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Plutonium-244
(詳細は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).
Accurate measurements, beginning in the early 1970s, have detected primordial plutonium-244,〔D. C. Hoffman, F. O. Lawrence, J. L. Mewherter, F. M. Rourke: "Detection of Plutonium-244 in Nature", in: ''Nature'' 1971, ''234'', 132–134; .〕 making it the shortest-lived primordial nuclide. The amount of 244Pu in the pre-Solar nebula (4.57×109 years ago) was estimated as 0.008 of amount of 238U.〔 As the age of the Earth is about 57 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 =1.0×10−18 g/g, which corresponded to the content in the Earth crust as low as 3×10−25 g/g〔 (i.e. the total mass of plutonium-244 in the Earth 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. Plutonium-244 is thus the shortest-lived and heaviest primordial isotope yet detected or theoretically predicted.
However, the detection of primordial 244Pu in 1971 is not confirmed by recent, more sensitive measurements〔J. Lachner et al. ''Attempt to detect primordial 244Pu on Earth''. Phys. Rev. C 85 (2012) 015801. 〕 using the method of accelerator mass spectrometry. In this 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: < 0.15×10−18 g/g, which is 370 (or less) atoms per gram of the sample, at least 7 times lower than the abundance measured by Hoffman ''et al.''.
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 halflife (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. However, a nuclear weapon explosion can produce some plutonium-244 by rapid successive neutron capture.
== References ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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