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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 ...
Although the thermal neutron fission cross section (σ f) of the resulting 233 U is comparable to 235 U and 239 Pu, it has a much lower capture cross section (σ γ) than the latter two fissile isotopes, providing fewer non-fissile neutron absorptions and improved neutron economy. The ratio of neutrons released per neutron absorbed (η) in 233 U
Because 135 Xe is a potent nuclear poison with the largest cross section for thermal neutron absorption, the buildup of 135 Xe in the fuel inside a power reactor can lower the reactivity greatly. If a power reactor is shut down or left running at a low power level, then large amounts of 135 Xe can build up through decay of 135 I.
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.
Uranium-234 has a neutron-capture cross section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral—the average of neutrons having a range of intermediate energies. In a nuclear reactor non-fissile isotopes 234 U and 238 U both capture a neutron, thereby breeding fissile isotopes 235 U and 239 Pu, respectively.
The capture cross section of such a neutron by the nucleus increases sharply. The energy at which the neutron-nucleus interaction cross section reaches a maximum is called the resonance energy. The resonance energy range is divided into two parts, the region of resolved and unresolved resonances.
However, the neutron capture cross section of 236 U is low, and this process does not happen quickly in a thermal reactor. Spent nuclear fuel typically contains about 0.4% 236 U. With a much greater cross-section, 237 Np may eventually absorb another neutron and become 238 Np, which quickly beta decays to plutonium-238 (another non-fissile ...