Neutron activation is the process by which neutron radiation induces radioactivity in materials. It occurs when nuclei capture free neutrons, becoming heavier nuclei in excited states. These nuclei are frequently radioactive, sometimes with very short half-lives, and so they and their decay products generally make the material radioactive. Thus unlike alpha, beta or gamma radiation, neutron radiation leaves the exposed materials emitting their own radiation long after exposure.
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 neutronactivation 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 neutron activation of the bomb itself and surrounding material.