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 (4.468 billion years), and thorium-232 (14.05 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. The amount of Pu in the pre-Solar nebula (4.57×10 years ago) was estimated as 0.8% the amount of U. 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.

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 (taken from the same mine as in the early study) were observed, so only an upper limit on the Pu content was obtained:  < 1.5×10 g/g: 370 (or fewer) 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 (about seven times lower than the 2012 study). 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 (~0.008), this upper limit is still 18x greater than the expected present Pu content in the bastnasite sample (1.2×10 g/g).

Trace amounts of Pu (that arrived on Earth within the last 10 million years) were found in rock from the Pacific ocean by a Japanese oil exploration company.

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. It is important to recall, however, that in order to be a primordial nuclide – one constituting the amalgam orbiting the Sun that ultimately coalesced into the Earth – plutonium-244 must have comprised some of the solar nebula, rather than having been replenished by extrasolar meteoritic dust. The presence of Pu in a meteor without evidence that the meteor originated in the Solar System's formational disc, supports the hypothesis that Pu was abundant enough to have been a part of that disc, if an extrasolar meteor contained it in some other gravitationally supported system, but such a meteor cannot prove the hypothesis. Only the unlikely discovery of live Pu within the Earth's composition could do that.

As an extinct radionuclide

Plutonium-244 is one of several extinct radionuclides that preceded the formation of the Solar System. Its half-life of 80 million years ensured its circulation across the Solar System before its extinction, and indeed, Pu has not yet been found in matter other than meteorites. Radionuclides such as Pu, decay to produce fissiogenic (i.e., arising from fission) 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 (Pu/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 (Pu/U) time dependence.

Production

Unlike plutonium-238, plutonium-239, plutonium-240, plutonium-241, and plutonium-242, plutonium-244 is not produced in quantity by the nuclear fuel cycle, because further neutron capture on plutonium-242 produces plutonium-243 which has a short half-life (5 hours) and quickly beta decays to americium-243 before having much opportunity to further capture neutrons in any but very high neutron flux environments. The global inventory of Pu is about 20 grams. Plutonium-244 is also a minor constituent of thermonuclear fallout, with a global Pu/Pu fallout ratio of (5.7 ± 1.0) × 10.

Applications

Plutonium-244 is used as an internal standard for isotope dilution mass spectrometry analysis of plutonium.

References

{{Isotopes of plutonium

Nuclear materials
Isotopes of plutonium