Plutonium-244
Plutonium-244 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, uranium-238, and thorium-232. 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. Since Pu cannot be easily produced by natural neutron capture in the low neutron activity environment of uranium ores, its presence cannot plausibly be explained by any other means than creation by r-process nucleosynthesis in supernovae or neutron star mergers.However, the detection of primordial Pu in 1971 is not confirmed by recent, more sensitive measurements using accelerator mass spectrometry. In a 2012 study, no traces of Pu in the samples of bastnasite were observed, so only an upper limit on the Pu content was obtained: < 1.5×10 g/g: 370 atoms per gram of the sample, at least seven times lower than the abundance measured by Hoffman et al. A 2022 study, once again using accelerator mass spectrometry, could not detect Pu in Bayan Obo bastnasite, finding an upper limit of < 2.1×10 g/g. Thus, the 1971 detection cannot have been a signal of primordial Pu. Considering the likely abundance ratio of Pu to U in the early solar system, this upper limit is still 18 times greater than the expected present Pu content in the bastnasite sample.
Live interstellar plutonium-244 has been detected in meteorite dust in marine sediments, though the levels detected are much lower than would be expected from current modelling of the in-fall from the interstellar medium. Trace amounts of Pu were also found in rock from the Pacific ocean by a Japanese oil exploration company. It is important to recall, however, that in order to be a primordial nuclide – one whose origin lay in the amalgam orbiting the Sun that ultimately coalesced into the Earth – the plutonium-244 must have comprised some of the solar nebula, rather than having been replenished by extrasolar meteoritic dust.
As an extinct radionuclide
Plutonium-244 is one of several extinct radionuclides that preceded the formation of the Solar System. Its half-life of 81.3 million years ensured its circulation across the Solar System before its extinction, and so evidence of it should also be found throughout the Solar System. Radionuclides such as Pu, decay to produce fissiogenic xenon isotopes that can then be used to time the events of the early Solar System. In fact, by analyzing data from Earth's mantle which indicates that about 30% of existing fissiogenic xenon is from Pu decay, it can be inferred that the Earth formed nearly 50–70 million years after the Solar System formed.Before the analysis of mass spectroscopy data from analyzing samples found in meteorites, it was inferential at best to credit Pu as being the nuclide responsible for the fissiogenic xenon found. However, an analysis of a laboratory sample of Pu compared with that of fissiogenic xenon gathered from the meteorites Pasamonte and Kapoeta produced matching spectra that immediately left little doubt as to the source of the isotopic xenon anomalies. Spectra data was further acquired for another actinide isotope, Cm, but such data proved contradictory and helped erase further doubts that the fission was appropriately attributed to Pu.
Both the examination of spectra data and study of fission tracks led to several findings of plutonium-244. In Western Australia, the analysis of the mass spectrum of xenon in 4.1–4.2-billion-year-old zircons was met with findings of diverse levels of Pu fission. Presence of Pu fission tracks can be established by using the initial ratio of Pu to U at a time T = years, when Xe formation first began in meteorites, and by considering how the ratio of Pu/U fission tracks varies over time. Examination of a whitlockite crystal within a lunar rock specimen brought by Apollo 14, established proportions of Pu/U fission tracks consistent with the time dependence.
Plutonium-244 is not detected from its decay products, as other extinct radionuclides are, as it would have become thorium-232, the only primordial isotope of its elements and so undetectable from isotopic analysis.