View source for Plutonium-244

{{Short description|Isotope of plutonium}}
{{main|Isotopes of plutonium}}
{{Infobox isotope
 | image = Pu-244 solution.png
 | image_caption = A concentrated solution of plutonium-244
 | alternate_names =
 | symbol =Pu
 | mass_number =244
 | mass = 244.0642044{{AME2016 II|ref}}
 | num_neutrons =150
 | num_protons =94
 | abundance = [[Trace radioisotope|Trace]]
 | halflife = {{val|8|e=7|u=years}}{{NUBASE2016|ref}}
 | decay_product = uranium-240
 | decay_symbol = U
 | decay_mass = 240
 | decay_mode1 = [[Alpha decay|α]] (99.879%)
 | decay_energy1 = &nbsp;
 | decay_mode2 = [[Spontaneous fission|SF]] (0.121%)
 | decay_energy2 = &nbsp;
 | decay_mode3 =
 | decay_energy3 =
 | decay_mode4 =
 | decay_energy4 =
 | parent = curium-248
 | parent_symbol = Cm
 | parent_mass = 248
 | parent_decay = α
 | parent2 = neptunium-244
 | parent2_symbol = Np
 | parent2_mass = 244
 | parent2_decay = [[Beta decay|β<sup>−</sup>]]
 | spin = 0+
 | excess_energy =
 | binding_energy =
}}
'''Plutonium-244''' ('''{{SimpleNuclide|Plutonium|244}}''') is an [[isotope of plutonium]] that has a [[half-life]] of 80 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 nuclide|primordial element]].

==Natural occurrence==
Accurate measurements, beginning in the early 1970s, appeared to detect [[Primordial nuclide|primordial]] plutonium-244,<ref name="PU244">{{cite journal | last1 = Hoffman | first1 = D. C. | last2 = Lawrence | first2 = F. O. | last3 = Mewherter | first3 = J. L. | last4 = Rourke | first4 = F. M. | year = 1971| title = Detection of Plutonium-244 in Nature | journal = [[Nature (journal)|Nature]] | volume =  234| issue = 5325 | pages =  132–134| doi = 10.1038/234132a0 | bibcode = 1971Natur.234..132H | s2cid = 4283169 }}</ref> making it the shortest-lived [[primordial nuclide]]. The amount of <sup>244</sup>Pu in the pre-[[Solar nebula]] (4.57×10<sup>9</sup>&nbsp;years ago) was estimated as 0.8% the amount of <sup>238</sup>U.<ref name="PRC"/> As the [[age of the Earth]] is about 57 half-lives of <sup>244</sup>Pu, the amount of plutonium-244 left should be very small; Hoffman et al. estimated its content in the rare-earth mineral [[bastnasite]] as {{math|''c''<sub>244</sub>}}&nbsp;=&nbsp;1.0×10<sup>−18</sup>&nbsp;g/g, which corresponded to the content in the [[Earth crust]] as low as 3×10<sup>−25</sup>&nbsp;g/g<ref name="PU244"/> (i.e. the total mass of plutonium-244 in Earth's crust is about 9&nbsp;g). Since plutonium-244 cannot be easily produced by natural [[neutron capture]] in the low neutron activity environment of [[uranium ore]]s (see below), its presence cannot plausibly be explained by any other means than creation by [[r-process]] [[stellar nucleosynthesis|nucleosynthesis]] in [[supernovae]] or [[neutron star merger]]s.

However, the detection of primordial <sup>244</sup>Pu in 1971 is not confirmed by recent, more sensitive measurements<ref name="PRC">{{cite journal|last=Lachner|first=J.|display-authors=etal|date=2012|title=Attempt to detect primordial <sup>244</sup>Pu on Earth|journal=Physical Review C|volume=85|issue=1 |pages=015801| doi=10.1103/PhysRevC.85.015801|bibcode=2012PhRvC..85a5801L}}</ref> 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 <sup>244</sup>Pu content was obtained: {{math|''c''<sub>244</sub>}}&nbsp;<&nbsp;1.5×10<sup>−19</sup>&nbsp;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.<ref name=PRC/> A 2022 study, once again using accelerator mass spectrometry, could not detect <sup>244</sup>Pu in [[Bayan Obo]] bastnasite, finding an upper limit of <&nbsp;2.1×10<sup>−20</sup>&nbsp;g/g (about seven times lower than the 2012 study). Thus, the 1971 detection cannot have been a signal of primordial <sup>244</sup>Pu. Considering the likely abundance ratio of <sup>244</sup>Pu to <sup>238</sup>U in the early solar system (~0.008), this upper limit is still 18&nbsp;times greater than the expected present <sup>244</sup>Pu content in the bastnasite sample (1.2×10<sup>−21</sup>&nbsp;g/g).<ref>{{cite journal |last1=Wu |first1=Yang |last2=Dai |first2=Xiongxin |first3=Shan |last3=Xing |first4=Maoyi |last4=Luo |first5=Marcus |last5=Christl |first6=Hans-Arno |last6=Synal |first7=Shaochun |last7=Hou |date=2022 |title=Direct search for primordial <sup>244</sup>Pu in Bayan Obo bastnaesite |url=http://www.ccspublishing.org.cn/article/doi/10.1016/j.cclet.2022.03.036?pageType=en |journal=Chinese Chemical Letters |volume=33 |issue=7 |pages=3522–3526 |doi=10.1016/j.cclet.2022.03.036 |access-date=29 January 2024}}</ref>

Trace amounts of <sup>244</sup>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.<ref>{{cite web |url=https://www.npr.org/2021/05/13/996499035/freshly-made-plutonium-from-outer-space-found-on-ocean-floor |title=Freshly-made plutonium from outer space found on ocean floor |last1=Greenfieldboyce |first1=Nell |date=May 13, 2021 |publisher=NPR |access-date=}}</ref>

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]].<ref name="WallnerFaestermann2015">{{cite journal|last1=Wallner|first1=A.|last2=Faestermann|first2=T.|last3=Feige|first3=J.|last4=Feldstein|first4=C.|last5=Knie|first5=K.|last6=Korschinek|first6=G.|last7=Kutschera|first7=W.|last8=Ofan|first8=A.|last9=Paul|first9=M.|last10=Quinto|first10=F.|last11=Rugel|first11=G.|last12=Steier|first12=P.|title=Abundance of live <sup>244</sup>Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis|journal=Nature Communications|volume=6|year=2015|pages=5956|issn=2041-1723|doi=10.1038/ncomms6956|pmid=25601158 |pmc=4309418 |arxiv=1509.08054|bibcode=2015NatCo...6.5956W}}</ref>  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 <sup>244</sup>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 <sup>244</sup>Pu within the Earth's composition could do that.

===As an extinct radionuclide===
[[File:FissionYield.png|thumb|left|300px|A comparison of the relative fissiogenic xenon yields found in the meteorites Pasamonte and Kapoeta with those of a laboratory sample of plutonium-244.<ref>{{Cite journal|last1=Alexander|first1=E. C.|last2=Lewis|first2=R. S.|last3=Reynolds|first3=J. H.|last4=Michel|first4=M. C.|date=1971-01-01|title=Plutonium-244: Confirmation as an Extinct Radioactivity|jstor=1731927|journal=Science|volume=172|issue=3985|pages=837–840|bibcode = 1971Sci...172..837A |doi = 10.1126/science.172.3985.837|pmid=17792940|s2cid=35389103 }}</ref>]]
Plutonium-244 is one of several [[extinct radionuclide]]s 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,<ref name=":1">{{Cite journal|title = Extinct <sup>244</sup>Pu in Ancient Zircons|jstor = 3839259|journal = Science|date = 2004-01-01|pages = 89–91|volume = 306|issue = 5693|first1 = Grenville|last1 = Turner|first2 = T. Mark|last2 = Harrison|first3 = Greg|last3 = Holland|first4 = Stephen J.|last4 = Mojzsis|first5 = Jamie|last5 = Gilmour|bibcode = 2004Sci...306...89T |doi = 10.1126/science.1101014 |pmid = 15459384| s2cid=11625563 }}</ref> and indeed, <sup>244</sup>Pu has not yet been found in matter other than meteorites.<ref name=":2">{{Cite journal|title = Plutonium-244 Fission Tracks: Evidence in a Lunar Rock 3.95 Billion Years Old|jstor = 1733798|journal = Science|date = 1972-01-01|pages = 909–911|volume = 176|issue = 4037|first1 = I. D.|last1 = Hutcheon|first2 = P. B.|last2 = Price|bibcode = 1972Sci...176..909H |doi = 10.1126/science.176.4037.909|pmid=17829301| s2cid=25831210 }}</ref> Radionuclides such as <sup>244</sup>Pu 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 <sup>244</sup>Pu 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.<ref>{{Cite journal|title = Plutonium-Fission Xenon Found in Earth's Mantle|jstor = 2896480|journal = Science|date = 1998-01-01|pages = 877–880|volume = 280|issue = 5365|first1 = Joachim|last1 = Kunz|first2 = Thomas|last2 = Staudacher|first3 = Claude J.|last3 = Allègre|bibcode = 1998Sci...280..877K |doi = 10.1126/science.280.5365.877 |pmid = 9572726}}</ref>

Preceding the analysis of [[mass spectroscopy|mass spectra]] data obtained by analyzing samples found in meteorites, it was inferential at best to accredit <sup>244</sup>Pu as being the nuclide responsible for the fissiogenic xenon found. However, an analysis of a laboratory sample of <sup>244</sup>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, <sup>244</sup>Cm, but such data proved contradictory and helped erase further doubts that the fission was appropriately attributed to <sup>244</sup>Pu.<ref>{{Cite journal|title = Plutonium-244: Confirmation as an Extinct Radioactivity|jstor = 1731927|journal = Science|date = 1971-01-01|pages = 837–840|volume = 172|issue = 3985|first1 = E. C.|last1 = Alexander|first2 = R. S.|last2 = Lewis|first3 = J. H.|last3 = Reynolds|first4 = M. C.|last4 = Michel|bibcode = 1971Sci...172..837A |doi = 10.1126/science.172.3985.837|pmid=17792940| s2cid=35389103 }}</ref>

Both the examination of spectra data and study of [[fission track]]s 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 <sup>244</sup>Pu fission.<ref name=":1" /> Presence of <sup>244</sup>Pu fission tracks can be established by using the initial ratio of <sup>244</sup>Pu to <sup>238</sup>U (Pu/U)<sub>0</sub> at a time T<sub>0</sub>&nbsp;=&nbsp;{{val|4.58|e=9}} 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)<sub>0</sub> time dependence.<ref name=":2" />

==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&nbsp;hours) and quickly [[beta decay]]s to [[americium-243]] before having much opportunity to further capture neutrons in any but very high neutron flux environments.<ref name="AnthroPu244">{{cite journal |last1=Armstrong|first1=Christopher R.|last2=Brant|first2=Heather A.|last3=Nuessle|first3=Patterson R.|last4=Hall|first4=Gregory|last5=Cadieux|first5=James R.|title=Anthropogenic plutonium-244 in the environment: Insights into plutonium's longest-lived isotope|journal=Scientific Reports|date=22 February 2016|volume=6|issue=1|page=21512 |eissn=2045-2322|doi=10.1038/srep21512|bibcode=2016NatSR...621512A|pmid=26898531|pmc=4761908|url=}}</ref>  The global inventory of <sup>244</sup>Pu is roughly 20 grams.<ref name="PreservingPu244">{{cite journal | last1=Patton | first1=Bradley D | last2=Alexander | first2=Charles W | last3=Benker | first3=Dennis | last4=Collins | first4=Emory D | last5=Romano | first5=Catherine E | last6=Wham | first6=Robert M | title=Preserving Plutonium-244 as a National Asset | website=osti.gov | date=January 2011 | osti=1024694 | url=https://www.osti.gov/biblio/1024694 | access-date=2022-10-02}}</ref>  Plutonium-244 is also a minor constituent of [[thermonuclear fallout]], with a global <sup>244</sup>Pu/<sup>239</sup>Pu fallout ratio of (5.7 ± 1.0) × 10<sup>−5</sup>.<ref name="AMSMinorPu">{{cite journal |last1=Steier|first1=P.|last2=Hrnecek|first2=E.|last3=Priller|first3=A.|last4=Quinto|first4=F.|last5=Srncik|first5=M.|last6=Wallner|first6=A.|last7=Wallner|first7=G.|last8=Winkler|first8=S.|title=AMS of the Minor Plutonium Isotopes|journal=Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms|date=January 2013|volume=294|issue=2 |pages=160–164 |issn=0168-583X|doi=10.1016/j.nimb.2012.06.017|bibcode=2013NIMPB.294..160S|pmid=23565016|pmc=3617651|url=}}</ref>

==Applications==
Plutonium-244 is used as an [[internal standard]] for [[isotope dilution]] mass spectrometry analysis of plutonium.<ref name="PreservingPu244"/>

== References ==
{{reflist}}
{{Isotopes of plutonium}}

[[Category:Actinides]]
[[Category:Nuclear materials]]
[[Category:Isotopes of plutonium]]