[Also: Half-Life Time, Radioactive Half-Life] The half-life is the time in which half the atoms (always a fraction, not a number) of a given radionuclide disintegrate from the amount of atoms present when measurement starts. From 200 atoms of a radionuclide with a half-life of one minute will 100 atoms disintegrate in the first minute, 50 in the second minute, etc. The half-life is a characteristic property of radioactive isotopes. The effective half-life includes all processes of elimination, including radioactive decay.
Different half-life terms:
- Physical Radioactive Half-Life
- Biological Radioactive Half-Life
- Effective Radioactive Half-Life.

See also Decay Constant, Decay.

Metastable isomers are produced through nuclear reactions. The excited state of the nucleus will de-excite by emission of gamma rays until the nuclear ground state is reached.
Isotopes in a metastable condition are designated with a 'm', for example 99mTc.

Neutron radiation is one type of ionizing radiation. Neutrons get emitted from an atom by the fission process or by decay processes. In the upper atmosphere neutron radiation is produced by the interaction of cosmic radiation with air. Neutron radiation is used for the production of medical isotopes and certain direct medical therapies.

See also Neutron Activation, Neutron Activation Analysis and Neutron Capture.

Fissile isotopes such as 233U, 235U or 239Pu get exposed to a thermal neutron beam. The resulting nuclear chain reaction is controlled by moderators such as graphite, Beryllium or heavy water.
For example Mo-99, source for Tc-99m, is produced by fission of highly enriched uranium (HEU) in a few reactors around the world (monitored and controlled by The International Atomic Energy Agency).

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.