Transient and secular radioactive equilibrium revisited
  The two definitions of radioactive equilibrium are revisited in this paper. The terms activity equilibrium and effective life equilibrium are proposed to take the place of currently used terms transient equilibrium and secular equilibrium. The proposed new definitions have the advantage of providing a clearer physics meaning. Besides the well known instant activity equilibrium, another class of exact effective life-time equilibrium is also discussed in this letter. 
Qinghui Zhang, Chandra Burman, Howard Amols
physics.med-ph