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Naturally occurring samarium (₆₂Sm) is composed of five stable isotopes, Sm, Sm, Sm, Sm and Sm, and two extremely long-lived radioisotopes, Sm (half life: 1.066×10 y) and Sm (6.3×10 y), with Sm being the most abundant (26.75% natural abundance). Sm (9.20×10 y) 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. 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 Sm, which has a half-life of 94.6 years, and Sm, which has a half-life of 340 days. All of the remaining radioisotopes, which range from Sm to Sm, 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_1/2 22.6 minutes), Sm (t_1/2 66 seconds) and Sm (t_1/2 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. A 2012 paper revising the estimated half-life of Sm from 10.3(5)×10 y to 6.8(7)×10 y was retracted in 2023.

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

List of isotopes

1. Sm – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. Bold half-life – nearly stable, half-life longer than age of universe.
5. ^ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
6. Modes of decay:

   IT:	Isomeric transition
   p:	Proton emission

7. Bold italics symbol as daughter – Daughter product is nearly stable.
8. Bold symbol as daughter – Daughter product is stable.
9. ( ) spin value – Indicates spin with weak assignment arguments.
10. Believed to undergo ββ decay to Nd
11. ^ Primordial radioisotope
12. ^ Fission product
13. Used in Samarium–neodymium dating
14. ^ Neutron poison in reactors
15. Believed to undergo α decay to Nd with a half-life over 2×10 years
16. Believed to undergo α decay to Nd
17. Believed to undergo α decay to Nd
18. Believed to undergo ββ decay to Gd with a half-life over 2.3×10 years

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 product of the decay chain from the fission product Nd (yield 1.0888%). Sm is 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 94.6 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 0.15 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.

• ANL factsheet

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.

References

• 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.

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2. ^ Chiera, Nadine M.; Sprung, Peter; Amelin, Yuri; Dressler, Rugard; Schumann, Dorothea; Talip, Zeynep (1 August 2024). "The Sm half-life re-measured: consolidating the chronometer for events in the early Solar System". Scientific Reports. 14 (1). doi:10.1038/s41598-024-64104-6. PMC 11294585.
3. "Standard Atomic Weights: Samarium". CIAAW. 2005.
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5. Samir Maji; et al. (2006). "Separation of samarium and neodymium: a prerequisite for getting signals from nuclear synthesis". Analyst. 131 (12): 1332–1334. Bibcode:2006Ana...131.1332M. doi:10.1039/b608157f. PMID 17124541.
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7. ^ Kinoshita, N.; Paul, M.; Kashiv, Y.; Collon, P.; Deibel, C. M.; DiGiovine, B.; Greene, J. P.; Henderson, D. J.; Jiang, C. L.; Marley, S. T.; Nakanishi, T.; Pardo, R. C.; Rehm, K. E.; Robertson, D.; Scott, R.; Schmitt, C.; Tang, X. D.; Vondrasek, R.; Yokoyama, A. (30 March 2012). "A Shorter 146Sm Half-Life Measured and Implications for 146Sm-142Nd Chronology in the Solar System". Science. 335 (6076): 1614–1617. arXiv:1109.4805. Bibcode:2012Sci...335.1614K. doi:10.1126/science.1215510. ISSN 0036-8075. PMID 22461609. S2CID 206538240. (Retracted, see doi:10.1126/science.adh7739, PMID 36996231,  Retraction Watch)
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11. https://www-nds.iaea.org/sgnucdat/c3.htm Cumulative Fission Yields, IAEA
12. Christophe Demazière. Reactor Physics Calculations on MOX Fuel in Boiling Water Reactors (BWRs) (PDF) (Report). OECD Nuclear Energy Agency. Figure 2, page 6
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Table of nuclides Categories: Isotopes Tables of nuclides Metastable isotopes Isotopes by element

• Isotopes of samarium
• Samarium
• Lists of isotopes by element