2.5 Disintegration series

Radioactive decay also often occurs in a series (or chain) with a number of daughter products, which are also radioactive, and terminates in a stable isotope. In a closed system, and starting with a specified amount of a mother element, the number of atoms of daughter elements and their activity grows gradually until radioactive equilibrium of the disintegration series is reached. At this point, the activities of all the radionuclides of the series are identical. Thus the measurement of the concentration of any daughter element can be used to estimate the concentration of any other element in the decay series. Under equilibrium conditions, this relationship can be expressed as follows:

λ₁N₁ = λ₂ N₂ = λ₃ N₃ = ... λ_iN_i

Examples of chain disintegration are the natural decay series ²³⁸U, ²³⁵U and ²³²Th.