where the half life
of the precursor isotope
is so long that the change of its activity
can be ignored during the period of interest and all activities remain constant.
PAC, 1982, 54, 1533
(Glossary of terms used in nuclear analytical chemistry (Provisional))
on page 1541
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook