From left to right: thorium, uranium, actinium and neptunium decay chain
A decay chain refers to a sequence of radioactive decays.
Three primordial decay chains have been observed in nature,
with starting nuclides
232Th (T½ = 1.405×1010 y),
238U (T½ = 4.468×109 y) and
235U (T½ = 4.07×108 y):
thorium series
uranium series
actinium series
In addition to these chains, there is a fourth one:
neptunium series
Due to the relatively short half-life of the starting nuclide
237Np (T½ = 1.14×106 y),
this chain is already extinct in nature.
