Polonium-210 has a half-life of 138 days and decays to the stable daughter isotope of lead, Pb.
The more parent isotopes there are - and the fewer daughter isotopes - the younger the sample.
Using the "age equation", nobody knows what D0 (number of atoms of the daughter isotope in the original composition) is.
We don't know because we don't know how much of the daughter isotope was there at the time of formation.
Francium-223 is the fifth product of the actinium decay series as the daughter isotope of actinium-227.
It is the ninth product of the neptunium decay series as a daughter isotope of actinium-225.
The steady decay of the uranium source into its daughter isotopes makes it a kind of radioactive clock.
A parent isotope is one that undergoes decay to form a daughter isotope.
The daughter of a daughter isotope is sometimes called a granddaughter isotope.
Pb is the only non-radiogenic lead isotope, therefore is not one of the daughter isotopes.
