secular equilibrium

https://doi.org/10.1351/goldbook.S05532
@R05084@ where the @H02717@ of the @P04805@ is so long that the change of its @A00114@ can be ignored during the period of interest and all activities remain constant.
Source:
PAC, 1982, 54, 1533. (Glossary of terms used in nuclear analytical chemistry (Provisional)) on page 1541 [Terms] [Paper]