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Neutron flux is the term applied to the measurement of neutrons passing though a given region of space.

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.

Neutron flux may refer to the number of neutrons passing through a unit area in unit time. It is most commonly measured in neutrons/(cm³·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.

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.

Neutrons are produced during nuclear fusion. While this effect is used in most modern nuclear weapons in various ways to achieve sometimes dramatic increases of yield, it is a major drawback for proposed applications of nuclear fusion as an energy source: As the particles do not carry a charge, they cannot be deflected by electric or magnetic fields but inevitably collide with the containment vessel, leaving it radioactive. As this is one of the main obstacles to fusion power generation, the International Fusion Materials Irradiation Facility has been founded as an initiative to invent a suitable containment vessel.

Within a nuclear 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.

• Free neutron
• Neutron radiation

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