Almost all radioactive isotopes emit characteristic gamma rays, and many chemical elements can be recognized by their gamma ray spectra.Neutron activation analysis is based on the detection and measurement of characteristic gamma rays emitted from radioactive isotopes produced in a sample upon irradiation with neutrons. Depending on the source of the neutrons, their energies and the treatment of the samples, the technique takes on several differing forms. Neutron activation analyses may employ thermal neutrons (with energies below 0.5 eV); epithermal neutrons (with energies between 0.5 eV and about 0.5 MeV); or fast neutrons (with energies above 0.5 MeV). Neutron activation analysis is used when the amount of an element in a specimen is so small that techniques of ordinary chemical analysis are impractical. Neutron activation analysis is capable of detecting the presence of as little as 10^-9 grams of an element in an unknown specimen.

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