Lämpöneutronien

Lämpöneutronit, sometimes referred to as thermal neutrons in English, are neutrons that have been slowed down to have kinetic energies comparable to the thermal energy of the surrounding medium. This typically occurs at room temperature, around 20 degrees Celsius, where their average kinetic energy is approximately 0.025 electronvolts (eV). In nuclear reactors, neutrons are produced during fission with much higher energies, often in the MeV range. To effectively sustain a chain reaction in many reactor designs, particularly those using unenriched uranium, these fast neutrons must be moderated.

Moderation is the process of reducing neutron energy through inelastic collisions with light nuclei. Materials commonly used as moderators include water (light water), heavy water, and graphite. These materials absorb some of the kinetic energy from the fast neutrons during each collision, gradually slowing them down. Once a neutron reaches thermal equilibrium with the moderator, it becomes a thermal neutron. Thermal neutrons are much more likely to cause fission in fissile isotopes like Uranium-235 compared to fast neutrons. This increased probability of fission at lower energies is quantified by the fission cross-section, which is significantly larger for thermal neutrons for many isotopes. The properties and behavior of thermal neutrons are crucial for understanding neutron diffusion and criticality in nuclear reactor physics.