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Is there any reason for the names of the decay chains? As shown in this chart (larger version here): enter image description here

only the Thorium chain starts on an isotope of the element it takes its name from, and it can also start from Uranium.

My bet is that they were given that name because it was thought those were the starting elements, and it was later discovered that there were parent isotopes. Am I right?

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It would be worth taking a look at the lifetime of the states above the element that gives each series it's name and reflecting on the ratio of equilibrium abundances... As always a online table of the isotopes is useful. –  dmckee Jan 17 at 21:34
Ra226 has a half life of 1601 years, and an abundance of 100%, and Uranium 238 has a half life of billions of years and abundance of 99.3%... I don't know what can we conclude from this. –  carllacan Jan 18 at 16:44

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