Transient Equilibrium is a condition reached when the t_phys of the parent is approximately 10 times greater than the t_phys of the daughter. A classical example is the ⁹⁹Mo/^99mTc Generator, where the ratio of the half-lives is 67 hr/6 hr = 11:1. During the 60 hr period representing 10 half-lives of ^99mTc , almost 50% of the ⁹⁹Mo has disappeared, as noted in diagram. This represents a very significant amount, unlike the negligible amount in secular equilibrium.

The relationship between Half-lives in transient equilibrium is the same as that in secular equilibrium:

        _{apparent                                       physical}

     t                                      =       t

          ^{daughter                                       parent}

 

There is a rule of thumb regarding how long it takes to reach equilibrium:

  Transient equilibrium is reached in ~4 t_½ of daughter. For Tc-99m, predicted length of time is 24 hours; actual time to equilibrium is 23 hr.

  Secular equilibrium is reached in ~6 t_½ of daughter