Plutonium-244 (244
Pu
) is an isotope of plutonium that has a half-life of 81.3 million years. This is longer than any of the other isotopes of plutonium and longer than any other actinide isotope 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 mathematics of the decay of plutonium-244, an exceedingly small amount should still be present in the Earth's composition, making plutonium a likely although unproven candidate as the shortest-lived primordial element.
 A concentrated solution of plutonium-244 | |
| General | |
|---|---|
| Symbol | 244Pu |
| Names | plutonium-244, 244Pu, Pu-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 (99.879%) | |
| SF (0.121%) | |
| Isotopes of plutonium Complete table of nuclides | |
Natural occurrence
editAccurate measurements, beginning in the early 1970s, appeared to detect primordial plutonium-244,[3] making it the shortest-lived primordial nuclide. The amount of 244Pu in the pre-Solar nebula (4.57×109 years ago) was estimated as 0.8% the amount of 238U.[4] As the age of the Earth is about 56 half-lives of 244Pu, the amount of plutonium-244 left should be very small; Hoffman et al. estimated its content in the rare-earth mineral bastnasite as c244 = 1.0×10−18 g/g, which corresponded to the content in the Earth crust as low as 3×10−25 g/g[3] (i.e. the total mass of plutonium-244 in Earth's crust is about 9 g). Since plutonium-244 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 244Pu in 1971 is not confirmed by recent, more sensitive measurements[4] using the method of accelerator mass spectrometry. In a 2012 study, no traces of plutonium-244 in the samples of bastnasite (taken from the same mine as in the early study) were observed, so only an upper limit on the 244Pu content was obtained: c244 < 1.5×10−19 g/g, which is 370 (or less) atoms per gram of the sample, at least seven times lower than the abundance measured by Hoffman et al.[4] A 2022 study, once again using accelerator mass spectrometry, could not detect 244Pu in Bayan 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 primordial 244Pu. Considering the likely abundance ratio of 244Pu to 238U in the early solar system (~0.008), this upper limit is still 18 times greater than the expected present 244Pu content in the bastnasite sample (1.2×10−21 g/g).[5]
Trace amounts of 244Pu (that arrived on Earth within the last 10 million years) were found in rock from the Pacific ocean by a Japanese oil exploration company.[6]
Live interstellar plutonium-244 has been detected in meteorite dust in marine sediments, although the levels detected are much lower than would be expected from current modelling of the in-fall from the interstellar medium.[7] 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 – that plutonium-244 must have comprised some of the solar nebula, rather than having been replenished by extrasolar meteoritic dust. The presence of plutonium-244 in meteoritic composition without evidence the meteor originated from the formational disc of the Solar System supports the hypothesis that 244Pu 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 244Pu within the Earth's composition could do that.
As an extinct radionuclide
editPlutonium-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,[9] and indeed, 244Pu has not yet been found in matter other than meteorites.[10] Radionuclides such as 244Pu undergo 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 the existing fissiogenic xenon is attributable to 244Pu decay, the timing of Earth's formation can be inferred to have occurred nearly 50–70 million years following the formation of the Solar System.[11]
Preceding the analysis of mass spectra data obtained by analyzing samples found in meteorites, it was inferential at best to accredit 244Pu as being the nuclide responsible for the fissiogenic xenon found. However, an analysis of a laboratory sample of 244Pu 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 to 244Pu.[12]
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 within 4.1–4.2 billion-year-old zircons was met with findings of diverse levels of 244Pu fission.[9] Presence of 244Pu fission tracks can be established by using the initial ratio of 244Pu to 238U (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 a whitlockite crystal within a lunar rock specimen brought over from the Apollo 14 mission established proportions of Pu/U fission tracks consistent with the (Pu/U)0 time dependence.[10]
Production
editUnlike 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.[13] The global inventory of 244Pu is roughly 20 grams.[14] Plutonium-244 is also a minor constituent of thermonuclear fallout, with a global 244Pu/239Pu fallout ratio of (5.7 ± 1.0) × 10−5.[15]
Applications
editPlutonium-244 is used as an internal standard for isotope dilution mass spectrometry analysis of plutonium.[14]
References
edit- ^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
- ^ Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 030003-1–030003-442. doi:10.1088/1674-1137/41/3/030003.
- ^ a b Hoffman, D. C.; Lawrence, F. O.; Mewherter, J. L.; Rourke, F. M. (1971). "Detection of Plutonium-244 in Nature". Nature. 234 (5325): 132–134. Bibcode:1971Natur.234..132H. doi:10.1038/234132a0. S2CID 4283169.
- ^ a b c Lachner, J.; et al. (2012). "Attempt to detect primordial 244Pu on Earth". Physical Review C. 85 (1): 015801. Bibcode:2012PhRvC..85a5801L. doi:10.1103/PhysRevC.85.015801.
- ^ Wu, Yang; Dai, Xiongxin; Xing, Shan; Luo, Maoyi; Christl, Marcus; Synal, Hans-Arno; Hou, Shaochun (2022). "Direct search for primordial 244Pu in Bayan Obo bastnaesite". Chinese Chemical Letters. 33 (7): 3522–3526. doi:10.1016/j.cclet.2022.03.036. Retrieved 29 January 2024.
- ^ Greenfieldboyce, Nell (May 13, 2021). "Freshly-made plutonium from outer space found on ocean floor". NPR.
- ^ Wallner, A.; Faestermann, T.; Feige, J.; Feldstein, C.; Knie, K.; Korschinek, G.; Kutschera, W.; Ofan, A.; Paul, M.; Quinto, F.; Rugel, G.; Steier, P. (2015). "Abundance of live 244Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis". Nature Communications. 6: 5956. arXiv:1509.08054. Bibcode:2015NatCo...6.5956W. doi:10.1038/ncomms6956. ISSN 2041-1723. PMC 4309418. PMID 25601158.
- ^ Alexander, E. C.; Lewis, R. S.; Reynolds, J. H.; Michel, M. C. (1971-01-01). "Plutonium-244: Confirmation as an Extinct Radioactivity". Science. 172 (3985): 837–840. Bibcode:1971Sci...172..837A. doi:10.1126/science.172.3985.837. JSTOR 1731927. PMID 17792940. S2CID 35389103.
- ^ a b Turner, Grenville; Harrison, T. Mark; Holland, Greg; Mojzsis, Stephen J.; Gilmour, Jamie (2004-01-01). "Extinct 244Pu in Ancient Zircons". Science. 306 (5693): 89–91. Bibcode:2004Sci...306...89T. doi:10.1126/science.1101014. JSTOR 3839259. PMID 15459384. S2CID 11625563.
- ^ a b Hutcheon, I. D.; Price, P. B. (1972-01-01). "Plutonium-244 Fission Tracks: Evidence in a Lunar Rock 3.95 Billion Years Old". Science. 176 (4037): 909–911. Bibcode:1972Sci...176..909H. doi:10.1126/science.176.4037.909. JSTOR 1733798. PMID 17829301. S2CID 25831210.
- ^ Kunz, Joachim; Staudacher, Thomas; Allègre, Claude J. (1998-01-01). "Plutonium-Fission Xenon Found in Earth's Mantle". Science. 280 (5365): 877–880. Bibcode:1998Sci...280..877K. doi:10.1126/science.280.5365.877. JSTOR 2896480. PMID 9572726.
- ^ Alexander, E. C.; Lewis, R. S.; Reynolds, J. H.; Michel, M. C. (1971-01-01). "Plutonium-244: Confirmation as an Extinct Radioactivity". Science. 172 (3985): 837–840. Bibcode:1971Sci...172..837A. doi:10.1126/science.172.3985.837. JSTOR 1731927. PMID 17792940. S2CID 35389103.
- ^ Armstrong, Christopher R.; Brant, Heather A.; Nuessle, Patterson R.; Hall, Gregory; Cadieux, James R. (22 February 2016). "Anthropogenic plutonium-244 in the environment: Insights into plutonium's longest-lived isotope". Scientific Reports. 6 (1): 21512. Bibcode:2016NatSR...621512A. doi:10.1038/srep21512. eISSN 2045-2322. PMC 4761908. PMID 26898531.
- ^ a b Patton, Bradley D; Alexander, Charles W; Benker, Dennis; Collins, Emory D; Romano, Catherine E; Wham, Robert M (January 2011). "Preserving Plutonium-244 as a National Asset". osti.gov. OSTI 1024694. Retrieved 2022-10-02.
- ^ Steier, P.; Hrnecek, E.; Priller, A.; Quinto, F.; Srncik, M.; Wallner, A.; Wallner, G.; Winkler, S. (January 2013). "AMS of the Minor Plutonium Isotopes". Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 294 (2): 160–164. Bibcode:2013NIMPB.294..160S. doi:10.1016/j.nimb.2012.06.017. ISSN 0168-583X. PMC 3617651. PMID 23565016.