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: 1.066 y) and ¹⁴⁸Sm (6.3 y), with ¹⁵²Sm being the most abundant (26.75% natural abundance). ¹⁴⁶Sm (9.20 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. Its instability is due to having 84 neutrons (two more than 82, which is a magic number corresponding to a stable neutron configuration), and so it may emit an alpha particle (which has 2 neutrons) to form neodymium-142 with 82 neutrons.

Other than those, the longest-lived radioisotopes are ¹⁵¹Sm, which has a half-life of 94.6 years, 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. The most stable of the known isomers is ^141mSm (half-life 22.6 minutes).

The long-lived isotopes, ¹⁴⁶Sm, ¹⁴⁷Sm, and ¹⁴⁸Sm, decay by alpha emission 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 A 2012 paper revising the estimated half-life of ¹⁴⁶Sm from 10.3(5)×10⁷ y to 6.8(7)×10⁷ y was retracted (due to an experimental mistake) in 2023, and the current, more accurate, value published subsequently.

The isotope ¹⁴⁷Sm is used in samarium–neodymium dating and as mentioned the extinct ¹⁴⁶Sm can also be used for dating.

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 the lightest element with even atomic number with no theoretically stable isotopes (all isotopes of it can energetically decay by the alpha, beta, or double-beta modes); other such elements are those with atomic numbers > 66 (dysprosium, which has the heaviest theoretically stable nuclide, ¹⁶⁴Dy).

List of isotopes

|-id=Samarium-128 | ¹²⁸Sm | style="text-align:right" | 62 | style="text-align:right" | 66 | 127.95797(54)# | 500#&nbsp;ms<br>[>310&nbsp;ns] | | | 0+ | | |-id=Samarium-129 | rowspan=2|¹²⁹Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 67 | rowspan=2|128.95456(54)# | rowspan=2|550(100)&nbsp;ms | β⁺ (?%) | ¹²⁹Pm | rowspan=2|(1/2+,3/2+) | rowspan=2| | rowspan=2| |- | β⁺, p (?%) | ¹²⁸Nd |-id=Samarium-130 | ¹³⁰Sm | style="text-align:right" | 62 | style="text-align:right" | 68 | 129.94879(43)# | 1#&nbsp;s | | | 0+ | | |-id=Samarium-131 | rowspan=2|¹³¹Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 69 | rowspan=2|130.94602(43)# | rowspan=2|1.2(2)&nbsp;s | β⁺ | ¹³¹Pm | rowspan=2|5/2+# | rowspan=2| | rowspan=2| |- | β⁺, p (?%) | ¹³⁰Nd |-id=Samarium-132 | ¹³²Sm | style="text-align:right" | 62 | style="text-align:right" | 70 | 131.94081(32)# | 4.0(3)&nbsp;s | β⁺ | ¹³²Pm | 0+ | | |-id=Samarium-133 | rowspan=2|¹³³Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 71 | rowspan=2|132.93856(32)# | rowspan=2|2.89(16)&nbsp;s | β⁺ (?%) | ¹³³Pm | rowspan=2|(5/2+) | rowspan=2| | rowspan=2| |- | β⁺, p (?%) | ¹³²Nd |-id=Samarium-133m | style="text-indent:1em" | ^133mSm | colspan="3" style="text-indent:2em" | 120(60)#&nbsp;keV | 3.5(4)&nbsp;s | β⁺ | ¹³³Pm | (1/2−) | | |-id=Samarium-134 | ¹³⁴Sm | style="text-align:right" | 62 | style="text-align:right" | 72 | 133.93411(21)# | 9.5(8)&nbsp;s | β⁺ | ¹³⁴Pm | 0+ | | |-id=Samarium-135 | rowspan=2|¹³⁵Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 73 | rowspan=2|134.93252(17) | rowspan=2|10.3(5)&nbsp;s | β⁺ (99.98%) | ¹³⁵Pm | rowspan=2|(7/2+) | rowspan=2| | rowspan=2| |- | β⁺, p (0.02%) | ¹³⁴Nd |-id=Samarium-136 | ¹³⁶Sm | style="text-align:right" | 62 | style="text-align:right" | 74 | 135.928276(13) | 47(2)&nbsp;s | β⁺ | ¹³⁶Pm | 0+ | | |-id=Samarium-136m | style="text-indent:1em" | ^136mSm | colspan="3" style="text-indent:2em" | 2264.7(11)&nbsp;keV | 15(1)&nbsp;μs | IT | ¹³⁶Sm | (8−) | | |-id=Samarium-137 | ¹³⁷Sm | style="text-align:right" | 62 | style="text-align:right" | 75 | 136.927008(31) | 45(1)&nbsp;s | β⁺ | ¹³⁷Pm | (9/2−) | | |-id=Samarium-138 | ¹³⁸Sm | style="text-align:right" | 62 | style="text-align:right" | 76 | 137.923244(13) | 3.1(2)&nbsp;min | β⁺ | ¹³⁸Pm | 0+ | | |-id=Samarium-139 | ¹³⁹Sm | style="text-align:right" | 62 | style="text-align:right" | 77 | 138.922297(12) | 2.57(10)&nbsp;min | β⁺ | ¹³⁹Pm | 1/2+ | | |-id=Samarium-139m | rowspan=2 style="text-indent:1em" | ^139mSm | rowspan=2 colspan="3" style="text-indent:2em" | 457.38(23)&nbsp;keV | rowspan=2|10.7(6)&nbsp;s | IT (93.7%) | ¹³⁹Sm | rowspan=2|11/2− | rowspan=2| | rowspan=2| |- | β⁺ (6.3%) | ¹³⁹Pm |-id=Samarium-140 | ¹⁴⁰Sm | style="text-align:right" | 62 | style="text-align:right" | 78 | 139.918995(13) | 14.82(12)&nbsp;min | β⁺ | ¹⁴⁰Pm | 0+ | | |-id=Samarium-141 | ¹⁴¹Sm | style="text-align:right" | 62 | style="text-align:right" | 79 | 140.9184815(92) | 10.2(2)&nbsp;min | β⁺ | ¹⁴¹Pm | 1/2+ | | |-id=Samarium-141m | rowspan=2 style="text-indent:1em" | ^141mSm | rowspan=2 colspan="3" style="text-indent:2em" | 175.9(3)&nbsp;keV | rowspan=2|22.6(2)&nbsp;min | β⁺ (99.69%) | ¹⁴¹Pm | rowspan=2|11/2− | rowspan=2| | rowspan=2| |- | IT (0.31%) | ¹⁴¹Sm |-id=Samarium-142 | rowspan=2|¹⁴²Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 80 | rowspan=2|141.9152094(20) | rowspan=2|72.49(5)&nbsp;min | EC (>95%) | rowspan=2|¹⁴²Pm | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β⁺ (<5%) |-id=Samarium-142m1 | style="text-indent:1em" | ^142m1Sm | colspan="3" style="text-indent:2em" | 2372.1(4)&nbsp;keV | 170(2)&nbsp;ns | IT | ¹⁴²Sm | 7− | | |-id=Samarium-142m2 | style="text-indent:1em" | ^142m2Sm | colspan="3" style="text-indent:2em" | 3662.2(7)&nbsp;keV | 480(60)&nbsp;ns | IT | ¹⁴²Sm | 10+ | | |-id=Samarium-143 | rowspan=2|¹⁴³Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 81 | rowspan=2|142.9146348(30) | rowspan=2|8.75(6)&nbsp;min | EC (60.0%) | ¹⁴³Pm | rowspan=2|3/2+ | rowspan=2| | rowspan=2| |- | β⁺ (40.0%) | ¹⁴³Pm |-id=Samarium-143m1 | rowspan=2 style="text-indent:1em" | ^143m1Sm | rowspan=2 colspan="3" style="text-indent:2em" | 753.99(16)&nbsp;keV | rowspan=2|66(2)&nbsp;s | IT (99.76%) | ¹⁴³Sm | rowspan=2|11/2− | rowspan=2| | rowspan=2| |- | β⁺ (0.24%) | ¹⁴³Pm |-id=Samarium-143m2 | style="text-indent:1em" | ^143m2Sm | colspan="3" style="text-indent:2em" | 2793.8(13)&nbsp;keV | 30(3)&nbsp;ms | IT | ¹⁴³Sm | 23/2− | | |-id=Samarium-144 | ¹⁴⁴Sm | style="text-align:right" | 62 | style="text-align:right" | 82 | 143.9120063(16) | colspan=3 align=center|Observationally stable | 0+ | 0.0308(4) | |-id=Samarium-144m | style="text-indent:1em" | ^144mSm | colspan="3" style="text-indent:2em" | 2323.60(8)&nbsp;keV | 880(25)&nbsp;ns | IT | ¹⁴⁴Sm | 6+ | | |-id=Samarium-145 | ¹⁴⁵Sm | style="text-align:right" | 62 | style="text-align:right" | 83 | 144.9134172(16) | 340(3)&nbsp;d | EC | ¹⁴⁵Pm | 7/2− | | |-id=Samarium-145m | style="text-indent:1em" | ^145mSm | colspan="3" style="text-indent:2em" | 8815(1)&nbsp;keV | 3.52(16)&nbsp;μs | IT | ¹⁴⁵Sm | 49/2+ | | |-id=Samarium-146 | ¹⁴⁶Sm | style="text-align:right" | 62 | style="text-align:right" | 84 | 145.9130468(33) | 9.20(26)&nbsp;y | α | ¹⁴²Nd | 0+ | Trace | |- | ¹⁴⁷Sm | style="text-align:right" | 62 | style="text-align:right" | 85 | 146.9149044(14) | 1.066(5)&nbsp;y | α | ¹⁴³Nd | 7/2− | 0.1500(14) | |-id=Samarium-148 | ¹⁴⁸Sm | style="text-align:right" | 62 | style="text-align:right" | 86 | 147.9148292(13) | 6.3(13)&nbsp;y | α | ¹⁴⁴Nd | 0+ | 0.1125(9) | |- | ¹⁴⁹Sm | style="text-align:right" | 62 | style="text-align:right" | 87 | 148.9171912(12) | colspan=3 align=center|Observationally stable | 7/2− | 0.1382(10) | |-id=Samarium-150 | ¹⁵⁰Sm | style="text-align:right" | 62 | style="text-align:right" | 88 | 149.9172820(12) | colspan=3 align=center|Observationally stable | 0+ | 0.0737(9) | |- | ¹⁵¹Sm | style="text-align:right" | 62 | style="text-align:right" | 89 | 150.9199389(12) | 94.6(6)&nbsp;y | β^− | ¹⁵¹Eu | 5/2− | | |-id=Samarium-151m | style="text-indent:1em" | ^151mSm | colspan="3" style="text-indent:2em" | 261.13(4)&nbsp;keV | 1.4(1)&nbsp;μs | IT | ¹⁵¹Sm | (11/2)− | | |-id=Samarium-152 | ¹⁵²Sm | style="text-align:right" | 62 | style="text-align:right" | 90 | 151.9197386(11) | colspan=3 align=center|Observationally stable | 0+ | 0.2674(9) | |- | ¹⁵³Sm | style="text-align:right" | 62 | style="text-align:right" | 91 | 152.9221036(11) | 46.2846(23)&nbsp;h | β^− | ¹⁵³Eu | 3/2+ | | |-id=Samarium-153m | style="text-indent:1em" | ^153mSm | colspan="3" style="text-indent:2em" | 98.39(10)&nbsp;keV | 10.6(3)&nbsp;ms | IT | ¹⁵³Sm | 11/2− | | |-id=Samarium-154 | ¹⁵⁴Sm | style="text-align:right" | 62 | style="text-align:right" | 92 | 153.9222158(14) | colspan=3 align=center|Observationally stable | 0+ | 0.2274(14) | |-id=Samarium-155 | ¹⁵⁵Sm | style="text-align:right" | 62 | style="text-align:right" | 93 | 154.9246466(14) | 22.18(6)&nbsp;min | β^− | ¹⁵⁵Eu | 3/2− | | |-id=Samarium-155m1 | style="text-indent:1em" | ^155m1Sm | colspan="3" style="text-indent:2em" | 16.5467(19)&nbsp;keV | 2.8(5)&nbsp;μs | IT | ¹⁵⁵Sm | 5/2+ | | |-id=Samarium-155m2 | style="text-indent:1em" | ^155m2Sm | colspan="3" style="text-indent:2em" | 538.03(19)&nbsp;keV | 1.00(8)&nbsp;μs | IT | ¹⁵⁵Sm | 11/2− | | |-id=Samarium-156 | ¹⁵⁶Sm | style="text-align:right" | 62 | style="text-align:right" | 94 | 155.9255382(91) | 9.4(2)&nbsp;h | β^− | ¹⁵⁶Eu | 0+ | | |-id=Samarium-156m | style="text-indent:1em" | ^156mSm | colspan="3" style="text-indent:2em" | 1397.55(9)&nbsp;keV | 185(7)&nbsp;ns | IT | ¹⁵⁶Sm | 5− | | |-id=Samarium-157 | ¹⁵⁷Sm | style="text-align:right" | 62 | style="text-align:right" | 95 | 156.9284186(48) | 8.03(7)&nbsp;min | β^− | ¹⁵⁷Eu | 3/2−# | | |-id=Samarium-158 | ¹⁵⁸Sm | style="text-align:right" | 62 | style="text-align:right" | 96 | 157.9299493(51) | 5.30(3)&nbsp;min | β^− | ¹⁵⁸Eu | 0+ | | |-id=Samarium-159 | ¹⁵⁹Sm | style="text-align:right" | 62 | style="text-align:right" | 97 | 158.9332171(64) | 11.37(15)&nbsp;s | β^− | ¹⁵⁹Eu | 5/2− | | |-id=Samarium-159m | style="text-indent:1em" | ^159mSm | colspan="3" style="text-indent:2em" | 1276.5(8)&nbsp;keV | 116(8)&nbsp;ns | IT | ¹⁵⁹Sm | (15/2+) | | |-id=Samarium-160 | ¹⁶⁰Sm | style="text-align:right" | 62 | style="text-align:right" | 98 | 159.9353370(21) | 9.6(3)&nbsp;s | β^− | ¹⁶⁰Eu | 0+ | | |-id=Samarium-160m1 | style="text-indent:1em" | ^160m1Sm | colspan="3" style="text-indent:2em" | 1361.3(4)&nbsp;keV | 120(46)&nbsp;ns | IT | ¹⁶⁰Sm | (5−) | | |-id=Samarium-160m2 | style="text-indent:1em" | ^160m2Sm | colspan="3" style="text-indent:2em" | 2757.3(4)&nbsp;keV | 1.8(4)&nbsp;μs | IT | ¹⁶⁰Sm | (11+) | | |-id=Samarium-161 | ¹⁶¹Sm | style="text-align:right" | 62 | style="text-align:right" | 99 | 160.9391601(73) | 4.8(4)&nbsp;s | β^− | ¹⁶¹Eu | 7/2+# | | |-id=Samarium-161m | style="text-indent:1em" | ^161mSm | colspan="3" style="text-indent:2em" | 1388.1(6)&nbsp;keV | 2.6(4)&nbsp;μs | IT | ¹⁶¹Sm | (17/2−) | | |-id=Samarium-162 | ¹⁶²Sm | style="text-align:right" | 62 | style="text-align:right" | 100 | 161.9416217(38) | 2.7(3)&nbsp;s | β^− | ¹⁶²Eu | 0+ | | |-id=Samarium-162m | style="text-indent:1em" | ^162mSm | colspan="3" style="text-indent:2em" | 1009.4(5)&nbsp;keV | 1.78(7)&nbsp;μs | IT | ¹⁶²Sm | (4−) | | |-id=Samarium-163 | rowspan=2|¹⁶³Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 101 | rowspan=2|162.9456791(79) | rowspan=2| | β^− | ¹⁶³Eu | rowspan=2|1/2−# | rowspan=2| | rowspan=2| |- | β^−, n (<0.1%) | ¹⁶²Eu |-id=Samarium-164 | rowspan=2|¹⁶⁴Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 102 | rowspan=2|163.9485501(44) | rowspan=2| | β^− | ¹⁶⁴Eu | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β^−, n (<0.7%) | ¹⁶³Eu |-id=Samarium-164m | style="text-indent:1em" | ^164mSm | colspan="3" style="text-indent:2em" | 1485.5(12)&nbsp;keV | 600(140)&nbsp;ns | IT | ¹⁶⁴Sm | (6−) | | |-id=Samarium-165 | rowspan=2|¹⁶⁵Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 103 | rowspan=2|164.95329(43)# | rowspan=2| | β^− (98.64%) | ¹⁶⁵Eu | rowspan=2|5/2−# | rowspan=2| | rowspan=2| |- | β^−, n (1.36%) | ¹⁶⁴Eu |-id=Samarium-166 | rowspan=2|¹⁶⁶Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 104 | rowspan=2|165.95658(43)# | rowspan=2| | β^− (95.62%) | ¹⁶⁶Eu | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β^−, n (4.38%) | ¹⁶⁵Eu |-id=Samarium-167 | rowspan=2|¹⁶⁷Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 105 | rowspan=2|166.96207(54)# | rowspan=2| | β^− | ¹⁶⁷Eu | rowspan=2|7/2−# | rowspan=2| | rowspan=2| |- | β^−, n (<16%) | ¹⁶⁶Eu |-id=Samarium-168 | rowspan=2|¹⁶⁸Sm | rowspan=2 style="text-align:right" | 62 | rowspan=2 style="text-align:right" | 106 | rowspan=2|167.96603(32)# | rowspan=2| | β^− | ¹⁶⁸Eu | rowspan=2|0+# | rowspan=2| | rowspan=2| |- | β^−, n (<21%) | ¹⁶⁷Eu

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

Samarium-151 (¹⁵¹Sm) 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 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. As this is still much shorter than its radioactive half-life, decay will hardly affect this equilibrium while in the reactor.

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. 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 46.285 hours, undergoing β^− decay into stable ¹⁵³Eu. 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.

See also

Daughter products other than samarium

• Isotopes of europium
• Isotopes of promethium
• Isotopes of neodymium

References