Neutron activation

Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when nuclei capture free neutrons, becoming heavier and entering excited states. Frequently, the excited nucleus can decay by emitting particles (neutrons, protons, alphas, etc.) producing equal in mass, or even lighter nuclei. The neutron capture (or the capture and subsequent emission of particles), results in the formation of a new (residual as it is usually called) nucleus, an activation product. Such nuclei are frequently radioactive. Their half-lives can range from very short (of the order of ms sometimes) to rather long ones (many days, weeks, or even years), making the material radioactive.

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In places with high neutron fluxes (primarily the cores of nuclear reactors) neutron activation contributes to material erosion, and the materials themselves must be disposed of as low-level radioactive waste. Some materials are more subject to neutron activation than others, so a suitably chosen low-activation material can significantly reduce these problems. One way to demonstrate that nuclear fusion is occurring inside a Farnsworth-Hirsch fusor is to use a Geiger counter to measure the radioactivity induced in a sheet of aluminum foil.

The lasting radiation from a nuclear weapon is in large part due to the neutron activation of the bomb itself and the surrounding material.

Neutron activation also has a practical use. It is one of the most sensitive and accurate methods of trace element analysis. It requires no sample preparation or solubilization and can therefore be applied to objects that need to be kept intact such as a valuable piece of art. Although the activation induces radioctivity in the object, its level is typically low and its lifetime may be short, so that its effects have soon disappeared. In this sense, neutron activation is a non-destructive analysis method.