Explain what is meant by ‘thermal neutron’?

A thermal neutron refers to a neutron that has been slowed down to an energy level that is in thermal equilibrium with its surrounding environment, typically at room temperature. In nuclear physics, thermal neutrons are significant because they are more likely to induce further fission reactions when they collide with fissile materials like uranium-235 or plutonium-239.

In reactors, neutrons produced by fission are initially fast neutrons, possessing high kinetic energy. However, these fast neutrons can be effectively slowed down through a process called moderation, where they lose energy by colliding with lighter nuclei, such as hydrogen or carbon. Once these neutrons reach lower energy states, they are classified as thermal neutrons.

This interaction is crucial for sustaining a chain reaction in nuclear reactors operated with thermal-neutron fission. The probability of fission occurring with thermal neutrons is significantly higher than with fast neutrons, making them essential for the effective operation of many nuclear reactors.