Neutron flux

Neutron flux is a term referring to the number of neutrons passing through an area over a span of time. It is most commonly measured in neutrons/(cm²·s). This is drawn from the mathematical definition of flux. The neutron fluence is defined as the neutron flux integrated over a certain time period and represents the number of neutrons per unit area that passed during this time.

Both within natural processes and in the experimental laboratory, neutron flux may be applied to atomic nuclei, in which nuclei are bombarded with neutrons at a steady rate. This can be used to produce different isotopes, including unstable, radioactive ones, of a given chemical element.

Natural neutron flux

Neutron flux in Asymptotic Giant Branch stars and in supernovae is responsible for most of the natural nucleosynthesis producing elements heavier than iron. In stars there is a relatively low neutron flux on the order of 10⁵ to 10¹¹ neutrons per cm² per second, resulting in nucleosynthesis by the s-process (slow-neutron-capture-process). By contrast, after a core-collapse supernova, there is an extremely high neutron flux, on the order of 10²² neutrons per cm² per second, resulting in nucleosynthesis by the r-process (rapid-neutron-capture-process).

Artificial neutron flux

Artificial neutron flux refers to neutron flux which is man-made, either as bi-products from weapons or nuclear energy production or for specific application such as from a Research reactor or by Spallation. A flow of neutrons is often used to initiate the fission of unstable large nuclei. The extra neutron(s) pushes the nuclide over the edge, causing it to split to form more stable products. This effect is essential in fission reactors and nuclear weapons.

Within a nuclear fission reactor the neutron flux is primarily the form of measurement used to control the reaction inside. The flux shape is the term applied to the density or relative strength of the flux as it moves around the reactor. Typically the strongest neutron flux occurs in the middle of the reactor core, becoming lower as you approach the edges. The higher the neutron flux the greater the chance of a nuclear reaction occurring as there are more neutrons going through an area.