| Abstrakt: |
Actinide elements are ubiquitous in nature. Uranium and thorium are present in the Earth's crust with average concentrations of 1-10 µg g−1, making them more abundant than Ag, Sb, Cd, or Hg. While U and Th can even be found as major or minor mineral constituents in a variety of geochemical environments, typically they are highly dispersed and present only at trace or ultra-trace concentrations in most natural materials. Because 238U, 235U, and 232Th are the parents of the three naturally occurring non-extinct radioactive decay chains, they are always accompanied by lower concentrations of their radioactive progeny, many of which are also actinides (Table 30.1). The chemical behavior of 238U, 235U, 232Th, and their progeny have been studied for years because accurate determination of their relative abundances can provide geochronological information on a variety of geological/environmental processes. Careful analyses have revealed that trace amounts of the transuranic actinides 237Np and 239Pu occur naturally and are present in U ores with mass ratios of <2 × 10−12 and <1 × 10−11 for 237Np/U and 239Pu/U, respectively (Levine and Seaborg, 1951; Peppard et al., 1952). Once considered extinct, the long-lived plutonium isotope 244Pu was first proposed to have existed in the early solar system by Kuroda (1960). The presence of this nuclide in the early solar system was first confirmed via its signature of fissiogenic Xe isotopes (Rowe and Kuroda, 1965). [ABSTRACT FROM AUTHOR] |
