Th-232 decay chain

Both decay chains involve a gaseous isotope. Rn-220 in the Th-232 decay chain is short-lived (<1 min) and is unlikely to diffuse from solids Rn-222 in the U-238 decay chain is longer-lived (3.8 d) and diffusion before daughters grow in is more credible. Short-lived daughters after the radon gap will exist for analytical purposes if their radon parents are present. It is noted that Pb-210 at the end of the U-238 chain is long-lived on the time scale of sample analysis (22 y), and will remain after sample collection even if Rn-222 is lost subsequently. 

Results for Th-232 Decay Chains. Considering Th-232 first, Pb-212 has been chosen as the reference nuclide since it has the largest number of measurements above detection in the dataset. The Pb-212 results from all three labs are closely similar and the means are not significantly different, thus these data are regarded as consistent. 

Pb-212 and Tl-208 are identified as y-emitting nuclides that can be used to make reliable estimates of the Th-232 decay chain. Th-232 average activity in secular equilibrium and assumed to be from natural materials is estimated as 0.027 Bq/g. 

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