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Naturally occurring europium (Eu) is composed of 2 isotopes, 151Eu and 153Eu, with 153Eu being the most abundant (52.2% natural abundance). While 153Eu is stable, 151Eu was recently found to be unstable and to undergo alpha decay with half-life of (4.62 ± 0.95(stat.) ± 0.68(syst.)) × 1018 y.〔 〕 Besides natural radioisotope 151Eu, 36 artificial radioisotopes have been characterized, with the most stable being 150Eu with a half-life of 36.9 years, 152Eu with a half-life of 13.516 years, and 154Eu with a half-life of 8.593 years. All of the remaining radioactive isotopes have half-lives that are less than 4.7612 years, and the majority of these have half-lives that are less than 12.2 seconds. This element also has 17 meta states, with the most stable being 150mEu (t1/2 12.8 hours), 152m1Eu (t1/2 9.3116 hours) and 152m2Eu (t1/2 96 minutes). The primary decay mode before the most abundant stable isotope, 153Eu, is electron capture, and the primary mode after is beta decay. The primary decay products before 153Eu are isotopes of samarium and the primary products after are isotopes of gadolinium. Standard atomic mass: 151.964(1) u. ==Europium-155== Europium-155 is a fission product with a half-life of 4.76 years. It has a maximum decay energy of 252 KeV. In a thermal reactor (almost all current nuclear power plants), it has a low fission product yield, about half of one percent as much as the most abundant fission products. 155Eu's large neutron capture cross section (about 3900 barns for thermal neutrons, 16000 resonance integral) means that most of even the small amount produced is destroyed in the course of the nuclear fuel's burnup. Yield, decay energy, and halflife are all far less than 137Cs and 90Sr, so 155Eu is not a significant contributor to nuclear waste. Some 155Eu is also produced by successive neutron capture on 153Eu (nonradioactive, 350 barns thermal, 1500 resonance integral, yield is about 5 times as great as 155Eu) and 154Eu (half-life 8.6 years, 1400 barns thermal, 1600 resonance integral, fission yield is extremely small because beta decay stops at 154Sm); however the differing cross sections mean that both 155Eu and 154Eu are destroyed faster than they are produced. 154Eu is a prolific emitter of gamma radiation.〔http://www-nds.ipen.br/sgnucdat/b2.pdf〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Isotopes of europium」の詳細全文を読む スポンサード リンク
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