Isotopes of samarium

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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 96.6 years,[2] 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 (t½ 22.6 minutes), 143m1Sm (t½ 66 seconds) and 139mSm (t½ 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

Samarium-149[edit]

149Sm is an observed stable isotope of samarium (predicted to decay, but no decays have ever been observed, giving it a half-life several orders of magnitude longer than the age of the universe), 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[edit]

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 96.6 .5314 77 β
Yield, % per fission[3]
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  ?


151Sm has a half-life of 96.6 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.[4] 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.

Table[edit]

nuclide
symbol
Z(p) N(n)  
isotopic mass (u)
 
half-life[n 1] decay
mode(s)[5][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) 96.6(24) 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[edit]

  • 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[edit]

  1. ^ 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. 
  2. ^ He, M.; Shen, H.; Shi, G.; Yin, X.; Tian, W.; Jiang, S. (2009). "Half-life of 151Sm remeasured". Physical Review C 80 (6). Bibcode:2009PhRvC..80f4305H. doi:10.1103/PhysRevC.80.064305.  edit
  3. ^ http://www-nds.iaea.org/sgnucdat/c3.htm Cumulative Fission Yields, IAEA
  4. ^ http://www.nea.fr/html/pt/docs/iem/jeju02/session5/SectionV-12.pdf Figure 2, page 6
  5. ^ http://www.nucleonica.net/unc.aspx


Isotopes of promethium Isotopes of samarium Isotopes of europium
Table of nuclides