Isotopes of samarium
Naturally occurring samarium (Sm) is composed of five stable isotopes, Sm, Sm, Sm, Sm and Sm, and two extremely long-lived radioisotopes, Sm (half life: 1.06 y) and Sm (7 y), with Sm being the most abundant (26.75% natural abundance). Sm is also fairly long-lived (6.8×10 y), but is not long-lived enough to have survived in significant quantities from the formation of the Solar System on Earth, although it remains useful in radiometric dating in the Solar System as an extinct radionuclide. Other than the naturally occurring isotopes, the longest-lived radioisotopes are Sm, which  has a half-life of 88.8 years, and Sm, 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 Sm (t 22.6 minutes), Sm (t 66 seconds) and Sm (t 10.7 seconds). The long lived isotopes, Sm, Sm, and Sm, 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. Sm is a medium-lived fission product and acts as a neutron poison in the nuclear fuel cycle. The stable fission product Sm is also a neutron poison. Samarium-149. Samarium-149 (Sm) is an observationally stable isotope of samarium (predicted to decay, but no decays have ever been observed, giving it a half-life at least 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 Xe. 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 Sm is stable, the concentration remains essentially constant during further reactor operation. This contrasts with xenon-135, which accumulates from the beta decay of iodine-135 (a short lived fission product) and has a high neutron cross section, but itself decays with a half-life of  9.2 hours (so does not remain in constant concentration long after the reactor shutdown), causing the so-called xenon pit. Samarium-151. Samarium-151 (Sm) has a half-life of 88.8 years, undergoing low-energy beta decay, and has a fission product yield of 0.4203% for thermal neutrons and U, about 39% of Sm's yield. The yield is somewhat higher for Pu. Its neutron absorption cross section for thermal neutrons is high at 15200 barns, about 38% of Sm's absorption cross section, or about 20 times that of U. Since the ratios between the production and absorption rates of Sm  and Sm are almost equal, the two isotopes should reach similar equilibrium concentrations. Since Sm reaches equilibrium in about 500 hours (20 days), Sm should reach equilibrium in about 50 days. Since nuclear fuel is used for several years (burnup) in a nuclear power plant, the final amount of Sm in the spent nuclear fuel at discharge is only a small fraction of the total Sm produced during the use of the fuel. According to one study, the mass fraction of Sm in spent fuel is about 0.0025 for heavy loading of MOX fuel and about half that for uranium  fuel, which is roughly two orders of magnitude less than the mass fraction of about .15 for the medium-lived fission product Cs. The decay energy of Sm is also about an order of magnitude less than that of Cs. The low yield, low survival rate, and low decay energy mean that Sm has insignificant nuclear waste impact compared to the two main medium-lived fission products Cs and Sr. Samarium-153. Samarium-153 (Sm) has a half-life of 46.3 hours, undergoing β decay into Eu. As a component of samarium lexidronam, it is used in palliation of bone cancer. It  is treated by the body in a similar manner to calcium, and it localizes selectively to bone.