fastneutron

A fast neutron is a neutron with kinetic energy typically above 1 MeV. Fast neutrons are produced in nuclear fission, fusion, and spallation reactions and are the primary product of fission events. Neutrons released by fission have an average energy near 2 MeV and are considered fast until they lose energy through collisions with surrounding matter.

Most nuclear reactors operate with thermal neutrons (approximately 0.025 eV) achieved by moderating the fast spectrum; however, fast reactors intentionally use a high-energy neutron spectrum with little or no moderation to sustain fission and breed fuel. In fast systems, fission can occur in both fissile and certain fertile isotopes, enabling breeding of new fissile material such as plutonium from uranium-238.

Interaction with matter: At fast energies, elastic scattering with nuclei is the primary mechanism for slowing neutrons through neutron cooling. Inelastic scattering and absorption also occur; capture cross sections tend to be lower than in the thermal region but exhibit resonances. Fast neutrons can travel long distances and penetrate shielding more readily than thermal neutrons.

Detection and measurement: Fast neutrons are detected indirectly via recoil protons in hydrogen-rich media or via scintillation from fast-neutron interactions; common detectors include organic scintillators, liquid scintillators, and fission chambers calibrated for fast neutrons. Techniques such as time-of-flight measure energy distribution.

Applications and safety: In nuclear engineering, fast neutrons enable fast reactors and certain transmutation or security applications. Shielding requires thick hydrogenous material to slow neutrons, followed by absorbers like boron or cadmium to capture them, due to their penetrating power and production of secondary radiation.