# Isotopes of europium

Naturally occurring europium ($_{63}$Eu) is composed of two isotopes, $^{151}$Eu and $^{153}$Eu, with $^{153}$Eu being the most abundant (52.2% natural abundance). While $^{153}$Eu is observationally stable (theoretically can undergo alpha decay with half-life over 5.5×10$^{17}$ years), $^{151}$Eu was found in 2007 to be unstable and undergo alpha decay. The half-life is measured to be (4.62 ± 0.95(stat.) ± 0.68(syst.)) × 10$^{18}$ years which corresponds to 1 alpha decay per two minutes in every kilogram of natural europium. Besides the natural radioisotope $^{151}$Eu, 36 artificial radioisotopes have been characterized, with the most stable being $^{150}$Eu with a half-life of 36.9 years, $^{152}$Eu with a half-life of 13.516 years, $^{154}$Eu with a half-life of 8.593 years, and $^{155}$Eu with a half-life of 4.7612 years. The majority of the remaining radioactive isotopes, which range from $^{130}$Eu to $^{170}$Eu, have half-lives that are less than 12.2 seconds. This element also has 18 metastable isomers, with the most stable being $^{150m}$Eu (t$_{1/2}$ 12.8 hours), $^{152m1}$Eu (t$_{1/2}$ 9.3116 hours) and $^{152m5}$Eu (t$_{1/2}$ 96 minutes).
The primary decay mode before the most abundant stable isotope, $^{153}$Eu, is electron capture, and the primary mode after is beta decay. The primary decay products before $^{153}$Eu are isotopes of samarium and the primary products after are isotopes of gadolinium.

## 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 |
| $^{130}$Eu | 63 | 67 | 129.96402(58)# | 1.0(4) ms | p | $^{129}$Sm | (1+) | | |
| $^{131}$Eu | 63 | 68 | 130.95763(43)# | 17.8(19) ms | p (89%) | $^{130}$Sm | 3/2+ | | |
| $^{131}$Eu | 63 | 68 | 130.95763(43)# | 17.8(19) ms | β$^+$ (?%) | $^{131}$Sm | 3/2+ | | |
| $^{131}$Eu | 63 | 68 | 130.95763(43)# | 17.8(19) ms | β$^+$, p (?%) | $^{130}$Pm | 3/2+ | | |
| $^{134}$Eu | 63 | 71 | 133.94654(32)# | 0.5(2) s | β$^+$ | $^{134}$Sm | | | |
| $^{134}$Eu | 63 | 71 | 133.94654(32)# | 0.5(2) s | β$^+$, p (?%) | $^{133}$Pm | | | |
| $^{135}$Eu | 63 | 72 | 134.94187(21)# | 1.5(2) s | β$^+$ | $^{135}$Sm | 5/2+# | | |
| $^{136}$Eu | 63 | 73 | 135.93962(21)# | 3.3(3) s | β$^+$ (99.91%) | $^{136}$Sm | 6+# | | |
| $^{136}$Eu | 63 | 73 | 135.93962(21)# | 3.3(3) s | β$^+$, p (0.09%) | $^{135}$Pm | 6+# | | |
| $^{136m}$Eu | 100(100)# keV | 100(100)# keV | 100(100)# keV | 3.8(3) s | β$^+$ (99.91%) | $^{136}$Sm | 1+# | | |
| $^{136m}$Eu | 100(100)# keV | 100(100)# keV | 100(100)# keV | 3.8(3) s | β$^+$, p (0.09%) | $^{135}$Pm | 1+# | | |
| $^{137}$Eu | 63 | 74 | 136.9354307(47) | 8.4(5) s | β$^+$ | $^{137}$Sm | 5/2+# | | |
| $^{138}$Eu | 63 | 75 | 137.933709(30) | 5# s | | | 2−# | | |
| $^{138m}$Eu | 100(50)# keV | 100(50)# keV | 100(50)# keV | 12.1(6) s | β$^+$ | $^{138}$Sm | 7−# | | |
| $^{139}$Eu | 63 | 76 | 138.929792(14) | 17.9(6) s | β$^+$ | $^{139}$Sm | (11/2)− | | |
| $^{139m}$Eu | 148.3(3) keV | 148.3(3) keV | 148.3(3) keV | 10(2) μs | IT | $^{139}$Eu | (7/2+) | | |
| $^{140}$Eu | 63 | 77 | 139.928088(55) | 1.51(2) s | β$^+$ (95.1%) | $^{140}$Sm | 1+ | | |
| $^{140}$Eu | 63 | 77 | 139.928088(55) | 1.51(2) s | EC (4.9%) | $^{140}$Sm | 1+ | | |
| $^{140m1}$Eu | 210(14) keV | 210(14) keV | 210(14) keV | 125(2) ms | IT (>99%) | $^{140}$Eu | (5−) | | |
| $^{140m1}$Eu | 210(14) keV | 210(14) keV | 210(14) keV | 125(2) ms | β$^+$ (>1%) | $^{140}$Sm | (5−) | | |
| $^{140m2}$Eu | 669(14) keV | 669(14) keV | 669(14) keV | 299.8(21) ns | IT | $^{140}$Eu | (8+) | | |
| $^{141}$Eu | 63 | 78 | 140.924932(14) | 40.7(7) s | β$^+$ | $^{141}$Sm | 5/2+ | | |
| $^{141m}$Eu | 96.45(7) keV | 96.45(7) keV | 96.45(7) keV | 2.7(3) s | IT (86%) | $^{141}$Eu | 11/2− | | |
| $^{141m}$Eu | 96.45(7) keV | 96.45(7) keV | 96.45(7) keV | 2.7(3) s | β$^+$ (14%) | $^{141}$Sm | 11/2− | | |
| $^{142}$Eu | 63 | 79 | 141.923447(32) | 2.36(10) s | β$^+$ (89.9%) | $^{142}$Sm | 1+ | | |
| $^{142}$Eu | 63 | 79 | 141.923447(32) | 2.36(10) s | EC (11.1%) | $^{142}$Sm | 1+ | | |
| $^{142m}$Eu | 450(30) keV | 450(30) keV | 450(30) keV | 1.223(8) min | β$^+$ | $^{142}$Sm | 8− | | |
| $^{143}$Eu | 63 | 80 | 142.920299(12) | 2.59(2) min | β$^+$ | $^{143}$Sm | 5/2+ | | |
| $^{143m}$Eu | 389.51(4) keV | 389.51(4) keV | 389.51(4) keV | 50.0(5) μs | IT | $^{143}$Eu | 11/2− | | |
| $^{144}$Eu | 63 | 81 | 143.918819(12) | 10.2(1) s | β$^+$ | $^{144}$Sm | 1+ | | |
| $^{144m}$Eu | 1127.6(6) keV | 1127.6(6) keV | 1127.6(6) keV | 1.0(1) μs | IT | $^{144}$Eu | 8− | | |
| $^{145}$Eu | 63 | 82 | 144.9162727(33) | 5.93(4) d | β$^+$ | $^{145}$Sm | 5/2+ | | |
| $^{145m}$Eu | 716.0(3) keV | 716.0(3) keV | 716.0(3) keV | 490(30) ns | IT | $^{145}$Eu | 11/2− | | |
| $^{146}$Eu | 63 | 83 | 145.9172109(65) | 4.61(3) d | β$^+$ | $^{146}$Sm | 4− | | |
| $^{146m}$Eu | 666.33(11) keV | 666.33(11) keV | 666.33(11) keV | 235(3) μs | IT | $^{146}$Eu | 9+ | | |
| $^{147}$Eu | 63 | 84 | 146.9167524(28) | 24.1(6) d | β$^+$ | $^{147}$Sm | 5/2+ | | |
| $^{147}$Eu | 63 | 84 | 146.9167524(28) | 24.1(6) d | α (0.0022%) | $^{143}$Pm | 5/2+ | | |
| $^{147m}$Eu | 625.27(5) keV | 625.27(5) keV | 625.27(5) keV | 765(15) ns | IT | $^{147}$Eu | 11/2− | | |
| $^{148}$Eu | 63 | 85 | 147.918091(11) | 54.5(5) d | β$^+$ | $^{148}$Sm | 5− | | |
| $^{148}$Eu | 63 | 85 | 147.918091(11) | 54.5(5) d | α (9.4×10$^{−7}$%) | $^{144}$Pm | 5− | | |
| $^{148m}$Eu | 720.4(3) keV | 720.4(3) keV | 720.4(3) keV | 162(8) ns | IT | $^{148}$Eu | 9+ | | |
| $^{149}$Eu | 63 | 86 | 148.9179369(42) | 93.1(4) d | EC | $^{149}$Sm | 5/2+ | | |
| $^{149m}$Eu | 496.386(2) keV | 496.386(2) keV | 496.386(2) keV | 2.45(5) μs | IT | $^{149}$Eu | 11/2− | | |
| $^{150}$Eu | 63 | 87 | 149.9197071(67) | 36.9(9) y | β$^+$ | $^{150}$Sm | 5− | | |
| $^{150m}$Eu | 41.7(10) keV | 41.7(10) keV | 41.7(10) keV | 12.8(1) h | β$^−$ (89%) | $^{150}$Gd | 0− | | |
| $^{150m}$Eu | 41.7(10) keV | 41.7(10) keV | 41.7(10) keV | 12.8(1) h | β$^+$ (11%) | $^{150}$Sm | 0− | | |
| $^{150m}$Eu | 41.7(10) keV | 41.7(10) keV | 41.7(10) keV | 12.8(1) h | IT (<5×10$^{−8}$%) | $^{150}$Eu | 0− | | |
| $^{151}$Eu | 63 | 88 | 150.9198566(13) | 4.6(12)×10$^{18}$ y | α | $^{147}$Pm | 5/2+ | 0.4781(6) | |
| $^{151m}$Eu | 196.245(10) keV | 196.245(10) keV | 196.245(10) keV | 58.9(5) μs | IT | $^{151}$Eu | 11/2− | | |
| $^{152}$Eu | 63 | 89 | 151.9217510(13) | 13.517(6) y | β$^+$ (72.08%) | $^{152}$Sm | 3− | | |
| $^{152}$Eu | 63 | 89 | 151.9217510(13) | 13.517(6) y | β$^−$ (27.92%) | $^{152}$Gd | 3− | | |
| $^{152m1}$Eu | 45.5998(4) keV | 45.5998(4) keV | 45.5998(4) keV | 9.3116(13) h | β$^−$ (73%) | $^{152}$Gd | 0− | | |
| $^{152m1}$Eu | 45.5998(4) keV | 45.5998(4) keV | 45.5998(4) keV | 9.3116(13) h | β$^+$ (27%) | $^{152}$Sm | 0− | | |
| $^{152m2}$Eu | 65.2969(4) keV | 65.2969(4) keV | 65.2969(4) keV | 940(80) ns | IT | $^{152}$Eu | 1− | | |
| $^{152m3}$Eu | 78.2331(4) keV | 78.2331(4) keV | 78.2331(4) keV | 165(10) ns | IT | $^{152}$Eu | 1+ | | |
| $^{152m4}$Eu | 89.8496(4) keV | 89.8496(4) keV | 89.8496(4) keV | 384(10) ns | IT | $^{152}$Eu | 4+ | | |
| $^{152m5}$Eu | 147.86(10) keV | 147.86(10) keV | 147.86(10) keV | 95.8(4) min | IT | $^{152}$Eu | 8− | | |
| $^{153}$Eu | 63 | 90 | 152.9212368(13) | Observationally Stable | Observationally Stable | Observationally Stable | 5/2+ | 0.5219(6) | |
| $^{153m}$Eu | 1771.0(4) keV | 1771.0(4) keV | 1771.0(4) keV | 475(10) ns | IT | $^{153}$Eu | 19/2− | | |
| $^{154}$Eu | 63 | 91 | 153.9229857(13) | 8.592(3) y | β$^−$ (99.98%) | $^{154}$Gd | 3− | | |
| $^{154}$Eu | 63 | 91 | 153.9229857(13) | 8.592(3) y | EC (0.018%) | $^{154}$Sm | 3− | | |
| $^{154m1}$Eu | 68.1702(4) keV | 68.1702(4) keV | 68.1702(4) keV | 2.2(1) μs | IT | $^{154}$Eu | 2+ | | |
| $^{154m2}$Eu | 145.3(3) keV | 145.3(3) keV | 145.3(3) keV | 46.3(4) min | IT | $^{154}$Eu | (8−) | | |
| $^{155}$Eu | 63 | 92 | 154.9228998(13) | 4.742(8) y | β$^−$ | $^{155}$Gd | 5/2+ | | |
| $^{156}$Eu | 63 | 93 | 155.9247630(38) | 15.19(8) d | β$^−$ | $^{156}$Gd | 0+ | | |
| $^{157}$Eu | 63 | 94 | 156.9254326(45) | 15.18(3) h | β$^−$ | $^{157}$Gd | 5/2+ | | |
| $^{158}$Eu | 63 | 95 | 157.9277822(22) | 45.9(2) min | β$^−$ | $^{158}$Gd | 1− | | |
| $^{159}$Eu | 63 | 96 | 158.9290995(46) | 18.1(1) min | β$^−$ | $^{159}$Gd | 5/2+ | | |
| $^{160}$Eu | 63 | 97 | 159.93183698(97) | 42.6(5) s | β$^−$ | $^{160}$Gd | (5−) | | |
| $^{160m}$Eu | 93.0(12) keV | 93.0(12) keV | 93.0(12) keV | 30.8(5) s | IT | $^{160}$Eu | (1−) | | |
| $^{161}$Eu | 63 | 98 | 160.933664(11) | 26.2(23) s | β$^−$ | $^{161}$Gd | 5/2+# | | |
| $^{162}$Eu | 63 | 99 | 161.9369583(14) | ~10 s | β$^−$ | $^{162}$Gd | 1+# | | |
| $^{162m}$Eu | 158.0(17) keV | 158.0(17) keV | 158.0(17) keV | 15.0(5) s | IT | $^{162}$Eu | (6+) | | |
| $^{163}$Eu | 63 | 100 | 162.93926551(97) | 7.7(4) s | β$^−$ | $^{163}$Gd | 5/2+# | | |
| $^{163m}$Eu | 964.5(5) keV | 964.5(5) keV | 964.5(5) keV | 911(24) ns | IT | $^{163}$Eu | (13/2−) | | |
| $^{164}$Eu | 63 | 101 | 163.9428529(22) | 4.16(19) s | β$^−$ | $^{164}$Gd | 3−# | | |
| $^{165}$Eu | 63 | 102 | 164.9455401(56) | 2.163+0.139 −0.120 s | β$^−$ | $^{165}$Gd | 5/2+# | | |
| $^{166}$Eu | 63 | 103 | 165.94981(11)# | 1.277+0.100 −0.145 s | β$^−$ (99.37%) | $^{166}$Gd | 0−# | | |
| $^{166}$Eu | 63 | 103 | 165.94981(11)# | 1.277+0.100 −0.145 s | β$^−$, n (0.63%) | $^{165}$Gd | 0−# | | |
| $^{167}$Eu | 63 | 104 | 166.95301(43)# | 852+76 −54 ms | β$^−$ (98.05%) | $^{167}$Gd | 5/2+# | | |
| $^{167}$Eu | 63 | 104 | 166.95301(43)# | 852+76 −54 ms | β$^−$, n (1.95%) | $^{166}$Gd | 5/2+# | | |
| $^{168}$Eu | 63 | 105 | 167.95786(43)# | 440+48 −47 ms | β$^−$ (96.05%) | $^{168}$Gd | 6−# | | |
| $^{168}$Eu | 63 | 105 | 167.95786(43)# | 440+48 −47 ms | β$^−$, n (3.95%) | $^{167}$Gd | 6−# | | |
| $^{169}$Eu | 63 | 106 | 168.96172(54)# | 389+92 −88 ms | β$^−$ (85.38%) | $^{169}$Gd | 5/2+# | | |
| $^{169}$Eu | 63 | 106 | 168.96172(54)# | 389+92 −88 ms | β$^−$, n (14.62%) | $^{168}$Gd | 5/2+# | | |
| $^{170}$Eu | 63 | 107 | 169.96687(54)# | 197+74 −71 ms | β$^−$ (>76%) | $^{170}$Gd | | | |
| $^{170}$Eu | 63 | 107 | 169.96687(54)# | 197+74 −71 ms | β$^−$, n (<24%) | $^{169}$Gd | | | |
| This table header & footer: view ||||||||||
* ↑ $^m$Eu – 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: α:Alpha decay β$^+$:Positron emission EC:Electron capture β$^−$:Beta 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.
* 1 2 Order of ground state and isomer is uncertain.
* ↑ primordial radionuclide
* 1 2 3 4 Fission product
* ↑ Believed to undergo α decay to $^{149}$Pm with a half-life over 5.5×10$^{17}$ years

## Europium-155

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 |
Europium-155 is a fission product with a half-life of 4.76 years. It has a maximum decay energy of 252 keV. In a thermal reactor (almost all current nuclear power plants), it has a low fission product yield, about half of one percent as much as the most abundant fission products.
$^{155}$Eu's large neutron capture cross section (about 3900 barns for thermal neutrons, 16000 resonance integral) means that most of even the small amount produced is destroyed in the course of the nuclear fuel's burnup. Yield, decay energy, and half-life are all far less than that of $^{137}$Cs and $^{90}$Sr, so $^{155}$Eu is not a significant contributor to nuclear waste.
Some $^{155}$Eu is also produced by successive neutron capture on $^{153}$Eu (nonradioactive, 350 barns thermal, 1500 resonance integral, yield is about 5 times as great as $^{155}$Eu) and $^{154}$Eu (half-life 8.6 years, 1400 barns thermal, 1600 resonance integral, fission yield is extremely small because beta decay stops at $^{154}$Sm). However, the differing cross sections mean that both $^{155}$Eu and $^{154}$Eu are destroyed faster than they are produced.
$^{154}$Eu is a prolific emitter of gamma radiation.
| Isotope | Half-life | Relative yield | Thermal neutron | Resonance integral |
| Eu-153 | Stable | 5 | 350 | 1500 |
| Eu-154 | 8.6 years | Nearly 0 | 1500 | 1600 |
| Eu-155 | 4.76 years | 1 | 3900 | 16000 |