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.
Yttrium
Yttrium in the Solar System was created through stellar nucleosynthesis, mostly by the s-process (≈72%), but also by the r-process (≈28%). The r-process consists of rapid neutron capture of lighter elements during supernova explosions. The s-process is a slow neutron capture of lighter elements inside pulsating red giant stars.
Yttrium isotopes are among the most common products of the nuclear fission of uranium in nuclear explosions and nuclear reactors. In the context of nuclear waste management, the most important isotopes of yttrium are Y and Y, with half-lives of 58.51 days and 64 hours, respectively. Though Y has a short half-life, it exists in secular equilibrium with its long-lived parent isotope, strontium-90 (Sr) with a half-life of 29 years.