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; beta decay) leading to the formation of stable ²⁰⁸Pb.