Secular equilibrium

Secular equilibrium in radioactive decay

Secular equilibrium can only occur in a radioactive decay chain if the half-life of the daughter radionuclide B is much shorter than the half-life of the parent radionuclide A. In such a situation, the decay rate of A, and hence the production rate of B, is approximately constant, because the half-life of A is very long compared to the timescales being considered. The quantity of radionuclide B builds up until the number of B atoms decaying per unit time becomes equal to the number being produced per unit time; the quantity of radionuclide B then reaches a constant, equilibrium value. Assuming the initial concentration of radionuclide B is zero, full equilibrium usually takes several half-lives of radionuclide B to establish.

The quantity of radionuclide B when secular equilibrium is reached is determined by the quantity of its parent A and the half-lives of the two radionuclide. This can be seen from the time rate of change of the number of atoms of radionuclide B:

where λ_A and λ_B are the decay constants of radionuclide A and B, related to their half-lives t_1/2 by λ = ln ⁡ ( 2 ) / t 1 / 2 , and N_A and N_B are the number of atoms of A and B at a given time.

Secular equilibrium occurs when d N B / d t = 0 , or

Over long enough times, comparable to the half-life of radionuclide A, the secular equilibrium is only approximate; N_A decays away according to

and the "equilibrium" quantity of radionuclide B declines in turn. For times short compared to the half-life of A, λ A t ≪ 1 and the exponential can be approximated as 1.