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.
Plutonium-242
Plutonium-242 is one of the isotopes of plutonium, the second longest-lived, with a half-life of 373,300 years.
Pu's halflife is about 15 times as long as Pu-239's halflife; therefore, it is one-fifteenth as radioactive and not one of the larger contributors to nuclear waste radioactivity.
Pu's gamma ray emissions are also weaker than those of the other isotopes.