Thorium decay series.

Unstable nuclei (radionuclides) do not necessarily decay directly into stable nuclides. They often decay into other radionuclides, which subsequently undergo further radioactive decay. The sequence of transformations between the original radionuclide and the final stable nuclide is known as a decay series.

Four principal radioactive decay series are recognised: the thorium, neptunium, uranium (or uranium-radium) and actinium (or uranium-actinium) series. They originate from 232Th, 237Np, 238U and 235U, respectively. Successive transformations within these series occur mainly through alpha and beta decay. In alpha decay, the mass number decreases by four, whereas in beta decay it remains unchanged. Gamma radiation may also be emitted as excited daughter nuclei lose excess energy.

Neptunium decay series.

Uranium decay series.

Actinium decay series.

Because alpha decay changes the mass number by four and beta decay does not change it, each series retains its characteristic mass-number relationship: 4n, 4n+1, 4n+2 or 4n+3. The four decay series therefore remain distinct and do not merge.

Decay series contain radionuclides with widely differing half-lives and describe their successive transformations. Some stages of a decay series can branch because a radionuclide may decay by more than one mode. Knowledge of decay series and the relationships between parent and daughter nuclides is important in geochronology and in studies of the origin and history of geological materials.

The neptunium decay series is virtually absent from nature today because its longest-lived member, 237Np, has a half-life of only about 2.14 million years. Any primordial 237Np has therefore long since decayed.