secular equilibrium

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sec·u·lar e·qui·lib·ri·um

a type of radioactive equilibrium in which the half-life of the precursor (parent) radioisotope is so much longer than that of the product (daughter) that the radioactivity of the daughter becomes equal to that of the parent with time.
References in periodicals archive ?
ANSTO established that the uranium was in secular equilibrium and therefore not adversely affecting gamma readings, and that thorium concentrations were probably not a major contributing factor to gamma readings.
8 day half-life) in secular equilibrium is the only standard worldwide that provides a National Institute of Standards and Technology (NIST) traceable calibration for F-18 and is the culmination of two years of collaborative research by NIST.