The actinide series contains 14 elements following 89Ac (actinium), with atomic numbers 90–103, and corresponding to the infilling of 5f and/or 6d orbitals (Table E19). All isotopes are radioactive and only Ac (actinium), Th (thorium), Pa(protoactinium), U (uranium), Np (neptunium) and Pu (plutonium) are of geochemical interest (Table E20). U and Th exist in significant amounts in terrestrial materials because some of their isotopes have sufficiently long half-lives (T > 108 years) to have survived in the solar system since nucleosynthesis. Ac and Pa, being daughter elements in the decay products of U and Th, also exist naturally. In addition, Np and Pu have been isolated in trace amounts in the products of natural nuclear reactors (e.g. Oklo reactor in Gabon; Cowan, 1976); they also are by-products of the nuclear industry. They are continuously formed by neutron capture reactions such as:
Neutrons of the first reaction arise from spontaneous fission of 235U or from (α, n) reactions...
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Villemant, B. (1998). Elements: actinide series. In: Geochemistry. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4496-8_95
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