radioactive equilibrium

Definition: The equilibrium reached by a radioactive series in which the rate of decay of each nuclide is equal to its rate of production. It follows that all rates of decay of the different nuclides within the sample are equal when radioactive equilibrium is achieved. For example, in the uranium series, uranium-238 decays to throium-234. Initially, the rate of production of thorium will exceed the rate at which it is decaying and the thorium content of the sample will rise. As the amount of thorium increases, its activity increases; eventually a situation is reached in which the rate of production of thorium is equal to its rate of decay. The proportion of thorium in the sample will then remain constant. Thorium decays to produce protactinium-234; some time after the stabilisation of the thorium content, the protactinium content will also stabilise. When the whole radioactive series attains stabilisation, the sample is said to be in radioactive equilibrium.