plutonium-244

Plutonium-244 (Pu) is an isotope of plutonium that has a half-life of 81.3 million years. This is longer than any other isotope of plutonium and longer than any other known isotope of an element beyond bismuth, except for the three naturally abundant ones: uranium-235 (704 million years), uranium-238 (4.463 billion years), and thorium-232 (14.0 billion years). Given the half-life of Pu, an exceedingly small amount should still be present on Earth, making plutonium a likely but unproven candidate as the shortest-lived primordial element.

==Natural occurrence== Accurate measurements, beginning in the early 1970s, appeared to detect primordial plutonium-244, making it the shortest-lived primordial nuclide. As the age of the Earth is about 56 half-lives of Pu, the amount of Pu left should be very small; Hoffman et al. estimated its content in the rare-earth mineral bastnasite as  = 1.0×10 g/g, which corresponded to the content in the Earth crust as low as 3×10 g/g (i.e. the total mass of plutonium-244 in Earth's crust is about 9 g). Since Pu cannot be easily produced by natural neutron capture in the low neutron activity environment of uranium ores (see below), its presence cannot plausibly be explained by any other means than creation by r-process nucleosynthesis in supernovae or neutron star mergers.