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.

[Beta Plus Decay] If an atom is unstable because there are too many protons in the nucleus, a proton is converted into a neutron and a positron is emitted. The atomic mass of the atom stays unchanged, but the number of protons increases by one, the number of neutrons decreases by one, which transforms the atom to a different element.

See also Beta Decay.

(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 sample is placed into a concentrated beam of neutrons. Through neutron-capture heavier nuclei become frequently unstable. This artificial radiation decays with a characteristic half-live consisting of alpha- and beta-particles and gamma-rays.

See Neutron Activation Analysis

Emitting is for example the release of particles (electrons, etc.) or rays (x-rays, etc.) as a consequence of e.g. a nuclear (decay, fission, or fusion) process.

See also Decay and Beta Radiation.