Among the members of the decay series of U-238 (see Figure 4.4) and Th-234 (see Figure 4.6), Ra-226, Rn-222, Pb-210, Bi-210, Po-210, and Ra-228, Th-228, Rn-220, Pb-212 isotopes, respectively, have been used in radiotracer studies. Their applications are discussed briefly here.

8.5.1.1 Th-234

As already discussed, the Th-232 isotope is the parent nuclide in a radioactive decay series. Its half-life and decay constant are 14 billion years and 5 X 1011/ years, respectively, so its radioactivity is also very low and can be measured only with difficulty. The half-life of the other thorium isotope, Th-234, is much shorter (24.1 days), so the same number of Th-234 nuclides gives 1011 times higher radio­activity than Th-232. Obviously, therefore, Th-234 is used for the labeling of Th-232.

The characteristic oxidation state of thorium is +4, so it has been used as a radioindicator of other elements with +4 oxidation state (e. g., Ce(IV)). These studies have been especially significant in colloid chemistry, where the effect of the tetravalent “ions” on the colloid processes (e. g., adsorption and coagulation) has been investigated.

The Th-234 isotope was separated from U-238 series by extraction with ether by Fajans and Goring.