Radioactive Decay

Three principal modes account for most decay. Alpha emission expels a helium nucleus, reducing the mass number by four and the atomic number by two, and is characteristic of the heaviest nuclides. Beta-minus emission converts a neutron into a proton with the ejection of an electron and an antineutrino, raising the atomic number by one at constant mass number, while beta-plus emission and electron capture move in the opposite direction. Gamma emission carries away excess energy from an excited nucleus without changing its composition. The process is statistical: the number of nuclei decaying in a given interval is proportional to the number present, which gives an exponential law and a constant half-life independent of the sample size. Successive decays of the heavy nuclides form the natural decay chains that terminate at stable lead isotopes.