Secular Equilibrium
The steady state reached when a long-lived parent supports a short-lived daughter at constant activity.
Formula / equation
λ₁N₁ = λ₂N₂
Unit
Bq
In depth
When the parent's half-life vastly exceeds the daughter's, the daughter is produced as fast as it decays and its activity becomes equal to the parent's. This is why uranium ores contain a fixed, tiny equilibrium amount of radium and radon.
Examples in the real world
Radium-226 supporting radon-222 in undisturbed uranium ore for millions of years.