(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:

A positron is a positively charged, with a resting energy of at least 511 keV, subatomic particle. A positron is the antiparticle of an electron, identical in mass and spin.
Positrons can be generated by positron decay or pair production.
Positron emission tomography detects positrons from the decay of radioactive tracers.

See also Beta Decay.

Characterization of atoms by their nuclear properties, as the number of protons and the number of neutrons. The different nuclides of an element are its isotopes (equal proton number, but different neutron numbers). Isomers of this particular nuclide are equal in the proton and mass numbers, but differ in their energy content. Unstable nuclides which are radioactive are called radionuclides.

See also Isotope, Isomer and Decay.

Henri Becquerel demonstrated beta particles in 1900. Identical with electrons is there negative charge at -1. Their mass is 549 millionths of one AMU, 1/2000 of the mass of a proton or neutron. Beta particles consist of high energetic electrons emitted by radioactive nuclei or neutrons. By the process of beta decay, one of the neutrons in the nucleus is transformed into a proton and a new atom is formed which has one less neutron but one more proton in the core. Beta decay is accompanied by the emission of a positron (the antiparticle of the electron), a positive charged antineutrino. Beta particles have a greater range of penetration than alpha particles but less than gamma rays or x-rays. The name beta was coined by Rutherford in 1897. The traveling speed of beta particles depends on their energy. Because of their small mass and charge beta particles travel deep into tissues and cause cellular damage and possible cancer.

See also Radiation Shielding.

Internal radiation is produced by the decay of radioactive material in the body.