Isotopes of samarium

Naturally occurring samarium (Sm) is composed of five stable isotopes, ¹⁴⁴Sm, ¹⁴⁹Sm, ¹⁵⁰Sm, ¹⁵²Sm and ¹⁵⁴Sm, and two extremely long-lived radioisotopes, ¹⁴⁷Sm (1.06×10¹¹y) and ¹⁴⁸Sm (7×10¹⁵y), with ¹⁵²Sm being the most abundant (26.75% natural abundance). ¹⁴⁶Sm is also fairly long-lived (1.03×10⁸y), but occurs naturally as only the tiniest trace remains from its original supernova nucleosynthesis.^[1]

Other than the naturally occurring isotopes, the longest-lived radioisotopes are ¹⁵¹Sm, which has a half-life of 90 years, and ¹⁴⁵Sm, which has a half-life of 340 days. All of the remaining radioisotopes 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_½ 22.6 minutes), ^143m1Sm (t_½ 66 seconds) and ^139mSm (t_½ 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 minus 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.

Standard atomic mass: 150.36(2) u

Contents 1 Samarium-149 2 Samarium-151 3 Table 3.1 Notes 4 References

Samarium-149

¹⁴⁹Sm is a stable isotope of samarium, and a fission product (yield 1.0888%) which is also 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.

Samarium-151

Medium-lived
fission products

Prop: Unit:	t½ a	Yield %	Q * keV	βγ *
155Eu	4.76	.0803	252	βγ
85Kr	10.76	.2180	687	βγ
113mCd	14.1	.0008	316	β
90Sr	28.9	4.505	2826	β
137Cs	30.23	6.337	1176	βγ
121mSn	43.9	.00005	390	βγ
151Sm	90	.5314	77	β

Chain yield, % per fission^[2]

	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	?

151
Sm has a half-life of 90 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 of¹⁵¹Sm 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 Sm-151 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 .15 for the medium-lived fission product Cs-137.^[3] The decay energy of¹⁵¹Sm 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.

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Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life[n 1]	decay mode(s)[4][n 2]	daughter isotope(s)[n 3]	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
	excitation energy
128Sm	62	66	127.95808(54)#	0.5# s			0+
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 4]			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[n 5]	62	84	145.913041(4)	1.03(5)×108 a	α	142Nd	0+	Trace
147Sm[n 5][n 6][n 7]	62	85	146.9148979(26)	1.06(2)×1011 a	α	143Nd	7/2-	0.1499(18)
148Sm[n 5]	62	86	147.9148227(26)	7(3)×1015 a	α	144Nd	0+	0.1124(10)
149Sm[n 6][n 8]	62	87	148.9171847(26)	Observationally Stable[n 9]			7/2-	0.1382(7)
150Sm	62	88	149.9172755(26)	Observationally Stable[n 10]			0+	0.0738(1)
151Sm[n 6][n 8]	62	89	150.9199324(26)	90(8) a	β-	151Eu	5/2-
151mSm	261.13(4) keV			1.4(1) µs			(11/2)-
152Sm[n 6]	62	90	151.9197324(27)	Observationally Stable[n 11]			0+	0.2675(16)
153Sm[n 6]	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 6]	62	92	153.9222093(27)	Observationally Stable[n 12]			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)#	4.8(8) s	β-	161Eu	7/2+#
162Sm	62	100	161.94122(54)#	2.4(5) s	β-	162Eu	0+
163Sm	62	101	162.94536(75)#	1# s	β-	163Eu	1/2-#
164Sm	62	102	163.94828(86)#	500# ms	β-	164Eu	0+
165Sm	62	103	164.95298(97)#	200# ms	β-	165Eu	5/2-#

1. ^ Bold for isotopes with half-lives longer than the age of the universe (nearly stable)
2. ^ Abbreviations:
   EC: Electron capture
   IT: Isomeric transition
3. ^ Bold for stable isotopes, bold italics for nearly stable isotopes (half-life longer than the age of the universe)
4. ^ Believed to undergo α decay to ¹⁴⁰Nd or β⁺β⁺ decay to ¹⁴⁴Nd
5. ^ ^{a b c} Primordial radioisotope
6. ^ ^{a b c d e f} Fission product
7. ^ Used in Samarium-neodymium dating
8. ^ ^{a b} Neutron poison in reactors
9. ^ Believed to undergo α decay to ¹⁴⁵Nd with a half-life over 2×10¹⁵ years
10. ^ Believed to undergo α decay to ¹⁴⁶Nd
11. ^ Believed to undergo α decay to ¹⁴⁸Nd
12. ^ Believed to undergo β^-β^- decay to ¹⁵⁴Gd with a half-life over 2.3×10¹⁸ years

Notes

• Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.

References

• Isotope masses from:
  • G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (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. http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf. 
• Isotopic compositions and standard atomic masses from:
  • J. R. de Laeter, J. K. Böhlke, P. De Bièvre, H. Hidaka, H. S. Peiser, K. J. R. Rosman and P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry 75 (6): 683–800. doi:10.1351/pac200375060683. http://www.iupac.org/publications/pac/75/6/0683/pdf/. 
  • M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry 78 (11): 2051–2066. doi:10.1351/pac200678112051. http://iupac.org/publications/pac/78/11/2051/pdf/. Lay summary. 
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
  • G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (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. http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf. 
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/. Retrieved September 2005. 
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0849304859. 

Isotopes of promethium	Isotopes of samarium	Isotopes of europium
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