# Isotopes of samarium

Naturally occurring samarium ($_{62}$Sm) is composed of five stable isotopes, $^{144}$Sm, $^{149}$Sm, $^{150}$Sm, $^{152}$Sm and $^{154}$Sm, and two extremely long-lived radioisotopes, $^{147}$Sm (half life: 1.066×10$^{11}$ y) and $^{148}$Sm (6.3×10$^{15}$ y), with $^{152}$Sm being the most abundant (26.75% natural abundance). $^{146}$Sm (9.20×10$^7$ y) 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. 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 $^{151}$Sm, which has a half-life of 94.6 years, and $^{145}$Sm, which has a half-life of 340 days. All of the remaining radioisotopes, which range from $^{129}$Sm to $^{168}$Sm, 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 $^{141m}$Sm (t$_{1/2}$ 22.6 minutes), $^{143m1}$Sm (t$_{1/2}$ 66 seconds) and $^{139m}$Sm (t$_{1/2}$ 10.7 seconds).
The long lived isotopes, $^{146}$Sm, $^{147}$Sm, and $^{148}$Sm, 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. A 2012 paper revising the estimated half-life of $^{146}$Sm from 10.3(5)×10$^7$ y to 6.8(7)×10$^7$ y was retracted in 2023.
Isotopes of samarium are used in samarium–neodymium dating for determining the age relationships of rocks and meteorites.
$^{151}$Sm is a medium-lived fission product and acts as a neutron poison in the nuclear fuel cycle. The stable fission product $^{149}$Sm 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 | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity | Natural abundance (mole fraction) | Natural abundance (mole fraction) |
| Nuclide | Excitation energy | Excitation energy | Excitation energy | Half-life | Decay mode | Daughter isotope | Spin and parity | Normal proportion | Range of variation |
| $^{129}$Sm | 62 | 67 | 128.95456(54)# | 550(100) ms | β$^+$ (?%) | $^{129}$Pm | (1/2+,3/2+) | | |
| $^{129}$Sm | 62 | 67 | 128.95456(54)# | 550(100) ms | β$^+$, p (?%) | $^{128}$Nd | (1/2+,3/2+) | | |
| $^{130}$Sm | 62 | 68 | 129.94879(43)# | 1# n | | | 0+ | | |
| $^{131}$Sm | 62 | 69 | 130.94602(43)# | 1.2(2) s | β$^+$ | $^{131}$Pm | 5/2+# | | |
| $^{131}$Sm | 62 | 69 | 130.94602(43)# | 1.2(2) s | β$^+$, p (?%) | $^{130}$Nd | 5/2+# | | |
| $^{132}$Sm | 62 | 70 | 131.94081(32)# | 4.0(3) s | β$^+$ | $^{132}$Pm | 0+ | | |
| $^{133}$Sm | 62 | 71 | 132.93856(32)# | 2.89(16) s | β$^+$ (?%) | $^{133}$Pm | (5/2+) | | |
| $^{133}$Sm | 62 | 71 | 132.93856(32)# | 2.89(16) s | β$^+$, p (?%) | $^{132}$Nd | (5/2+) | | |
| $^{133m}$Sm | 120(60)# keV | 120(60)# keV | 120(60)# keV | 3.5(4) s | β$^+$ | $^{133}$Pm | (1/2−) | | |
| $^{134}$Sm | 62 | 72 | 133.93411(21)# | 9.5(8) s | β$^+$ | $^{134}$Pm | 0+ | | |
| $^{135}$Sm | 62 | 73 | 134.93252(17) | 10.3(5) s | β$^+$ (99.98%) | $^{135}$Pm | (7/2+) | | |
| $^{135}$Sm | 62 | 73 | 134.93252(17) | 10.3(5) s | β$^+$, p (0.02%) | $^{134}$Nd | (7/2+) | | |
| $^{136}$Sm | 62 | 74 | 135.928276(13) | 47(2) s | β$^+$ | $^{136}$Pm | 0+ | | |
| $^{136m}$Sm | 2264.7(11) keV | 2264.7(11) keV | 2264.7(11) keV | 15(1) μs | IT | $^{136}$Sm | (8−) | | |
| $^{137}$Sm | 62 | 75 | 136.927008(31) | 45(1) s | β$^+$ | $^{137}$Pm | (9/2−) | | |
| $^{138}$Sm | 62 | 76 | 137.923244(13) | 3.1(2) min | β$^+$ | $^{138}$Pm | 0+ | | |
| $^{139}$Sm | 62 | 77 | 138.922297(12) | 2.57(10) min | β$^+$ | $^{139}$Pm | 1/2+ | | |
| $^{139m}$Sm | 457.38(23) keV | 457.38(23) keV | 457.38(23) keV | 10.7(6) s | IT (93.7%) | $^{139}$Sm | 11/2− | | |
| $^{139m}$Sm | 457.38(23) keV | 457.38(23) keV | 457.38(23) keV | 10.7(6) s | β$^+$ (6.3%) | $^{139}$Pm | 11/2− | | |
| $^{140}$Sm | 62 | 78 | 139.918995(13) | 14.82(12) min | β$^+$ | $^{140}$Pm | 0+ | | |
| $^{141}$Sm | 62 | 79 | 140.9184815(92) | 10.2(2) min | β$^+$ | $^{141}$Pm | 1/2+ | | |
| $^{141m}$Sm | 175.9(3) keV | 175.9(3) keV | 175.9(3) keV | 22.6(2) min | β$^+$ (99.69%) | $^{141}$Pm | 11/2− | | |
| $^{141m}$Sm | 175.9(3) keV | 175.9(3) keV | 175.9(3) keV | 22.6(2) min | IT (0.31%) | $^{141}$Sm | 11/2− | | |
| $^{142}$Sm | 62 | 80 | 141.9152094(20) | 72.49(5) min | EC (>95%) | $^{142}$Pm | 0+ | | |
| $^{142}$Sm | 62 | 80 | 141.9152094(20) | 72.49(5) min | β$^+$ (<5%) | $^{142}$Pm | 0+ | | |
| $^{142m1}$Sm | 2372.1(4) keV | 2372.1(4) keV | 2372.1(4) keV | 170(2) ns | IT | $^{142}$Sm | 7− | | |
| $^{142m2}$Sm | 3662.2(7) keV | 3662.2(7) keV | 3662.2(7) keV | 480(60) ns | IT | $^{142}$Sm | 10+ | | |
| $^{143}$Sm | 62 | 81 | 142.9146348(30) | 8.75(6) min | EC (60.0%) | $^{143}$Pm | 3/2+ | | |
| $^{143}$Sm | 62 | 81 | 142.9146348(30) | 8.75(6) min | β$^+$ (40.0%) | $^{143}$Pm | 3/2+ | | |
| $^{143m1}$Sm | 753.99(16) keV | 753.99(16) keV | 753.99(16) keV | 66(2) s | IT (99.76%) | $^{143}$Sm | 11/2− | | |
| $^{143m1}$Sm | 753.99(16) keV | 753.99(16) keV | 753.99(16) keV | 66(2) s | β$^+$ (0.24%) | $^{143}$Pm | 11/2− | | |
| $^{143m2}$Sm | 2793.8(13) keV | 2793.8(13) keV | 2793.8(13) keV | 30(3) ms | IT | $^{143}$Sm | 23/2− | | |
| $^{144}$Sm | 62 | 82 | 143.9120063(16) | Observationally stable | Observationally stable | Observationally stable | 0+ | 0.0308(4) | |
| $^{144m}$Sm | 2323.60(8) keV | 2323.60(8) keV | 2323.60(8) keV | 880(25) ns | IT | $^{144}$Sm | 6+ | | |
| $^{145}$Sm | 62 | 83 | 144.9134172(16) | 340(3) d | EC | $^{145}$Pm | 7/2− | | |
| $^{145m}$Sm | 8815(1) keV | 8815(1) keV | 8815(1) keV | 3.52(16) μs | IT | $^{145}$Sm | 49/2+ | | |
| $^{146}$Sm | 62 | 84 | 145.9130468(33) | 9.20(26)×10$^7$ y | α | $^{142}$Nd | 0+ | Trace | |
| $^{147}$Sm | 62 | 85 | 146.9149044(14) | 1.066(5)×10$^{11}$ y | α | $^{143}$Nd | 7/2− | 0.1500(14) | |
| $^{148}$Sm | 62 | 86 | 147.9148292(13) | 6.3(13)×10$^{15}$ y | α | $^{144}$Nd | 0+ | 0.1125(9) | |
| $^{149}$Sm | 62 | 87 | 148.9171912(12) | Observationally stable | Observationally stable | Observationally stable | 7/2− | 0.1382(10) | |
| $^{150}$Sm | 62 | 88 | 149.9172820(12) | Observationally stable | Observationally stable | Observationally stable | 0+ | 0.0737(9) | |
| $^{151}$Sm | 62 | 89 | 150.9199389(12) | 94.6(6) y | β$^−$ | $^{151}$Eu | 5/2− | | |
| $^{151m}$Sm | 261.13(4) keV | 261.13(4) keV | 261.13(4) keV | 1.4(1) μs | IT | $^{151}$Sm | (11/2)− | | |
| $^{152}$Sm | 62 | 90 | 151.9197386(11) | Observationally stable | Observationally stable | Observationally stable | 0+ | 0.2674(9) | |
| $^{153}$Sm | 62 | 91 | 152.9221036(11) | 46.2846(23) h | β$^−$ | $^{153}$Eu | 3/2+ | | |
| $^{153m}$Sm | 98.39(10) keV | 98.39(10) keV | 98.39(10) keV | 10.6(3) ms | IT | $^{153}$Sm | 11/2− | | |
| $^{154}$Sm | 62 | 92 | 153.9222158(14) | Observationally stable | Observationally stable | Observationally stable | 0+ | 0.2274(14) | |
| $^{155}$Sm | 62 | 93 | 154.9246466(14) | 22.18(6) min | β$^−$ | $^{155}$Eu | 3/2− | | |
| $^{155m1}$Sm | 16.5467(19) keV | 16.5467(19) keV | 16.5467(19) keV | 2.8(5) μs | IT | $^{155}$Sm | 5/2+ | | |
| $^{155m2}$Sm | 538.03(19) keV | 538.03(19) keV | 538.03(19) keV | 1.00(8) μs | IT | $^{155}$Sm | 11/2− | | |
| $^{156}$Sm | 62 | 94 | 155.9255382(91) | 9.4(2) h | β$^−$ | $^{156}$Eu | 0+ | | |
| $^{156m}$Sm | 1397.55(9) keV | 1397.55(9) keV | 1397.55(9) keV | 185(7) ns | IT | $^{156}$Sm | 5− | | |
| $^{157}$Sm | 62 | 95 | 156.9284186(48) | 8.03(7) min | β$^−$ | $^{157}$Eu | 3/2−# | | |
| $^{158}$Sm | 62 | 96 | 157.9299493(51) | 5.30(3) min | β$^−$ | $^{158}$Eu | 0+ | | |
| $^{159}$Sm | 62 | 97 | 158.9332171(64) | 11.37(15) s | β$^−$ | $^{159}$Eu | 5/2− | | |
| $^{159m}$Sm | 1276.5(8) keV | 1276.5(8) keV | 1276.5(8) keV | 116(8) ns | IT | $^{159}$Sm | (15/2+) | | |
| $^{160}$Sm | 62 | 98 | 159.9353370(21) | 9.6(3) s | β$^−$ | $^{160}$Eu | 0+ | | |
| $^{160m1}$Sm | 1361.3(4) keV | 1361.3(4) keV | 1361.3(4) keV | 120(46) ns | IT | $^{160}$Sm | (5−) | | |
| $^{160m2}$Sm | 2757.3(4) keV | 2757.3(4) keV | 2757.3(4) keV | 1.8(4) μs | IT | $^{160}$Sm | (11+) | | |
| $^{161}$Sm | 62 | 99 | 160.9391601(73) | 4.349+0.425 −0.441 s | β$^−$ | $^{161}$Eu | 7/2+# | | |
| $^{161m}$Sm | 1388.1(6) keV | 1388.1(6) keV | 1388.1(6) keV | 2.6(4) μs | IT | $^{161}$Sm | (17/2−) | | |
| $^{162}$Sm | 62 | 100 | 161.9416217(38) | 3.369+0.200 −0.303 s | β$^−$ | $^{162}$Eu | 0+ | | |
| $^{162m}$Sm | 1009.4(5) keV | 1009.4(5) keV | 1009.4(5) keV | 1.78(7) μs | IT | $^{162}$Sm | (4−) | | |
| $^{163}$Sm | 62 | 101 | 162.9456791(79) | 1.744+0.180 −0.204 s | β$^−$ | $^{163}$Eu | 1/2−# | | |
| $^{163}$Sm | 62 | 101 | 162.9456791(79) | 1.744+0.180 −0.204 s | β$^−$, n (<0.1%) | $^{162}$Eu | 1/2−# | | |
| $^{164}$Sm | 62 | 102 | 163.9485501(44) | 1.422+0.54 −0.59 s | β$^−$ | $^{164}$Eu | 0+ | | |
| $^{164}$Sm | 62 | 102 | 163.9485501(44) | 1.422+0.54 −0.59 s | β$^−$, n (<0.7%) | $^{163}$Eu | 0+ | | |
| $^{164m}$Sm | 1485.5(12) keV | 1485.5(12) keV | 1485.5(12) keV | 600(140) ns | IT | $^{164}$Sm | (6−) | | |
| $^{165}$Sm | 62 | 103 | 164.95329(43)# | 592+51 −55 ms | β$^−$ (98.64%) | $^{165}$Eu | 5/2−# | | |
| $^{165}$Sm | 62 | 103 | 164.95329(43)# | 592+51 −55 ms | β$^−$, n (1.36%) | $^{164}$Eu | 5/2−# | | |
| $^{166}$Sm | 62 | 104 | 165.95658(43)# | 396+56 −63 ms | β$^−$ (95.62%) | $^{166}$Eu | 0+ | | |
| $^{166}$Sm | 62 | 104 | 165.95658(43)# | 396+56 −63 ms | β$^−$, n (4.38%) | $^{165}$Eu | 0+ | | |
| $^{167}$Sm | 62 | 105 | 166.96207(54)# | 334+83 −78 ms | β$^−$ | $^{167}$Eu | 7/2−# | | |
| $^{167}$Sm | 62 | 105 | 166.96207(54)# | 334+83 −78 ms | β$^−$, n (<16%) | $^{166}$Eu | 7/2−# | | |
| $^{168}$Sm | 62 | 106 | 167.96603(32)# | 353+210 −164 ms | β$^−$ | $^{168}$Eu | 0+# | | |
| $^{168}$Sm | 62 | 106 | 167.96603(32)# | 353+210 −164 ms | β$^−$, n (<21%) | $^{167}$Eu | 0+# | | |
| This table header & footer: view ||||||||||
* ↑ $^m$Sm – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* ↑ Bold half-life – nearly stable, half-life longer than age of universe.
* 1 2 3 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ Modes of decay: IT:Isomeric transition p:Proton emission
* ↑ Bold italics symbol as daughter – Daughter product is nearly stable.
* ↑ Bold symbol as daughter – Daughter product is stable.
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* ↑ Believed to undergo β$^+$β$^+$ decay to $^{144}$Nd
* 1 2 Primordial radioisotope
* 1 2 3 4 5 6 Fission product
* ↑ Used in Samarium–neodymium dating
* 1 2 Neutron poison in reactors
* ↑ Believed to undergo α decay to $^{145}$Nd with a half-life over 2×10$^{15}$ years
* ↑ Believed to undergo α decay to $^{146}$Nd
* ↑ Believed to undergo α decay to $^{148}$Nd
* ↑ Believed to undergo β$^−$β$^−$ decay to $^{154}$Gd with a half-life over 2.3×10$^{18}$ years

## Samarium-149

Samarium-149 ($^{149}$Sm) 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 $^{149}$Nd (yield 1.0888%). $^{149}$Sm is a neutron-absorbing nuclear poison with significant effect on nuclear reactor operation, second only to $^{135}$Xe. 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 $^{149}$Sm 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 productsvte :
| | t$_½$ (year) | Yield (%) | Q (keV) | βγ |
| $^{155}$Eu | 4.76 | 0.0803 | 252 | βγ |
| $^{85}$Kr | 10.76 | 0.2180 | 687 | βγ |
| $^{113m}$Cd | 14.1 | 0.0008 | 316 | β |
| $^{90}$Sr | 28.9 | 4.505 | 2826 | β |
| $^{137}$Cs | 30.23 | 6.337 | 1176 | βγ |
| $^{121m}$Sn | 43.9 | 0.00005 | 390 | βγ |
| $^{151}$Sm | 94.6 | 0.5314 | 77 | β |
Yield, % per fission :
| | Thermal | Fast | 14 MeV |
| $^{232}$Th | not fissile | 0.399 ± 0.065 | 0.165 ± 0.035 |
| $^{233}$U | 0.333 ± 0.017 | 0.312 ± 0.014 | 0.49 ± 0.11 |
| $^{235}$U | 0.4204 ± 0.0071 | 0.431 ± 0.015 | 0.388 ± 0.061 |
| $^{238}$U | not fissile | 0.810 ± 0.012 | 0.800 ± 0.057 |
| $^{239}$Pu | 0.776 ± 0.018 | 0.797 ± 0.037 | ? |
| $^{241}$Pu | 0.86 ± 0.24 | 0.910 ± 0.025 | ? |
Samarium-151 ($^{151}$Sm) has a half-life of 94.6 years, undergoing low-energy beta decay, and has a fission product yield of 0.4203% for thermal neutrons and $^{235}$U, about 39% of $^{149}$Sm's yield. The yield is somewhat higher for $^{239}$Pu.
Its neutron absorption cross section for thermal neutrons is high at 15200 barns, about 38% of $^{149}$Sm's absorption cross section, or about 20 times that of $^{235}$U. Since the ratios between the production and absorption rates of $^{151}$Sm and $^{149}$Sm are almost equal, the two isotopes should reach similar equilibrium concentrations. Since $^{149}$Sm reaches equilibrium in about 500 hours (20 days), $^{151}$Sm 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 $^{151}$Sm in the spent nuclear fuel at discharge is only a small fraction of the total $^{151}$Sm produced during the use of the fuel. According to one study, the mass fraction of $^{151}$Sm 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 $^{137}$Cs. The decay energy of $^{151}$Sm is also about an order of magnitude less than that of $^{137}$Cs. The low yield, low survival rate, and low decay energy mean that $^{151}$Sm has insignificant nuclear waste impact compared to the two main medium-lived fission products $^{137}$Cs and $^{90}$Sr.
* ANL factsheet

## Samarium-153

Samarium-153 ($^{153}$Sm) has a half-life of 46.3 hours, undergoing β$^−$ decay into $^{153}$Eu. As a component of samarium lexidronam, it is used in palliation of bone cancer. It is treated by the body in a similar manner to calcium, and it localizes selectively to bone.