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Naturally occurring samarium (Sm) is composed of five stable isotopes, 144Sm, 149Sm, 150Sm, 152Sm and 154Sm, and two extremely long-lived radioisotopes, 147Sm (1.06 y) and 148Sm (7 y), with 152Sm being the most abundant (26.75% natural abundance). 146Sm is also fairly long-lived (1.03×108 y), but occurs naturally as only the tiniest trace remains from its original supernova nucleosynthesis. Other than the naturally occurring isotopes, the longest-lived radioisotopes are 151Sm, which has a half-life of 88.8 years, and 145Sm, which has a half-life of 340 days. All of the remaining radioisotopes have half-lives that are less than two days, and the majority of these have half-lives that are less than 48 seconds. This element also has twelve known isomers with the most stable being 141mSm (t1/2 22.6 minutes), 143m1Sm (t1/2 66 seconds) and 139mSm (t1/2 10.7 seconds). The long lived isotopes,146Sm, 147Sm, and 148Sm primarily decay by alpha decay to isotopes of neodymium. Lighter unstable isotopes of samarium primarily decay by electron capture to isotopes of promethium, while heavier ones decay by beta decay to isotopes of europium Isotopes of samarium are used in samarium-neodymium dating for determining the age relationships of rocks and meteorites. 151Sm is a medium-lived fission product and acts as a neutron poison in the nuclear fuel cycle. The stable fission product 149Sm is also a neutron poison. Standard atomic mass: 150.36(2) u. ==Samarium-149== 149Sm is an observed stable isotope of samarium (predicted to decay, but no decays have ever been observed, giving it a half-life several orders of magnitude longer than the age of the universe), and a fission product (yield 1.0888%), which is also a neutron-absorbing nuclear poison with significant effect on nuclear reactor operation, second only to 135Xe. Its neutron cross section is 40140 barns for thermal neutrons. The equilibrium concentration (and thus the poisoning effect) builds to an equilibrium value in about 500 hours (about 20 days) of reactor operation, and since 149Sm is stable, the concentration remains essentially constant during further reactor operation. 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 samarium」の詳細全文を読む スポンサード リンク
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