 A concentrated solution of plutonium-244 | |
| General | |
|---|---|
| Symbol | 244Pu |
| Names | plutonium-244 |
| Protons(Z) | 94 |
| Neutrons(N) | 150 |
| Nuclide data | |
| Natural abundance | Trace |
| Half-life(t1/2) | 8.13×107 years[1] |
| Isotope mass | 244.0642044[2]Da |
| Spin | 0+ |
| Parent isotopes | 248Cm (α) 244Np (β−) |
| Decay products | 240U |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| α (99.877%) | 4.666[3] |
| SF (0.123%) | |
| Isotopes of plutonium Complete table of nuclides | |
Plutonium-244 (244Pu) is anisotope of plutonium that has ahalf-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), andthorium-232 (14.0 billion years). Given the half-life of244Pu, an exceedingly small amount should still be present on Earth, making plutonium a likely but unproven candidate as the shortest-livedprimordial element.
Accurate measurements, beginning in the early 1970s, appeared to detect primordial plutonium-244,[4] making it the shortest-livedprimordial nuclide. As theage of the Earth is about 56 half-lives of244Pu, the amount of244Pu left should be very small; Hoffman et al. estimated its content in the rare-earth mineralbastnasite asc244 = 1.0×10−18 g/g, which corresponded to the content in theEarth crust as low as 3×10−25 g/g[4] (i.e. the total mass of plutonium-244 in Earth's crust is about 9 g). Since244Pu cannot be easily produced by naturalneutron capture in the low neutron activity environment ofuranium ores (see below), its presence cannot plausibly be explained by any other means than creation byr-processnucleosynthesis insupernovae orneutron star mergers.
However, the detection of primordial244Pu in 1971 is not confirmed by recent, more sensitive measurements[5] usingaccelerator mass spectrometry. In a 2012 study, no traces of244Pu in the samples ofbastnasite (taken from the same mine as in the early study) were observed, so only an upper limit on the244Pu content was obtained:c244 < 1.5×10−19 g/g: 370 (or fewer) atoms per gram of the sample, at least seven times lower than the abundance measured by Hoffman et al.[5] A 2022 study, once again using accelerator mass spectrometry, could not detect244Pu inBayan Obo bastnasite, finding an upper limit of < 2.1×10−20 g/g (about seven times lower than the 2012 study). Thus, the 1971 detection cannot have been a signal of primordial244Pu. Considering the likely abundance ratio of244Pu to238U in the early solar system (~0.008),[5] this upper limit is still 18 times greater than the expected present244Pu content in the bastnasite sample (1.2×10−21 g/g).[6]
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 theinterstellar medium.[7] Trace amounts of244Pu were also found in rock from the Pacific ocean by a Japaneseoil exploration company.[8] It is important to recall, however, that in order to be aprimordial nuclide – one whose origin lay in the amalgam orbiting theSun 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.
Plutonium-244 is one of severalextinct radionuclides that preceded the formation of the Solar System. Its half-life of 81.3 million years ensured its circulation across theSolar System before its extinction,[10] and so evidence of it should also be found throughout the Solar System.[11] Radionuclides such as244Pu, 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 from244Pu decay, it can be inferred that the Earth formed nearly 50–70 million years after the Solar System formed.[12]
Before the analysis ofmass spectroscopy data from analyzing samples found in meteorites, it was inferential at best to credit244Pu as being the nuclide responsible for the fissiogenic xenon found. However, an analysis of a laboratory sample of244Pu 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,244Cm, but such data proved contradictory and helped erase further doubts that the fission was appropriately attributed to244Pu.[9]
Both the examination of spectra data and study offission tracks led to several findings of plutonium-244. InWestern 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 of244Pu fission.[10] Presence of244Pu fission tracks can be established by using the initial ratio of244Pu to238U (Pu/U)0 at a time T0 = 4.58×109 years, when Xe formation first began in meteorites, and by considering how the ratio of Pu/U fission tracks varies over time. Examination of awhitlockite crystal within a lunar rock specimen brought byApollo 14, established proportions of Pu/U fission tracks consistent with the (Pu/U)0 time dependence.[11]
Plutonium-244 is not detected from its decay products, as other extinct radionuclides are, as it would have becomethorium-232, the only primordial isotope of its elements and so undetectable from isotopic analysis.
Unlikeplutonium-238,plutonium-239,plutonium-240,plutonium-241, andplutonium-242, plutonium-244 is not produced in quantity by thenuclear fuel cycle, because furtherneutron capture on plutonium-242 produces plutonium-243 which has a short half-life (~5 hours) and quicklybeta decays toamericium-243 before having much opportunity to further capture neutrons in any but very high neutron flux environments.[13] The global inventory of244Pu is about 20 grams.[14] Plutonium-244 is also a minor constituent ofthermonuclear fallout, with a global244Pu/239Pu fallout ratio of (5.7 ± 1.0) × 10−5.[15]
Plutonium-244 is used as aninternal standard forisotope dilution mass spectrometry analysis of plutonium.[14]