Secular equilibrium

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In nuclear physics, secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate (e.g., due to decay of a parent isotope) is equal to its decay rate.

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:

{\frac {dN_{B}}{dt}}=\lambda _{A}N_{A}-\lambda _{B}N_{B} ,

where λA and λB are the decay constants of radionuclide A and B, related to their half-lives t1/2 by \lambda =ln(2)/t_{{1/2}}, and NA and NB are the number of atoms of A and B at a given time.

Secular equilibrium occurs when dN_{B}/dt=0, or

N_{B}={\frac {\lambda _{A}}{\lambda _{B}}}N_{A} .

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

N_{A}(t)=N_{A}(0)e^{{-\lambda _{A}t}} ,

and the "equilibrium" quantity of radionuclide B declines in turn. For times short compared to the half-life of A, \lambda _{A}t\ll 1 and the exponential can be approximated as 1.

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References

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