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Encyclopedia > Thermal neutron

A thermal neutron is a free neutron with a kinetic energy level of ca. 0.025 eV (approx. 4.0e-21 J; 2.4 MJ/kg, hence a speed of 2.2 km/s). They are named 'thermal' as this level of kinetic energy is similar to the average kinetic energy of the molecules in a room-temperature gas (see kinetic theory for energies and rms speeds of molecules). After a number of collisions with nuclei, neutrons arrive at this energy level, provided that they are not absorbed. A free neutron is a neutron that exists outside of an atomic nucleus. ... Kinetic energy is energy that a body has as a result of its speed. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... The joule (symbol: J) is the SI unit of energy, or work. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... The kinetic theory of gases is a theory that explains the macroscopic properties of gases by consideration of their composition at a molecular level. ... Properties In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939. ...

Thermal neutrons have a much larger effective cross-section than fast 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. A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (10 TJ/kg, hence a speed of 14,000 km/s. ... A stylized representation of a lithium atom. ... A radionuclide is an atom with an unstable nucleus. ... Isotopes are forms of an element whose nuclei have the same atomic number–-the number of protons in the nucleus--but different atomic masses because they contain different numbers of neutrons. ... A chemical element, often called simply element, is a chemical substance that cannot be divided or changed into other chemical substances by any ordinary chemical technique. ...

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 fast neutrons directly. Core of a nuclear reactor A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate (as opposed to a nuclear explosion, where the chain reaction occurs in a split second). ... In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction. ... Sketch of induced nuclear fission, a neutron (n) strikes a uranium nucleus which splits into similar products (F. P.), and releases more neutrons to continue the process, and energy in the form of gamma and other radiation. ... The fast breeder or fast breeder reactor (FBR) is a type of fast neutron reactor that produces more fissile material than it consumes. ...

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