Radioactivity is a spontaneous and irreversible transformation of the atomic nucleus. It is the source of ionizing particles (α particles, comprising two protons and two neutrons; β− rays or electrons; and strong electromagnetic radiations (γ rays)). Radioactive decay is archetypal of Poisson processes, which take place with a constant probability per unit of time. Radioactivity powers the energy flow of planetary interiors. It is also the theoretical basis of dating methods that are used to measure the age of rocky planets (geochronology).
After a serendipitous discovery by French physicist Henri Becquerel in 1896, radioactive elements were methodically investigated by Marie Curie. In 1928, George Gamow demonstrated that α decay was due to quantum mechanical tunneling, and in 1934 Enrico Fermi provided a successful explanation of the β− decay involving neutrinos.