Neutron activation

Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emitting gamma rays, or particles such as beta particles, alpha particles, fission products, and neutrons (in nuclear fission). Thus, the process of neutron capture, even after any intermediate decay, often results in the formation of an unstable activation product. Such radioactive nuclei can exhibit half-lives ranging from small fractions of a second to many years.

Neutron activation is the only common way that a stable material can be made radioactive. All naturally occurring materials, including air, water, and soil, can be induced (activated) by neutron capture into some amount of radioactivity in varying degrees, as a result of the production of neutron-rich radioisotopes. Some atoms require more than one neutron to become unstable, which makes them harder to activate because a double or triple capture by a nucleus is less likely than a single capture. Water, for example, is made up of hydrogen and oxygen. Hydrogen (most common isotope, ¹H) requires two captures to attain instability as tritium (hydrogen-3), while oxygen (most common isotope, ¹⁶O) requires three captures to become unstable oxygen-19. Thus, water is difficult to activate, unlike sodium chloride (NaCl), in which both sodium and chlorine become unstable with one capture each (see Isotopes of sodium; Isotopes of chlorine). These facts were experienced at the Operation Crossroads atomic test series in 1946.