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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.
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.
The prompt neutron lifetime, , is the average time between the emission of a neutron and either its absorption or escape from the system. [17] The neutrons that occur directly from fission are called prompt neutrons, and the ones that are a result of radioactive decay of fission fragments are called delayed neutrons.
The number of neutrons produced per fission is multiplicatively modified by the dominant eigenvalue. The resulting value of this eigenvalue reflects the time dependence of the neutron density in a multiplying medium. k eff < 1, subcritical: the neutron density is decreasing as time passes; k eff = 1, critical: the neutron density remains ...
A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (1.6 × 10 −13 J), hence a speed of ~ 14 000 km/s (~ 5% of the speed of light). 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.
As the neutron energy increases, the neutron cross section of most isotopes decreases. The boron isotope 10 B is responsible for the majority of the neutron absorption. Boron-containing materials can also be used as neutron shielding, to reduce the activation of material close to a reactor core.