A neutron is a subatomic particle, a constituent of atomic nuclei, along with protons. Unlike protons, which carry a positive electric charge, neutrons are electrically neutral, meaning they have no net electric charge. This characteristic allows them to penetrate atomic nuclei relatively unimpeded by electrostatic forces, making them crucial in nuclear reactions, particularly nuclear fission.
Neutrons are classified as baryons and are composed of three quarks (one up quark and two down quarks). Key properties include:
The neutron's role in nuclear fission is paramount. When a neutron strikes the nucleus of a fissile atom, such as Uranium-235 (), it can cause the nucleus to split into smaller fragments, releasing a significant amount of energy and, critically, emitting more neutrons. This process is represented as:
These newly released neutrons can then go on to strike other fissile nuclei, perpetuating a nuclear chain reaction. The careful control of neutron population is fundamental to the safe and efficient operation of a nuclear reactor.
Neutrons are typically classified by their kinetic energy:
Neutrons interact with matter primarily through scattering and absorption. In a reactor, materials like graphite or heavy water act as neutron moderators to slow down fast neutrons. Control rods, often made of cadmium or boron, are used to absorb excess neutrons, thereby regulating the reaction rate. The neutron absorption cross-section of a material for a thermal neutron is key to its effectiveness as a control material.
Neutron radiation also poses a significant safety concern. When neutrons are absorbed by stable isotopes, they can transform them into radioactive isotopes through a process known as neutron activation. This can lead to the production of radioactive waste and the activation of reactor components, necessitating robust shielding and stringent radiological protection programmes. Understanding neutron behaviour is central to designing effective shielding and ensuring operational safety.