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.
Xenon-135
Iodine-135 is a fission product of uranium with a yield of about 6% (counting also the iodine-135 produced almost immediately from decay of fission-produced tellurium-135). This I decays with a 6.57 hour half-life to Xe. Thus, in an operating nuclear reactor, Xe is being continuously produced. Xe has a very large neutron absorption cross-section, so in the high neutron flux environment of a nuclear reactor core, the Xe soon absorbs a neutron and becomes almost-stable Xe. Thus, in about 50 hours, the Xe concentration reaches equilibrium where its creation by I decay is balanced with its destruction by neutron absorption.