderated, to increase the likelihood of their interaction with fissile material (such as uranium-235 or plutonium-239) and to sustain a chain reaction. There are a few reasons why this is done:
Cross-Section Enhancement: Neutrons have a higher probability of inducing fission in fissile nuclei when they are moving at lower energies. This phenomenon is known as the neutron cross-section. At higher energies, neutrons tend to pass through atomic nuclei without inducing fission. Slowing down neutrons increases their chances of interacting with the fissile material, enhancing the probability of fission reactions.
Resonance Absorption: Some fissile nuclei have resonances, which are specific neutron energy ranges where the probability of absorption is significantly higher. By slowing down the neutrons, they can be brought into resonance with the fissile nuclei, maximizing the absorption and fission probabilities.
Thermal Neutron Spectrum: Slowing down neutrons leads to the creation of a thermal neutron spectrum, where most of the neutrons have energies comparable to the thermal energy of the surrounding medium. In this energy range, the probability of inducing fission in fissile nuclei is highest. By achieving a thermal neutron spectrum, it becomes easier to sustain a self-sustaining chain reaction, as more neutrons are available to induce fission.
To slow down neutrons, a material called a moderator is used. Common moderators include light elements such as water (H2O), heavy water (D2O), or graphite (carbon). These materials have a high probability of elastic scattering with neutrons, causing them to lose energy through collisions and ultimately slowing them down to thermal energies.
It's important to control the neutron population and their energy distribution in a nuclear reactor to ensure a sustainable and controlled chain reaction, as well as to optimize the production of energy or other desired outcomes.