Isotopes of promethium

Main isotopes of Chemistry:promethium (61Pm) | Iso­tope | Decay

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| abun­dance | half-life (t1/2) | mode | pro­duct
145Pm  | trace | 17.7 y  | ε | 145Nd
146Pm  | syn | 5.53 y  | ε  | 146Nd
β− | 146Sm
147Pm  | trace  | 2.6234 y  | β− | 147Sm
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Promethium (61Pm) is an artificial element, except in trace quantities as a product of spontaneous fission of 238U and 235U and alpha decay of 151Eu, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was first synthesized in 1945.

The known isotopes run from 128Pm to 166Pm, 39 in all; the most stable are 145Pm with a half-life of 17.7 years, 146Pm with a half-life of 5.53 years, and 147Pm (the common isotope) with a half-life of 2.6234 years. 143Pm and 144Pm also have lengthy if poorly-known lives on the order of a year, but all the others have half-lives that are less than six days, with the majority less than a few minutes. There are also 24 known meta states with the most stable being 148mPm at a half-life of 41.29 days.

The primary decay mode for isotopes lighter than 146Pm is electron capture resulting in isotopes of neodymium, and the primary decay mode heavier than 146Pm is beta decay giving isotopes of samarium; promethium-146 itself decays both ways.

## Contents

* 1 List of isotopes
* 2 Stability of promethium isotopes
* 3 Promethium-147
* 4 See also
* 5 References

## List of isotopes

Nuclide
[n 1] | Z | N | Isotopic mass (u)
[n 2][n 3] | Half-life
[n 4] | Decay
mode
[n 5] | Daughter
isotope
[n 6][n 7] | Spin and
parity
[n 8][n 4] | Isotopic
abundance

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Excitation energy[n 4]
125Pm[1] | 61  | 64  |  | >310 ns  |  |  |  |
126Pm[1] | 61  | 65  | 125.95733(54)#  | 500# ms
[>310 ns]  |  |  |  |
127Pm[1] | 61  | 66  | 126.95136(43)#  | 1# s
[>310 ns]  |  |  | 3/2+#  |
128Pm  | 61  | 67  | 127.94823(32)#  | 1.0(3) s  | β+ (?%)  | 128Nd  | 4+#  |
β+, p (?%)  | 127Pr
129Pm  | 61  | 68  | 128.94291(32)#  | 2.4(9) s  | β+ | 129Nd  | 5/2+#  |
130Pm  | 61  | 69  | 129.94045(22)#  | 2.6(2) s  | β+ (?%)  | 130Nd  | (5+, 6+, 4+)  |
β+, p (?%)  | 129Pr
131Pm  | 61  | 70  | 130.93583(22)#  | 6.3(8) s  | β+ | 131Nd  | (11/2−)  |
132Pm  | 61  | 71  | 131.93384(16)#  | 6.2(6) s  | β+ | 132Nd  | (3+)  |
β+, p (5×10−5%)  | 131Pr
133Pm  | 61  | 72  | 132.929782(54)  | 13.5(21) s  | β+ | 133Nd  | (3/2+)  |
133mPm  | 129.7(7) keV  | 8# s  |  |  | (11/2−)  |
134Pm  | 61  | 73  | 133.928326(45)  | 22(1) s  | β+ | 134Nd  | (5+)  |
134m1Pm  | 50(50)# keV[n 9] | ~5 s  | β+ | 134Nd  | (2+)  |
134m2Pm  | 120(50)# keV  | 20(1) μs  | IT  | 134Pm  | (7−)  |
135Pm  | 61  | 74  | 134.924785(89)  | 49(3) s  | β+ | 135Nd  | (3/2+, 5/2+)  |
135mPm  | 240(100)# keV  | 40(3) s  | β+ | 135Nd  | (11/2−)  |
136Pm  | 61  | 75  | 135.923596(74)  | 107(6) s  | β+ | 136Nd  | 7+#  |
136m1Pm[n 9] | 100(120) keV  | 90(35) s  | β+ | 136Nd  | 2+#  |
136m2Pm  | 42.7(2) keV  | 1.5(1) μs  | IT  | 136Pm  | 7−#  |
137Pm  | 61  | 76  | 136.920480(14)  | 2# min  |  |  | 5/2−#  |
137mPm  | 160(50) keV  | 2.4(1) min  | β+ | 137Nd  | 11/2−  |
138Pm  | 61  | 77  | 137.919576(12)  | 3.24(5) min  | β+ | 138Nd  | 3−#  |
139Pm  | 61  | 78  | 138.916799(15)  | 4.15(5) min  | β+ | 139Nd  | (5/2)+  |
139mPm  | 188.7(3) keV  | 180(20) ms  | IT  | 139Pm  | (11/2)−  |
140Pm  | 61  | 79  | 139.916036(26)  | 9.2(2) s  | β+ | 140Nd  | 1+  |
140mPm  | 429(28) keV  | 5.95(5) min  | β+ | 140Nd  | 8−  |
141Pm  | 61  | 80  | 140.913555(15)  | 20.90(5) min  | β+ | 141Nd  | 5/2+  |
141m1Pm  | 628.62(7) keV  | 630(20) ns  | IT  | 141Pm  | 11/2−  |
141m2Pm  | 2530.75(17) keV  | >2 μs  | IT  | 141Pm  | (23/2+)  |
142Pm  | 61  | 81  | 141.912891(25)  | 40.5(5) s  | β+ (77.1%)  | 142Nd | 1+  |
EC (22.9%)
142m1Pm  | 883.17(16) keV  | 2.0(2) ms  | IT  | 142Pm  | (8)−  |
142m2Pm  | 2828.7(6) keV  | 67(5) μs  | IT  | 142Pm  | (13−)  |
143Pm  | 61  | 82  | 142.9109381(32)  | 265(7) d  | EC  | 143Nd | 5/2+  |
β+ (<5.7×10−6%)
144Pm  | 61  | 83  | 143.9125962(31)  | 363(14) d  | EC  | 144Nd | 5−  |
β+ (<8×10−5%)
144m1Pm  | 840.90(5) keV  | 780(200) ns  | IT  | 144Pm  | (9)+  |
144m2Pm  | 8595.8(22) keV  | ~2.7 μs  | IT  | 144Pm  | (27+)  |
145Pm  | 61  | 84  | 144.9127557(30)  | 17.7(4) y  | EC  | 145Nd | 5/2+  |
α (2.8×10−7%)  | 141Pr
146Pm  | 61  | 85  | 145.9147022(46)  | 5.53(5) y  | EC (66.0%)  | 146Nd | 3−  |
β− (34.0%)  | 146Sm
147Pm[n 10] | 61  | 86  | 146.9151449(14)  | 2.6234(2) y  | β− | 147Sm | 7/2+  | Trace[n 11]
148Pm  | 61  | 87  | 147.9174811(61)  | 5.368(7) d  | β− | 148Sm | 1−  |
148mPm  | 137.9(3) keV  | 41.29(11) d  | β− (95.8%)  | 148Sm | 5−, 6−  |
IT (4.2%)  | 148Pm
149Pm[n 10] | 61  | 88  | 148.9183415(23)  | 53.08(5) h  | β− | 149Sm | 7/2+  |
149mPm  | 240.214(7) keV  | 35(3) μs  | IT  | 149Pm  | 11/2−  |
150Pm  | 61  | 89  | 149.920990(22)  | 2.698(15) h  | β− | 150Sm | (1−)  |
151Pm[n 10] | 61  | 90  | 150.9212166(49)  | 28.40(4) h  | β− | 151Sm  | 5/2+  |
152Pm  | 61  | 91  | 151.923505(28)  | 4.12(8) min  | β− | 152Sm | 1+  |
152mPm  | 140(90) keV[n 9] | 7.52(8) min  | β− | 152Sm | 4(−)  |
153Pm  | 61  | 92  | 152.9241563(97)  | 5.25(2) min  | β− | 153Sm  | 5/2−  |
154Pm  | 61  | 93  | 153.926713(27)  | 2.68(7) min  | β− | 154Sm | (4+)  |
154mPm[n 9] | −230(50) keV  | 1.73(10) min  | β− | 154Sm | (1−)  |
155Pm  | 61  | 94  | 154.9281370(51)  | 41.5(2) s  | β− | 155Sm  | (5/2−)  |
156Pm  | 61  | 95  | 155.9311141(13)  | 27.4(5) s  | β− | 156Sm  | 4+  |
156mPm  | 150.30(10) keV  | 2.3(20) s  | IT (98%)  | 156Pm  | 1+#  |
β− (2%)  | 156Sm
157Pm  | 61  | 96  | 156.9331213(75)  | 10.56(10) s  | β− | 157Sm  | (5/2−)  |
158Pm  | 61  | 97  | 157.93654695(95)  | 4.8(5) s  | β− | 158Sm  | (0+,1+)#  |
158mPm  | 150(50)# keV  | >16 μs  | IT  | 158Pm  | 5+#  |
159Pm  | 61  | 98  | 158.939286(11)  | 1.648+0.043
−0.042 s[2] | β− | 159Sm  | (5/2−)  |
159mPm  | 1465.0(5) keV  | 4.42(17) μs  | IT  | 159Pm  | 17/2+#  |
β−, n (<0.6%)[2] | 158Sm
160Pm  | 61  | 99  | 159.9432153(22)  | 874+16
−12 ms[2] | β− | 160Sm  | 6−#  |
β−, n (<0.1%)[2] | 159Sm
160mPm  | 191(11) keV  | >700 ms  |  |  | 1−#  |
161Pm  | 61  | 100  | 160.9462298(97)  | 724+20
−12 ms[2] | β− (98.91%)  | 161Sm  | (5/2−)  |
β−, n (1.09%)[2] | 160Sm
161mPm  | 965.9(9) keV  | 890(90) ns  | IT  | 161Pm  | (13/2+)  |
162Pm  | 61  | 101  | 161.95057(32)#  | 467+38
−18 ms[2] | β− (98.21%)  | 162Sm  | 2+#  |
β−, n (1.79%)[2] | 161Sm
163Pm  | 61  | 102  | 162.95388(43)#  | 362+42
−30 ms[2] | β− (95%)  | 163Sm  | 5/2−#  |
β−, n (5.00%)[2] | 162Sm
164Pm  | 61  | 103  | 163.95882(43)#  | 280+38
−33 ms[2] | β− (93.82%)  | 164Sm  | 5−#  |
β−, n (6.18%)[2] | 163Sm
165Pm  | 61  | 104  | 164.96278(54)#  | 297+111
−101 ms[2] | β− (86.74%)  | 165Sm  | 5/2−#  |
β−, n (13.26%)[2] | 164Sm
166Pm  | 61  | 105  |  | 228+131
−112 ms[2] | β− | 166Sm  |  |
β−, n (<52%)[2] | 165Sm

1. ↑ mPm - Excited nuclear isomer.
2. ↑ ( ) - Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. ↑ # - Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. ↑ 4.0 4.1 4.2 # - Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
5. ↑ Modes of decay:  EC: | Electron capture

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IT: | Isomeric transition
n: | Neutron emission
p: | Proton emission
6. ↑ Bold italics symbol as daughter - Daughter product is nearly stable.
7. ↑ Bold symbol as daughter - Daughter product is stable.
8. ↑ ( ) spin value - Indicates spin with weak assignment arguments.
9. ↑ 9.0 9.1 9.2 9.3 Order of ground state and isomer is uncertain.
10. ↑ 10.0 10.1 10.2 Fission product
11. ↑ Spontaneous fission product of 232Th, 235U, 238U and alpha decay daughter of primordial 151Eu

## Stability of promethium isotopes

Promethium is one of the two elements of the first 82 elements that has no stable isotopes. This is a rarely occurring effect of the liquid drop model. Namely, promethium does not have any beta-stable isotopes, as for any mass number, it is energetically favorable for a promethium isotope to undergo positron emission or beta decay, respectively forming a neodymium or samarium isotope which has a higher binding energy per nucleon. The other element for which this happens is technetium (Z = 43).

## Promethium-147

Promethium-147 beta decays to the long-lived primordial radioisotope samarium-147 with a half-life of 2.6234 years, emitting low-energy beta radiation without gamma emission. It is a common fission product, produced in nuclear reactors and in trace quantities in nature, where it is also produced by the alpha decay of europium-151.[3]

In the reactor environment, it is almost exclusively produced through beta decay of neodymium-147 as usual for fission products. The isotopes 142-146Nd, 148Nd, and 150Nd are all stable with respect to beta decay, so the isotopes of promethium with those masses are not produced by beta decay and are therefore not significant fission products (as they could only be produced directly, rather than through a beta-decay chain). 149Pm and 151Pm are, but have half-lives of only 53.08 and 28.40 hours, so are not found in spent nuclear fuel that has been cooled for months or years.

Promethium-147 is used as a beta particle source and a radioisotope thermoelectric generator (RTG) fuel; its power density is about 2 watts per gram. Mixed with a phosphor, it was used to illuminate the Apollo Lunar Module electrical switch tips and the control panels of the Lunar Roving Vehicle.[4] For luminescent applications, it has generally been replaced by tritium, which is even safer and has a longer half-life (12.32 years).

## See also

Daughter products other than promethium

* Isotopes of samarium
* Isotopes of neodymium
* Isotopes of praseodymium

## References

1. ↑ 1.0 1.1 1.2 Suzuki, H; Fukuda, N; Takeda, H; Shimizu, Y; Yoshimoto, M; Togano, Y; Sato, H; Kitamura, N et al. (7 November 2025). "Discovery of Proton-Rich Radioactive Isotopes in the Z = 60 Region Produced by the Projectile Fragmentation of a 345-MeV/Nucleon 238U Beam". Progress of Theoretical and Experimental Physics 2025 (11). doi:10.1093/ptep/ptaf149.
2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 Kiss, G. G.Expression error: Unrecognized word "et". (2022). "Measuring the β-decay properties of neutron-rich exotic Pm, Sm, Eu, and Gd isotopes to constrain the nucleosynthesis yields in the rare-earth region". The Astrophysical Journal 936 (107): 107. doi:10.3847/1538-4357/ac80fc. Bibcode: 2022ApJ...936..107K.
3. ↑ Belli, P.; Bernabei, R.; Cappella, F. et al. (2007). "Search for α decay of natural Europium". Nuclear Physics A 789 (1–4): 15–29. doi:10.1016/j.nuclphysa.2007.03.001. Bibcode: 2007NuPhA.789...15B.
4. ↑ "Apollo Experience Report - Protection Against Radiation". NASA. http://www.hq.nasa.gov/alsj/tnD7080RadProtect.pdf.

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