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Naturally occurring xenon (Xe) is made of eight stable isotopes and one very long-lived isotope. (124Xe, 126Xe, and 134Xe are predicted to undergo double beta decay, but this has never been observed in these isotopes, so they are considered to be stable.)〔(Status of ββ-decay in Xenon ), Roland Lüscher, accessed on line September 17, 2007. 〕 Xenon has the second highest number of stable isotopes. Only tin, with 10 stable isotopes, has more. Beyond these stable forms, there are over 30 unstable isotopes and isomers that have been studied, the longest-lived of which is 136Xe, which undergoes double beta decay with a half-life of with the next longest lived being 127Xe with a half-life of 36.345 days. Of known isomers, the longest-lived is 131mXe with a half-life of 11.934 days. 129Xe is produced by beta decay of 129I (half-life: 16 million years); 131mXe, 133Xe, 133mXe, and 135Xe are some of the fission products of both 235U and 239Pu, and therefore used as indicators of nuclear explosions. The artificial isotope 135Xe is of considerable significance in the operation of nuclear fission reactors. 135Xe has a huge cross section for thermal neutrons, 2.65×106 barns, so it acts as a neutron absorber or "poison" that can slow or stop the chain reaction after a period of operation. This was discovered in the earliest nuclear reactors built by the American Manhattan Project for plutonium production. Fortunately the designers had made provisions in the design to increase the reactor's reactivity (the number of neutrons per fission that go on to fission other atoms of nuclear fuel). Relatively high concentrations of radioactive xenon isotopes are also found emanating from nuclear reactors due to the release of this fission gas from cracked fuel rods or fissioning of uranium in cooling water. The concentrations of these isotopes are still usually low compared to the naturally occurring radioactive noble gas 222Rn. Because xenon is a tracer for two parent isotopes, Xe isotope ratios in meteorites are a powerful tool for studying the formation of the solar system. The I-Xe method of dating gives the time elapsed between nucleosynthesis and the condensation of a solid object from the solar nebula (Xenon being a gas, only that part of it that formed after condensation will be present inside the object). Xenon isotopes are also a powerful tool for understanding terrestrial differentiation. Excess 129Xe found in carbon dioxide well gases from New Mexico was believed to be from the decay of mantle-derived gases soon after Earth's formation. Standard atomic mass: 131.293(6) u. All other isotopes have half-lives less than 12 days, most less than 20 hours. The shortest-lived isotope is 148Xe with a half-life of 408 ns. Its 41 isotopes have mass numbers ranging from 108 to 148. 108Xe (disc. 2011) is the second heaviest nuclide with equal numbers of protons and neutrons, after 112Ba. ==Xenon-133== Xenon-133 (sold as a drug under the brand name Xeneisol, ATC code ) is an isotope of xenon. It is a radionuclide that was inhaled to assess pulmonary function, and to image the lungs. It is also used to image blood flow, particularly in the brain. 133Xe is also an important fission product. 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 xenon」の詳細全文を読む スポンサード リンク
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