Radioactive decay of a parent nuclide through a sequence of radioactive daughter nuclides to a final, stable daughter nuclide. Uranium has three naturally occurring isotopes: 238U, 235U, and 234U; all are radioactive. Thorium exists mainly as one isotope (232Th) which is also radioactive. In addition, five radioactive isotopes of thorium occur in nature as short-lived, intermediate daughters of 238U, 235U, and 232Th. Each of these isotopes is the parent of a chain (decay series) of radioactive daughters, each ending with stable isotopes of lead. Decay of 238U gives rise to the ‘uranium series’ which includes 234U as an intermediate daughter and ends in stable 206Pb. The decay of 235U gives rise to the ‘actinium series’ which ends in stable 207Pb. The decay of 232Th results in the emission of six alpha and four beta particles (see alpha decay; and beta decay) leading to the formation of stable 208Pb.
Subjects: Ecology and Conservation — Earth Sciences and Geography.