Neutron cross section

In <a href="/facts/Nuclear_physics/fzQbt1aF">nuclear physics</a>, the concept of a neutron cross section is used to express the likelihood of interaction between an incident <a href="/facts/Neutron/oRCM1geb">neutron</a> and a target nucleus. The neutron cross section σ can be defined as the area in cm2 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 target nuclei. In conjunction with the <a href="/facts/Neutron_flux/nDE9Nlrx">neutron flux</a>, it enables the calculation of the reaction rate, for example to derive the thermal <a href="/facts/Power_(physics)/mkaEyRnn">power</a> of a <a href="/facts/Nuclear_power_plant/IJj5pJUn">nuclear power plant</a>. The standard unit for measuring the cross section is the <a href="/facts/Barn_(unit)/0yCEtKJ9">barn</a>, which is equal to 10−28 m2 or 10−24 cm2. The larger the neutron cross section, the more likely a neutron will react with the nucleus.
An <a href="/facts/Isotope/KD9B1y6o">isotope</a> (or <a href="/facts/Nuclide/97Ykvzr3">nuclide</a>) can be classified according to its neutron cross section and how it reacts to an incident neutron. Nuclides that tend to absorb a neutron and either <a href="/facts/Radioactive_decay/Ya6FVjt4">decay</a> or keep the neutron in its nucleus are <a href="/facts/Neutron_absorber/bkruR3yz">neutron absorbers</a> and will have a capture cross section for that reaction. Isotopes that undergo <a href="/facts/Nuclear_fission/LfkIylvm">fission</a> are <a href="/facts/Fissile_material/XJR9qoxW">fissionable</a> 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 <a href="/facts/Uranium-238/QhcHm6q8">uranium-238</a>, have nonzero cross sections of all three.
Isotopes which have a large scatter cross section and a low mass are good <a href="/facts/Neutron_moderator/1qa6YvDD">neutron moderators</a> (see chart below). Nuclides which have a large absorption cross section are <a href="/facts/Neutron_poison/bkruR3yz">neutron poisons</a> if they are neither fissile nor undergo decay. A poison that is purposely inserted into a nuclear reactor for controlling its <a href="/facts/Effective_neutron_multiplication_factor/3l0lHBM4">reactivity</a> in the long term and improve its <a href="/facts/Shutdown_(nuclear_reactor)/gQr5uJrc">shutdown margin</a> is called a burnable poison.

Neutron cross section open-in-new

Neutron cross section