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Isotopes of samarium

Naturally occurring samarium (₆₂Sm) is composed of five stable isotopes, ¹⁴⁴Sm, ¹⁴⁹Sm, ¹⁵⁰Sm, ¹⁵²Sm and ¹⁵⁴Sm, and two extremely long-lived radioisotopes, ¹⁴⁷Sm (half life: 1.06×10¹¹ y) and ¹⁴⁸Sm (6.3×10¹⁵ y), with ¹⁵²Sm being the most abundant (26.75% natural abundance). ¹⁴⁶Sm is also fairly long-lived, 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extinct radionuclide.^[4][5] A 2012 paper revising the estimated half-life of ¹⁴⁶Sm from 10.3(5)×10⁷ y to 6.8(7)×10⁷ y was retracted in 2023.^[5][6] It is the longest-lived nuclide that has not yet been confirmed to be primordial.

Other than the naturally occurring isotopes, the longest-lived radioisotopes are ¹⁵¹Sm, which has a half-life of 94.6 years,^[7] and ¹⁴⁵Sm, which has a half-life of 340 days. All of the remaining radioisotopes, which range from ¹²⁹Sm to ¹⁶⁸Sm, 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 (t_1/2 22.6 minutes), ^143m1Sm (t_1/2 66 seconds) and ^139mSm (t_1/2 10.7 seconds).

The long lived isotopes, ¹⁴⁶Sm, ¹⁴⁷Sm, and ¹⁴⁸Sm, primarily decay by alpha decaytoisotopes of neodymium. Lighter unstable isotopes of samarium primarily decay by electron capturetoisotopes of promethium, while heavier ones decay by beta decaytoisotopes 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.

Samarium is theoretically the lightest element with even atomic number with no stable isotopes (all isotopes of it can theoretically go either alpha decayorbeta decayordouble beta decay), other such elements are those with atomic numbers > 66 (dysprosium, which is the heaviest theoretically stable nuclide).

List of isotopes[edit]

Samarium-149[edit]

Samarium-149 (¹⁴⁹Sm) 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 product of the decay chain from the fission product ¹⁴⁹Nd (yield 1.0888%). ¹⁴⁹Sm is 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. 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[edit]

Samarium-151 (¹⁵¹Sm) 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 ²³⁵U, about 39% of ¹⁴⁹Sm's yield. The yield is somewhat higher for ²³⁹Pu.

Its neutron absorption cross section for thermal neutrons is high at 15200 barns, about 38% of ¹⁴⁹Sm's absorption cross section, or about 20 times that of ²³⁵U. Since the ratios between the production and absorption rates of ¹⁵¹Sm and ¹⁴⁹Sm are almost equal, the two isotopes should reach similar equilibrium concentrations. Since ¹⁴⁹Sm reaches equilibrium in about 500 hours (20 days), ¹⁵¹Sm 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 ¹⁵¹Sm in the spent nuclear fuel at discharge is only a small fraction of the total ¹⁵¹Sm produced during the use of the fuel. According to one study, the mass fraction of ¹⁵¹Sm 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 0.15 for the medium-lived fission product ¹³⁷Cs.^[11] The decay energyof¹⁵¹Sm is also about an order of magnitude less than that of ¹³⁷Cs. The low yield, low survival rate, and low decay energy mean that ¹⁵¹Sm has insignificant nuclear waste impact compared to the two main medium-lived fission products ¹³⁷Cs and ⁹⁰Sr.

• ANL factsheet

Samarium-153[edit]

Samarium-153 (¹⁵³Sm) has a half-life of 46.3 hours, undergoing β⁻ decay into ¹⁵³Eu. As a component of samarium lexidronam, it is used in palliation of bone cancer.^[12] It is treated by the body in a similar manner to calcium, and it localizes selectively to bone.

References[edit]

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Isotopic compositions and standard atomic masses from:
  • de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
  • Wieser, Michael E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051.
• "News & Notices: Standard Atomic Weights Revised". International Union of Pure and Applied Chemistry. 19 October 2005.
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.