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The absorption neutron cross section of an isotope of a chemical element is the effective cross-sectional area that an atom of that isotope presents to absorption and is a measure of the probability of neutron capture. It is usually measured in barns. Absorption cross section is often highly dependent on neutron energy. In general, the ...
In nuclear physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. The neutron cross section σ can be defined as the area in cm 2 for which the number of neutron-nuclei reactions taking place is equal to the product of the number of incident neutrons that would pass through the area and the number of ...
A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (about 4.0×10 −21 J or 2.4 MJ/kg, hence a speed of 2.19 km/s), which is the energy corresponding to the most probable speed at a temperature of 290 K (17 °C or 62 °F), the mode of the Maxwell–Boltzmann distribution for this temperature, E peak = k T.
The symbols are defined as: [3], and are the average number of neutrons produced per fission in the medium (2.43 for uranium-235). and are the microscopic fission and absorption thermal cross sections for fuel, respectively.
The symbols are defined as: [2], and are the average number of neutrons produced per fission in the medium (2.43 for uranium-235). and are the microscopic fission and absorption cross sections for fuel, respectively.
Quantity (common name/s) (Common) symbol/s Defining equation SI units Dimension Number of atoms N = Number of atoms remaining at time t. N 0 = Initial number of atoms at time t = 0
This is only slightly modified in a real moderator due to the speed (energy) dependence of the absorption cross-section of most materials, so that low-speed neutrons are preferentially absorbed, [5] [6] so that the true neutron velocity distribution in the core would be slightly hotter than predicted.
Some reactions are only possible with fast neutrons: (n,2n) reactions produce small amounts of protactinium-231 and uranium-232 in the thorium cycle which is otherwise relatively free of highly radioactive actinide products. 9 Be + n → 2α + 2n can contribute some additional neutrons in the beryllium neutron reflector of a nuclear weapon.