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Although the Po-210 has a shorter half life than Pu-239 and
Am-241, it decays directly to stable Pb-206. The Am-241 and Pu-239 are parents
of much longer decay chains. The longer lived parents of these decay chains
support the continued generation of daughter products which are all radioactive,
and decay by alpha, beta, and gamma emission. Once a separated Po-210 atom
decays, that is the end, but the radioactive daughters are generated as long as
there is still parent material present. Po-210 is only available because it can
be separated from natural uranium that has established secular equilibrium with
all the daughter products. Because some of the daughter products of the
long-lived parents are fairly long-lived by our time standards (1599 y for
Ra-226 which is a daughter of U-238). Once secular equilibrium is established,
every decay of the parent is followed by a decay of every daughter down the line
until stable Pb-206 is reached.
As long as there is parent material that has reached secular
equilibrium, there will be the same activity of every daughter in the decay
chain somewhere in the universe, no matter if they are separated or still in the
same location. If a daughter is separated from its parent, it will decay with
its own half life, and the daughter will grow in from the parent at exactly the
same rate (half way to equilibrium after one half-life of the daughter--just as
1/2 of the daughter is gone after one half life).
Michael A. Kay, ScD, CHMM
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