Uranium–uranium dating
U, with a half-life of about 4.5 billion years, decays to U through emission of an alpha particle to thorium-234 (Th), which is comparatively unstable with a half-life of just 24 days. Th then decays through beta particle emission to protactinium-234 (Pa)). This decays with a half-life of 6.7 hours, again through emission of a beta particle, to U. This isotope has a half-life of about 245,000 years. The next decay product, thorium-230 (Th), has a half-life of about 75,000 years and is used in the uranium-thorium technique. Although analytically simpler, in practice U/U requires knowledge of the ratio at the time the material under study was formed and is generally used only for samples older than the ca. 450,000 year upper limit of the Th/U technique. For those materials (principally marine carbonates) for which these conditions apply, it remains a superior technique.
Europium
Eu (half-life 13.516 years) and Eu (half-life 8.593 years) cannot be beta decay products because Sm and Sm are non-radioactive, but Eu is the only long-lived "shielded" nuclide, other than Cs, to have a fission yield of more than 2.5 parts per million fissions. A larger amount of Eu is produced by neutron activation of a significant portion of the non-radioactive Eu; however, much of this is further converted to Eu.