Radioactive equilibrium

With regard to a decay series, radioactive equilibrium refers to a condition in which the (activity) ratio between the parent nuclide and each of its radioactive decay products is constant.

This means that each daughter nuclide (decay product) is decaying at the same rate as it is being newly created. The activity ratios between the individual radionuclides of the decay series therefore remain constant.

A radioactive equilibrium can only come about if the following conditions are met:

• The half-life of the parent nuclide is longer than the longest half-life of the decay products.
• Sufficient time has passed for an equilibrium to develop.
• There are no interfering factors that affect the radionuclides of a decay series in different ways.