The uranium and thorium decay series have numerous radionuclides with half-lives ranging from fractions of a second to a few years. The former include many of the isotopes of radon, polonium, bismuth, lead and thallium and these are generally too short to show appreciable disequilibrium with respect to their precursors in the decay series. Others, however, have half-lives that make them useful in tracing a variety of oceanic processes that occur on short time scales. This group includes isotopes of thorium: 234Th (half-life = 24.1 d), 227Th (18.6 d), 228Th (1.9 y), radium: 224Ra (3.64 d),...

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