Isotopes of samarium

Isotopes of samarium (₆₂Sm)
Standard atomic weight Ar°(Sm)
	150.36±0.02; 150.36±0.02 (abridged);
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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extinct radionuclide.^[4]^[5] A 2012 paper revising the estimated half-life of ¹⁴⁶Sm from 10.3(5)×10⁷ y to 6.8(7)×10⁷ y was retracted in 2023.^[5]^[6] It is the longest-lived nuclide that has not yet been confirmed to be primordial.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 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)
Nuclide ^{[n 1]}	Excitation energy^{[n 5]}			Half-life ^{[n 4]}^{[n 5]}	Decay mode ^{[n 6]}	Daughter isotope ^{[n 7]}^{[n 8]}	Spin and parity ^{[n 9]}^{[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+#
¹³¹Sm	62	69	130.94611(32)#	1.2(2) s	β⁺, p (rare)	¹³⁰Nd	5/2+#
¹³²Sm	62	70	131.94069(32)#	4.0(3) s	β⁺	¹³²Pm	0+
¹³²Sm	62	70	131.94069(32)#	4.0(3) s	β⁺, p	¹³¹Nd	0+
¹³³Sm	62	71	132.93867(21)#	2.90(17) s	β⁺	¹³³Pm	(5/2+)
¹³³Sm	62	71	132.93867(21)#	2.90(17) s	β⁺, p	¹³²Nd	(5/2+)
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+)
¹³⁵Sm	62	73	134.93252(17)	10.3(5) s	β⁺, p (.02%)	¹³⁴Nd	(7/2+)
^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−
^139mSm	457.40(22) keV			10.7(6) s	β⁺ (6.3%)	¹³⁹Pm	11/2−
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−
^141mSm	176.0(3) keV			22.6(2) min	IT (.31%)	¹⁴¹Sm	11/2−
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−
^143m1Sm	753.99(16) keV			66(2) s	β⁺ (.24%)	¹⁴³Pm	11/2−
^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^{[n 11]}	α	¹⁴²Nd	0+	Trace
¹⁴⁷Sm^{[n 12]}^{[n 13]}^{[n 14]}	62	85	146.9148979(26)	(1.066±0.5)×10¹¹ y	α	¹⁴³Nd	7/2−	0.1499(18)
¹⁴⁸Sm^{[n 12]}	62	86	147.9148227(26)	(6.3±1.3)×10¹⁵ y	α	¹⁴⁴Nd	0+	0.1124(10)
¹⁴⁹Sm^{[n 13]}^{[n 15]}	62	87	148.9171847(26)	Observationally stable^{[n 16]}			7/2−	0.1382(7)
¹⁵⁰Sm	62	88	149.9172755(26)	Observationally stable^{[n 17]}			0+	0.0738(1)
¹⁵¹Sm^{[n 13]}^{[n 15]}	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 13]}	62	90	151.9197324(27)	Observationally stable^{[n 18]}			0+	0.2675(16)
¹⁵³Sm^{[n 13]}	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 13]}	62	92	153.9222093(27)	Observationally stable^{[n 19]}			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^[9]	β⁻	¹⁶¹Eu	7/2+#
¹⁶²Sm	62	100	161.94122(54)#	3.369+0.200 −0.303 s^[9]	β⁻	¹⁶²Eu	0+
¹⁶³Sm	62	101	162.94536(75)#	1.744+0.180 −0.204 s^[9]	β⁻	¹⁶³Eu	1/2−#
¹⁶⁴Sm	62	102	163.94828(86)#	1.422+0.54 −0.59 s^[9]	β⁻	¹⁶⁴Eu	0+
¹⁶⁵Sm	62	103	164.95298(97)#	592+51 −55 ms^[9]	β⁻ (98.64%)	¹⁶⁵Eu	5/2−#
¹⁶⁵Sm	62	103	164.95298(97)#	592+51 −55 ms^[9]	β⁻, n (1.36%)	¹⁶⁴Eu	5/2−#
¹⁶⁶Sm	62	104		396+56 −63 ms^[9]	β⁻ (95.62%)	¹⁶⁶Eu	0+
¹⁶⁶Sm	62	104		396+56 −63 ms^[9]	β⁻, n (4.38%)	¹⁶⁵Eu	0+
¹⁶⁷Sm	62	105		334+83 −78 ms^[9]	β⁻	¹⁶⁷Eu
¹⁶⁷Sm	62	105		334+83 −78 ms^[9]	β⁻, n	¹⁶⁶Eu
¹⁶⁸Sm	62	106		353+210 −164 ms^[9]	β⁻	¹⁶⁸Eu	0+
¹⁶⁸Sm	62	106		353+210 −164 ms^[9]	β⁻, n	¹⁶⁷Eu	0+
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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]}
	t_½ (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^[10]
	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.^[11] 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.

ANL factsheet

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.^[12] 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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