(NAA) Neutron activation analysis is a very sensitive analytical technique to determine even very low concentration of chemical elements, trace elements for example, in small biological samples.
NAA becomes commercial available in the USA in 1960.
In the activation process stable nuclides in the sample, which is placed in a neutron beam (neutron flux, 90-95% are thermal neutron with low energy levels under 0.5 eV), will change to radioactive nuclides through neutron capture (artificial radioactivity). These radioactive nuclides decay by emitting alpha-, beta-particles and gamma-rays with a unique half-life. Qualitative and quantitative analysis of the sample is done with a high-resolution gamma-ray spectrometer.
NAA is subdivided into the following techniques:

Radioactive decay is the change of instable atoms to a more stable state. This change to a different nuclide by the spontaneous emission of radiation such as alpha or beta particles, gamma rays, or by electron capture follows an element-specific decay chain. Each step in the decay chain has a definite half-life.
Sometimes also the reduction of excitation energy of the nucleus by e.g. internal conversion is mentioned as radioactive decay.

See also Decay Chain, Radioisotope.

Law which describes the reduction in radioactivity of a material in dependence of time.

See also Decay and Half-Life.