Isotopes of samarium (62Sm)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
144Sm 3.08% stable
145Sm synth 340 d ε 145Pm
146Sm trace 1.03×108 y α 142Nd
147Sm 15% 1.07×1011 y α 143Nd
148Sm 11.3% 6.3×1015 y α 144Nd
149Sm 13.8% stable
150Sm 7.37% stable
151Sm synth 94.6 y β− 151Eu
152Sm 26.7% stable
153Sm synth 46.28 h β− 153Eu
154Sm 22.7% stable
Standard atomic weight Ar°(Sm)
  • 150.36±0.02
  • 150.36±0.02 (abridged)[2][3]

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extinct radionuclide.[4][5] A 2012 paper revising the estimated half-life of 146Sm from 10.3(5)×107 y to 6.8(7)×107 y was retracted in 2023.[5][6] It is the longest-lived nuclide that has not yet been confirmed to be primordial.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 (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.

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

Nuclide
[n 1]		 Z N Isotopic mass (Da)
[n 2][n 3]		 Half-life
[n 4][n 5]		 Decay
mode
[n 6]		 Daughter
isotope
[n 7][n 8]		 Spin and
parity
[n 9][n 5]		 Natural abundance (mole fraction)
Excitation energy[n 5] Normal proportion Range of variation
129Sm 62 67 128.95464(54)# 550(100) ms 5/2+#
130Sm 62 68 129.94892(43)# 1# s β+ 130Pm 0+
131Sm 62 69 130.94611(32)# 1.2(2) s β+ 131Pm 5/2+#
β+, p (rare) 130Nd
132Sm 62 70 131.94069(32)# 4.0(3) s β+ 132Pm 0+
β+, p 131Nd
133Sm 62 71 132.93867(21)# 2.90(17) s β+ 133Pm (5/2+)
β+, p 132Nd
134Sm 62 72 133.93397(21)# 10(1) s β+ 134Pm 0+
135Sm 62 73 134.93252(17) 10.3(5) s β+ (99.98%) 135Pm (7/2+)
β+, p (.02%) 134Nd
135mSm 0(300)# keV 2.4(9) s β+ 135Pm (3/2+, 5/2+)
136Sm 62 74 135.928276(13) 47(2) s β+ 136Pm 0+
136mSm 2264.7(11) keV 15(1) μs (8−)
137Sm 62 75 136.92697(5) 45(1) s β+ 137Pm (9/2−)
137mSm 180(50)# keV 20# s β+ 137Pm 1/2+#
138Sm 62 76 137.923244(13) 3.1(2) min β+ 138Pm 0+
139Sm 62 77 138.922297(12) 2.57(10) min β+ 139Pm 1/2+
139mSm 457.40(22) keV 10.7(6) s IT (93.7%) 139Sm 11/2−
β+ (6.3%) 139Pm
140Sm 62 78 139.918995(13) 14.82(12) min β+ 140Pm 0+
141Sm 62 79 140.918476(9) 10.2(2) min β+ 141Pm 1/2+
141mSm 176.0(3) keV 22.6(2) min β+ (99.69%) 141Pm 11/2−
IT (.31%) 141Sm
142Sm 62 80 141.915198(6) 72.49(5) min β+ 142Pm 0+
143Sm 62 81 142.914628(4) 8.75(8) min β+ 143Pm 3/2+
143m1Sm 753.99(16) keV 66(2) s IT (99.76%) 143Sm 11/2−
β+ (.24%) 143Pm
143m2Sm 2793.8(13) keV 30(3) ms 23/2(−)
144Sm 62 82 143.911999(3) Observationally stable[n 10] 0+ 0.0307(7)
144mSm 2323.60(8) keV 880(25) ns 6+
145Sm 62 83 144.913410(3) 340(3) d EC 145Pm 7/2−
145mSm 8786.2(7) keV 990(170) ns
[0.96(+19−15) μs]		 (49/2+)
146Sm 62 84 145.913041(4) 1.03(5)×108 y[8] α 142Nd 0+ Trace
147Sm[n 11][n 12][n 13] 62 85 146.9148979(26) (1.066±0.5)×1011 y α 143Nd 7/2− 0.1499(18)
148Sm[n 11] 62 86 147.9148227(26) (6.3±1.3)×1015 y α 144Nd 0+ 0.1124(10)
149Sm[n 12][n 14] 62 87 148.9171847(26) Observationally stable[n 15] 7/2− 0.1382(7)
150Sm 62 88 149.9172755(26) Observationally stable[n 16] 0+ 0.0738(1)
151Sm[n 12][n 14] 62 89 150.9199324(26) 94.6±0.6 y β− 151Eu 5/2−
151mSm 261.13(4) keV 1.4(1) μs (11/2)−
152Sm[n 12] 62 90 151.9197324(27) Observationally stable[n 17] 0+ 0.2675(16)
153Sm[n 12] 62 91 152.9220974(27) 46.2846±0.0023 h β− 153Eu 3/2+
153mSm 98.37(10) keV 10.6(3) ms IT 153Sm 11/2−
154Sm[n 12] 62 92 153.9222093(27) Observationally stable[n 18] 0+ 0.2275(29)
155Sm 62 93 154.9246402(28) 22.3(2) min β− 155Eu 3/2−
156Sm 62 94 155.925528(10) 9.4(2) h β− 156Eu 0+
156mSm 1397.55(9) keV 185(7) ns 5−
157Sm 62 95 156.92836(5) 8.03(7) min β− 157Eu (3/2−)
158Sm 62 96 157.92999(8) 5.30(3) min β− 158Eu 0+
159Sm 62 97 158.93321(11) 11.37(15) s β− 159Eu 5/2−
160Sm 62 98 159.93514(21)# 9.6(3) s β− 160Eu 0+
161Sm 62 99 160.93883(32)# 4.349+0.425
−0.441 s[10]		 β− 161Eu 7/2+#
162Sm 62 100 161.94122(54)# 3.369+0.200
−0.303 s[10]		 β− 162Eu 0+
163Sm 62 101 162.94536(75)# 1.744+0.180
−0.204 s[10]		 β− 163Eu 1/2−#
164Sm 62 102 163.94828(86)# 1.422+0.54
−0.59 s[10]		 β− 164Eu 0+
165Sm 62 103 164.95298(97)# 592+51
−55 ms[10]		 β− (98.64%) 165Eu 5/2−#
β−, n (1.36%) 164Eu
166Sm 62 104 396+56
−63 ms[10]		 β− (95.62%) 166Eu 0+
β−, n (4.38%) 165Eu
167Sm 62 105 334+83
−78 ms[10]		 β− 167Eu
β−, n 166Eu
168Sm 62 106 353+210
−164 ms[10]		 β− 168Eu 0+
β−, n 167Eu
This table header & footer:
  1. ↑ mSm – 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. 1 2 3 # – 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. ↑ Possibly undergoing β+β+ decay to 144Nd[1]
  11. 1 2 Primordial radioisotope
  12. 1 2 3 4 5 6 Fission product
  13. ↑ Used in Samarium–neodymium dating
  14. 1 2 Neutron poison in reactors
  15. ↑ Believed to undergo α decay to 145Nd with a half-life over 2×1015 years[1][9]
  16. ↑ Possibly to undergo α decay to 146Nd[9]
  17. ↑ Possibly to undergo α decay to 148Nd[9]
  18. ↑ Believed to undergo β−β− decay to 154Gd with a half-life over 2.3×1018 years[1]

Samarium-149

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

Medium-lived
fission products
t½
(year)		 Yield
(%)		 Q
(keV)		 βγ
155Eu  4.76 0.0803  252 βγ
85Kr 10.76 0.2180  687 βγ
113mCd 14.1  0.0008  316 β
90Sr 28.9  4.505   2826 β
137Cs 30.23 6.337   1176 βγ
121mSn 43.9  0.00005 390 βγ
151Sm 88.8  0.5314  77 β
Yield, % per fission[11]
Thermal Fast 14 MeV
232Th not fissile 0.399 ± 0.065 0.165 ± 0.035
233U 0.333 ± 0.017 0.312 ± 0.014 0.49 ± 0.11
235U 0.4204 ± 0.0071 0.431 ± 0.015 0.388 ± 0.061
238U not fissile 0.810 ± 0.012 0.800 ± 0.057
239Pu 0.776 ± 0.018 0.797 ± 0.037  ?
241Pu 0.86 ± 0.24 0.910 ± 0.025  ?

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.[12] The decay energy of 151Sm 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.

Samarium-153

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.[13] It is treated by the body in a similar manner to calcium, and it localizes selectively to bone.

References

  1. 1 2 3 4 Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  2. ↑ "Standard Atomic Weights: Samarium". CIAAW. 2005.
  3. ↑ Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; et al. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
  4. ↑ 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.
  5. 1 2 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.
  6. ↑
  7. ↑ He, M.; Shen, H.; Shi, G.; Yin, X.; Tian, W.; Jiang, S. (2009). "Half-life of 151Sm remeasured". Physical Review C. 80 (6): 064305. Bibcode:2009PhRvC..80f4305H. doi:10.1103/PhysRevC.80.064305.
  8. ↑ See retraction note above
  9. 1 2 3 Belli, P.; Bernabei, R.; Danevich, F. A.; Incicchitti, A.; Tretyak, V. I. (2019). "Experimental searches for rare alpha and beta decays". European Physical Journal A. 55 (140): 4–6. arXiv:1908.11458. Bibcode:2019EPJA...55..140B. doi:10.1140/epja/i2019-12823-2. S2CID 201664098.
  10. 1 2 3 4 5 6 7 8 Kiss, G. G.; Vitéz-Sveiczer, A.; Saito, Y.; et al. (2022). "Measuring the β-decay properties of neutron-rich exotic Pm, Sm, Eu, and Gd isotopes to constrain the nucleosynthesis yields in the rare-earth region". The Astrophysical Journal. 936 (107): 107. Bibcode:2022ApJ...936..107K. doi:10.3847/1538-4357/ac80fc. hdl:2117/375253.
  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
  13. ↑ Ballantyne, Jane C; Fishman, Scott M; Rathmell, James P. (2009-10-01). Bonica's Management of Pain. Lippincott Williams & Wilkins. pp. 655–. ISBN 978-0-7817-6827-6. Retrieved 19 July 2011.
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