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A free neutron is a neutron that exists outside of an atomic nucleus. While neutrons can be stable when bound inside nuclei, free neutrons are unstable and decay with a lifetime of just under 15 minutes (885.7 ± 0.8 s). The only possible decay mode is via the weak nuclear force, is into a proton, an electron, and an electron antineutrino, the proton and electron forming a hydrogen atom: Even though it is not a chemical element, the free neutron is often included in tables of nuclides. It is then considered to have an atomic number of zero and a mass number of one.
Production Various nuclides become more stable by expelling neutrons as a decay mode; this is known as neutron decay, and happens commonly during spontaneous fission. Cosmic radiation interacting the earth's atmosphere continuously generates neutrons that can be detected at the surface. Nuclear fission reactors naturally produce free neutrons; their role is to sustain the energy-producing chain reaction. The intense neutron radiation is can also be used to produce various radioisotopes through the process of neutron activation. Experimental nuclear fusion reactors produce free neutrons as a waste product. However, it is these neutrons that possess most of the energy, and converting that energy to a useful form has proved a difficult engineering challenge to nuclear physicists. This also explains why this form of energy is likely to create huge amounts of radioactive waste. * Thermal neutron A '''thermal neutron''' is a free neutron that is Boltzmann distributed with kT = 0.024 eV (4.0×10-21 J) at room temperature. This gives characteristic (not average, or median) speed of 2.2 km/s. The name 'thermal' comes from their energy being that of the room temperature gas or material they are permeating. (see kinetic theory for energies and speeds of molecules). After a number of collisions (often in the range of 10–20) with nuclei, neutrons arrive at this energy level, provided that they are not absorbed. In many substances, thermal neutrons have a much larger effective cross-section than faster neutrons, and can therefore be absorbed more easily by any atomic nuclei that they collide with, creating a heavier — and often unstable — isotope of the chemical element as a result. Most fission reactors use a neutron moderator to slow down, or thermalize the neutrons that are emitted by nuclear fission so that they are more easily captured, causing further fission. Others, called fast breeder reactors, use fission energy neutrons directly. Cold neutrons These neutrons are thermal neutrons that have been equilibrated in a very cold substances such as liquid deuterium. These are produced in neutron scattering research facilities. Fission energy neutron A '''fast neutron''' is a free neutron with a kinetic energy level close to 2 MeV (20 TJ/kg), hence a speed of 28,000 km/s. They are named fission energy or fast neutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes such as nuclear fission. Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a neutron moderator. In reactors, typically heavy water, light water, or graphite are used to moderate neutrons. Intermediate neutrons A fission energy neutron that is slowing down is often said to have intermediate energy. There are not many non-elastic reactions in this energy region, so most of what happens is just slowing to thermal speeds before eventual capture. High-energy neutrons These neutrons have more energy than fission energy neutrons and generated in accelerators or in the atmosphere from cosmic particles. They can have energies as high as tens of joules per neutron. See also See also | |||||||||||||||||||||||||||||||||||||||||||||||||
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