Isotopes of promethium

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.

Isotopes of promethium (₆₁Pm)

Main isotopes[1] Decay Isotope abun­dance half-life (t1/2) mode pro­duct 143Pm synth 265 d ε 143Nd 144Pm synth 363 d ε 144Nd 145Pm synth 17.7 y ε 145Nd α 141Pr 146Pm synth 5.53 y ε 146Nd β− 146Sm 147Pm trace 2.6234 y β− 147Sm

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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
	Excitation energy[n 4]
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[3]	β−	159Sm	(5/2−)
159mPm	1465.0(5) keV			4.42(17) μs	IT	159Pm	17/2+#
					β−, n (<0.6%)[3]	158Sm
160Pm	61	99	159.9432153(22)	874+16 −12 ms[3]	β−	160Sm	6−#
					β−, n (<0.1%)[3]	159Sm
160mPm	191(11) keV			>700 ms			1−#
161Pm	61	100	160.9462298(97)	724+20 −12 ms[3]	β− (98.91%)	161Sm	(5/2−)
					β−, n (1.09%)[3]	160Sm
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+#
					β−, n (1.79%)[3]	161Sm
163Pm	61	102	162.95388(43)#	362+42 −30 ms[3]	β− (95%)	163Sm	5/2−#
					β−, n (5.00%)[3]	162Sm
164Pm	61	103	163.95882(43)#	280+38 −33 ms[3]	β− (93.82%)	164Sm	5−#
					β−, n (6.18%)[3]	163Sm
165Pm	61	104	164.96278(54)#	297+111 −101 ms[3]	β− (86.74%)	165Sm	5/2−#
					β−, n (13.26%)[3]	164Sm
166Pm	61	105		228+131 −112 ms[3]	β−	166Sm
					β−, n (<52%)[3]	165Sm
This table header & footer: view

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. # – 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
   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. Order of ground state and isomer is uncertain.
10. Fission product
11. Spontaneous fission product of ²³²Th, ²³⁵U, ²³⁸U and alpha decay daughter of primordial ¹⁵¹Eu

Stability of promethium isotopes

See also: Isotopes of technetium § Stability of technetium 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.^[4]

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.^[5] 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