RADIOACTIVITYDOE-HDBK-1019/1-93 Atomic and Nuclear PhysicsNP-01Page 40Rev. 0Figure 14 Approach of Sodium-24 to Equilibrium The development of the equation to calculate how the amount of sodium-24 changes over timeas it approaches the equilibrium value is beyond the scope of this handbook. However, theequation is presented below.This equation can be used to calculate the values of the amount of sodium-24 present at differenttimes. As the time increases, the exponential term approaches zero, and the number of atomspresent will approach R/. A plot of the approach of sodium-24 to equilibrium is shown inFigure 14.TransientRadioactiveEquilibriumTransient radioactive equilibriumoccurs when the parent nuclide and the daughter nuclidedecay at essentially the same rate.For transient equilibrium to occur, the parent must have a long half-life when compared to thedaughter. An example of this type of compound decay process is barium-140, which decays bybeta emission to lanthanum-140, which in turn decays by beta emission to stable cerium-140.