Isotopes of promethium

Isotopes of promethium (₆₁Pm)
α	141Pr
β−	146Sm
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Last updated September 03, 2025

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.

List of isotopes

Nuclide ^{[n 1]}	Z	N	Isotopic mass (Da)^[2] ^{[n 2]}^{[n 3]}	Half-life ^[1] ^{[n 4]}	Decay mode ^[1] ^{[n 5]}	Daughter isotope ^{[n 6]}^{[n 7]}	Spin and parity ^[1] ^{[n 8]}^{[n 4]}	Isotopic abundance
Nuclide ^{[n 1]}	Excitation energy^{[n 4]}			Half-life ^[1] ^{[n 4]}	Decay mode ^[1] ^{[n 5]}	Daughter isotope ^{[n 6]}^{[n 7]}	Spin and parity ^[1] ^{[n 8]}^{[n 4]}	Isotopic abundance
¹²⁸Pm	61	67	127.94823(32)#	1.0(3) s	β⁺ (?%)	¹²⁸Nd	4+#
¹²⁸Pm	61	67	127.94823(32)#	1.0(3) s	β⁺, p (?%)	¹²⁷Pr	4+#
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+)
¹³⁰Pm	61	69	129.94045(22)#	2.6(2) s	β⁺, p (?%)	¹²⁹Pr	(5+, 6+, 4+)
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+)
¹³²Pm	61	71	131.93384(16)#	6.2(6) s	β⁺, p (5×10⁻⁵%)	¹³¹Pr	(3+)
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+
¹⁴²Pm	61	81	141.912891(25)	40.5(5) s	EC (22.9%)	¹⁴²Nd	1+
^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+
¹⁴³Pm	61	82	142.9109381(32)	265(7) d	β⁺ (<5.7×10⁻⁶%)	¹⁴³Nd	5/2+
¹⁴⁴Pm	61	83	143.9125962(31)	363(14) d	EC	¹⁴⁴Nd	5−
¹⁴⁴Pm	61	83	143.9125962(31)	363(14) d	β⁺ (<8×10⁻⁵%)	¹⁴⁴Nd	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+
¹⁴⁵Pm	61	84	144.9127557(30)	17.7(4) y	α (2.8×10⁻⁷%)	¹⁴¹Pr	5/2+
¹⁴⁶Pm	61	85	145.9147022(46)	5.53(5) y	EC (66.0%)	¹⁴⁶Nd	3−
¹⁴⁶Pm	61	85	145.9147022(46)	5.53(5) y	β⁻ (34.0%)	¹⁴⁶Sm	3−
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−
^148mPm	137.9(3) keV			41.29(11) d	IT (4.2%)	¹⁴⁸Pm	5−, 6−
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+#
^156mPm	150.30(10) keV			2.3(20) s	β⁻ (2%)	¹⁵⁶Sm	1+#
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^[3]	β⁻	¹⁵⁹Sm	(5/2−)
^159mPm	1465.0(5) keV			4.42(17) μs	IT	¹⁵⁹Pm	17/2+#
^159mPm	1465.0(5) keV			4.42(17) μs	β⁻, n (<0.6%)^[3]	¹⁵⁸Sm	17/2+#
¹⁶⁰Pm	61	99	159.9432153(22)	874+16 −12 ms^[3]	β⁻	¹⁶⁰Sm	6−#
¹⁶⁰Pm	61	99	159.9432153(22)	874+16 −12 ms^[3]	β⁻, n (<0.1%)^[3]	¹⁵⁹Sm	6−#
^160mPm	191(11) keV			>700 ms			1−#
¹⁶¹Pm	61	100	160.9462298(97)	724+20 −12 ms^[3]	β⁻ (98.91%)	¹⁶¹Sm	(5/2−)
¹⁶¹Pm	61	100	160.9462298(97)	724+20 −12 ms^[3]	β⁻, n (1.09%)^[3]	¹⁶⁰Sm	(5/2−)
^161mPm	965.9(9) keV			890(90) ns	IT	¹⁶¹Pm	(13/2+)
¹⁶²Pm	61	101	161.95057(32)#	467+38 −18 ms^[3]	β⁻ (98.21%)	¹⁶²Sm	2+#
¹⁶²Pm	61	101	161.95057(32)#	467+38 −18 ms^[3]	β⁻, n (1.79%)^[3]	¹⁶¹Sm	2+#
¹⁶³Pm	61	102	162.95388(43)#	362+42 −30 ms^[3]	β⁻ (95%)	¹⁶³Sm	5/2−#
¹⁶³Pm	61	102	162.95388(43)#	362+42 −30 ms^[3]	β⁻, n (5.00%)^[3]	¹⁶²Sm	5/2−#
¹⁶⁴Pm	61	103	163.95882(43)#	280+38 −33 ms^[3]	β⁻ (93.82%)	¹⁶⁴Sm	5−#
¹⁶⁴Pm	61	103	163.95882(43)#	280+38 −33 ms^[3]	β⁻, n (6.18%)^[3]	¹⁶³Sm	5−#
¹⁶⁵Pm	61	104	164.96278(54)#	297+111 −101 ms^[3]	β⁻ (86.74%)	¹⁶⁵Sm	5/2−#
¹⁶⁵Pm	61	104	164.96278(54)#	297+111 −101 ms^[3]	β⁻, n (13.26%)^[3]	¹⁶⁴Sm	5/2−#
¹⁶⁶Pm	61	105		228+131 −112 ms^[3]	β⁻	¹⁶⁶Sm
¹⁶⁶Pm	61	105		228+131 −112 ms^[3]	β⁻, n (<52%)^[3]	¹⁶⁵Sm
This table header & footer: view

↑ ^mPm – 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).
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:
EC: Electron capture
IT: Isomeric transition
n: Neutron emission
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 3 4 Order of ground state and isomer is uncertain.
1 2 3 Fission product
↑ 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 has a half-life of 2.62 years, and is a fission product produced in nuclear reactors via beta decay from neodymium-147. The isotopes ¹⁴²Nd, ¹⁴³Nd, ¹⁴⁴Nd, ¹⁴⁵Nd, ¹⁴⁶Nd, ¹⁴⁸Nd, and ¹⁵⁰Nd are all stable with respect to beta decay, so the isotopes of promethium with those masses cannot be produced by beta decay and therefore are not fission products in significant quantities (they could only be produced directly, rather than along a beta-decay chain). ¹⁴⁹Pm and ¹⁵¹Pm 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. It is found naturally mostly from the spontaneous fission of uranium-238 and less often from the alpha decay of europium-151.^[4]

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 Apollo Lunar Module electrical switch tips and painted on control panels of the Lunar Roving Vehicle.^[5]

References

1 2 3 4 Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3) 030001. doi:10.1088/1674-1137/abddae.
↑ Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3) 030003. doi:10.1088/1674-1137/abddaf.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Kiss, G. G.; Vitéz-Sveiczer, A.; Saito, Y.; et al. (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. Bibcode:2022ApJ...936..107K. doi: 10.3847/1538-4357/ac80fc . hdl: 2117/375253 .
↑ Belli, P.; Bernabei, R.; Cappella, F.; et al. (2007). "Search for α decay of natural Europium". Nuclear Physics A. 789 (1–4): 15–29. Bibcode:2007NuPhA.789...15B. doi:10.1016/j.nuclphysa.2007.03.001.
↑ "Apollo Experience Report - Protection Against Radiation" (PDF). NASA. Archived from the original (PDF) on 14 November 2014. Retrieved 9 December 2011.

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[2] Pm – Excited nuclear isomer.

[4] ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.

[TMS-5] # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).

[TNN-6] 1 2 3 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).

[7] Modes of decay:
EC: Electron capture
IT: Isomeric transition
n: Neutron emission
p: Proton emission

[8] Bold italics symbol as daughter – Daughter product is nearly stable.

[9] Bold symbol as daughter – Daughter product is stable.

[10] ( ) spin value – Indicates spin with weak assignment arguments.

[order-11] 1 2 3 4 Order of ground state and isomer is uncertain.

[FP-12] 1 2 3 Fission product

[13] Spontaneous fission product of ²³²Th, ²³⁵U, ²³⁸U and alpha decay daughter of primordial ¹⁵¹Eu

[NUBASE2020-1] 1 2 3 4 Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3) 030001. doi:10.1088/1674-1137/abddae.

[AME2020_II-3] Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3) 030003. doi:10.1088/1674-1137/abddaf.

[Ln922-14] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Kiss, G. G.; Vitéz-Sveiczer, A.; Saito, Y.; et al. (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. Bibcode:2022ApJ...936..107K. doi: 10.3847/1538-4357/ac80fc . hdl: 2117/375253 .

[15] Belli, P.; Bernabei, R.; Cappella, F.; et al. (2007). "Search for α decay of natural Europium". Nuclear Physics A. 789 (1–4): 15–29. Bibcode:2007NuPhA.789...15B. doi:10.1016/j.nuclphysa.2007.03.001.

[16] "Apollo Experience Report - Protection Against Radiation" (PDF). NASA. Archived from the original (PDF) on 14 November 2014. Retrieved 9 December 2011.

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[n 1]

[2]

[n 2]

[n 3]

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[n 9]

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[n 11]

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Isotopes of promethium

Contents

List of isotopes

Stability of promethium isotopes

Promethium-147

See also

References

EC:	Electron capture
IT:	Isomeric transition
n:	Neutron emission
p:	Proton emission