Equilibrium is a condition established in a parent/daughter mixture when both parent and daughter are radioactive and when the daughter’s half-life is shorter than that of the parent. If the daughter’s half-life exceeds that of the parent, equilibrium will never be reached.

There are two types of equilibrium:

• Secular Equilibrium  
  

•  Transient Equilibrium

The basic difference between these two types of equilibrium is the following: during 10 half-lives of the daughter, essentially no parent decay takes place during secular equilibrium, but significant decay takes place during transient equilibrium.

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.