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 (7×10¹⁵ y), with ¹⁵²Sm being the most abundant (26.75% natural abundance). ¹⁴⁶Sm is also fairly long-lived (6.8×10⁷ 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.^[3][4]

Isotopes of samarium (₆₂Sm)

Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct ^144Sm 3.08% stable ^145Sm syn 340 d ε ^145Pm ^146Sm syn 6.8×10^7 y α ^142Nd ^147Sm 15.00% 1.06×10^11 y α ^143Nd ^148Sm 11.25% 7×10^15 y α ^144Nd ^149Sm 13.82% stable ^150Sm 7.37% stable ^151Sm syn 94.6 y β^− ^151Eu ^152Sm 26.74% stable ^153Sm syn 46.284 h β^− ^153Eu ^154Sm 22.74% stable
Standard atomic weight Ar°(Sm)
	150.36±0.02 150.36±0.02 (abridged)^[1]^[2]
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Other than the naturally occurring isotopes, the longest-lived radioisotopes are ¹⁵¹Sm, which has a half-life of 88.8 years,^[5] 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 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.

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)	6.8(7)×10^7 y	α	^142Nd	0+	Trace
^147Sm^[n 11]^[n 12]^[n 13]	62	85	146.9148979(26)	1.06(2)×10^11 y	α	^143Nd	7/2−	0.1499(18)
^148Sm^[n 11]	62	86	147.9148227(26)	7(3)×10^15 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)	88.8(24) 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)	46.284(4) 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^[8]	β^−	^161Eu	7/2+#
^162Sm	62	100	161.94122(54)#	3.369+0.200 −0.303 s^[8]	β^−	^162Eu	0+
^163Sm	62	101	162.94536(75)#	1.744+0.180 −0.204 s^[8]	β^−	^163Eu	1/2−#
^164Sm	62	102	163.94828(86)#	1.422+0.54 −0.59 s^[8]	β^−	^164Eu	0+
^165Sm	62	103	164.95298(97)#	592+51 −55 ms^[8]	β^− (98.64%)	^165Eu	5/2−#
					β^−, n (1.36%)	^164Eu
^166Sm	62	104		396+56 −63 ms^[8]	β^− (95.62%)	^166Eu	0+
					β^−, n (4.38%)	^165Eu
^167Sm	62	105		334+83 −78 ms^[8]	β^−	^167Eu
					β^−, n	^166Eu
^168Sm	62	106		353+210 −164 ms^[8]	β^−	^168Eu	0+
					β^−, n	^167Eu
This table header & footer: view

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. # – 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 β⁺β⁺ decay to ¹⁴⁴Nd^[6]
11. Primordial radioisotope
12. Fission product
13. Used in Samarium–neodymium dating
14. Neutron poison in reactors
15. Believed to undergo α decay to ¹⁴⁵Nd with a half-life over 2×10¹⁵ years^[6][7]
16. Believed to undergo α decay to ¹⁴⁶Nd^[7]
17. Believed to undergo α decay to ¹⁴⁸Nd^[7]
18. Believed to undergo β⁻β⁻ decay to ¹⁵⁴Gd with a half-life over 2.3×10¹⁸ years^[6]

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^{[further explanation needed]}

	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^[9]

	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.^[10] 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.^[11] 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.

1. "Standard Atomic Weights: Samarium". CIAAW. 2005.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
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). doi:10.3847/1538-4357/ac80fc.
9. https://www-nds.iaea.org/sgnucdat/c3.htm Cumulative Fission Yields, IAEA
10. Christophe Demazière. Reactor Physics Calculations on MOX Fuel in Boiling Water Reactors (BWRs) (PDF) (Report). OECD Nuclear Energy Agency. Figure 2, page 6
11. 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.