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In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power of a nuclear power plant. The standard unit for measuring the cross section is the barn, which is equal to 10−28 m2 or 10−24 cm2. The larger neutron cross section, the more likely a neutron will react with the nucleus. An isotope (or nuclide) can be classified according to its neutron cross section and how it reacts to an incident neutron. Radionuclides that tend to absorb a neutron and either decay or keep the neutron in its nucleus are neutron absorbers and will have a ''capture cross section'' for that reaction. Isotopes that fission, are fissile fuels and have a corresponding ''fission cross section''. The remaining isotopes will simply scatter the neutron, and have a ''scatter cross section''. Some isotopes, like uranium-238, have nonzero cross sections of all three. Isotopes with a large scatter cross section and have a low mass are good neutron moderators (see chart below). Nuclides which have a large absorption cross section are neutron poisons if they are neither fissile nor undergo decay. A poison that is purposely inserted into a nuclear reactor for controlling its reactivity in the long term and improve its shutdown margin is called a ''burnable'' poison. == Parameters of interest == The neutron cross section, and therefore the probability of an interaction, depends on: * the target type (hydrogen, uranium…), * the type of nuclear reaction (scattering, fission…). * the incident particle energy, also called speed or temperature (thermal, fast…), and, to a lesser extent, of: * its relative angle between the incident neutron and the target nuclide, * the target nuclide temperature. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「neutron cross section」の詳細全文を読む スポンサード リンク
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