Secular equilibrium: a condition reached when the tphys of the parent is many times greater than the tphys of the daughter, e.g., 100-1000 times greater. To keep things in perspective, during 10 half-lives of the daughter, decay of the parent is negligible. Decay of the parent is represented by the flat line in the diagram below.
Transient equilibrium : a condition reached when the tphys of the parent is approximately 10 times greater than the tphys of the daughter. A classical example is the Mo-99/Tc-99m Generator. During the 67 hr period representing 11 half-lives of Tc-99m, 50% of the Mo-99 has disappeared, as noted in the following diagram. In both transient and secular equilibrium, apparent half-life of the daughter = physical half-life of the parent
|Stephen Karesh, PhD.||
Last Updated: August 14, 1996