radioactivity

Radioactivity is a general term for the behavior of some atomic nuclei which eventually spontaneously change, at the same time emitting one or more particles, a type of event termed radioactive decay. The emissions are dangerous to life and referred to as radiation, but confusingly, the term radiation also encompasses electromagnetic radiation (EMR), much of which is a very ordinary and necessary part of our lives.

Many configurations of protons and neutrons (nuclear isomers) are unstable and some have a tendency to undergo such decay, forming a different configuration. A nucleus's "tendency to decay" has a constant probability, with any specific configuration having the same probability of a particular decay over any given second, but for different configurations, the likelihood differs. A particular configuration's likelihood is quantified as a half-life, the expected time for half of a large number of such nuclei to decay. One classification of such decays is by the particle(s) emitted, among them alpha decay (emission of an alpha particle, identical to a helium-4 nucleus), beta decay (emission of a beta particle, i.e., an electron or positron), and gamma decay (emission of a gamma-ray photon). Some numerical combinations of protons and neutrons have no stable configuration, and some of these radioactive decays adjust these counts. One pattern of such decays is a nucleus undergoing alpha or beta decay changing its numbers of protons and neutrons, then very soon undergoing gamma decay changing to a (more) stable arrangement of these neutrons and protons, the emitted photon taking away excess energy.

Nucleosynthesis (such as Big Bang nucleosynthesis or that within stars and supernovae) produces such radioactive nuclei (as well as stable nuclei), after which such decays occur until the result is stable nuclei. Characteristics of supernovae are explored through their EMR showing the spectral signature of various radioactive elements within various stages of their light curves.