Isotopes of americium

Isotopes of americium (₉₅Am)
SF	–
α	238Np
SF	–
SF	–
	view; talk; edit;

Americium (₉₅Am) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no known stable isotopes. The first isotope to be synthesized was ²⁴¹Am in 1944. The artificial element decays by ejecting alpha particles. Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom). Despite ²⁴³
Am being an order of magnitude longer lived than ²⁴¹
Am, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.

Quick Facts Main isotopes, Decay ...

Close

Eighteen radioisotopes of americium, ranging from ²²⁹Am to ²⁴⁷Am with the exception of ²³¹Am, have been characterized; another isotope, ²²³Am, has also been reported but is unconfirmed. The most stable isotopes are ²⁴³Am with a half-life of 7,370 years and ²⁴¹Am with a half-life of 432.2 years. All of the remaining radioactive isotopes have half-lives that are less than 51 hours, and the majority of these have half-lives that are less than 100 minutes. This element also has 8 meta states, with the most stable being ^242m1Am (t_1/2 = 141 years). This isomer is unusual in that its half-life is far longer than that of the ground state of the same isotope.

More information Nuclide, Z ...

Nuclide ^{[n 1]}	Z	N	Isotopic mass (Da) ^{[n 2]}^{[n 3]}	Half-life^[1]	Decay mode^[1] ^{[n 4]}	Daughter isotope	Spin and parity^[1] ^{[n 5]}^{[n 6]}
Nuclide ^{[n 1]}	Excitation energy^{[n 6]}			Half-life^[1]	Decay mode^[1] ^{[n 4]}	Daughter isotope	Spin and parity^[1] ^{[n 5]}^{[n 6]}
²²³Am^{[n 7]}	95	128	223.04584(32)#	10(9) ms	α	²¹⁹Np	9/2–#
²²⁹Am	95	134	229.04528(11)	1.8(15) s	α	²²⁵Np	5/2–#
²³⁰Am	95	135	230.04603(15)#	40(9) s	β⁺ (<70%)	²³⁰Pu	1–#
²³⁰Am	95	135	230.04603(15)#	40(9) s	β⁺SF (>30%)	(various)	1–#
²³²Am	95	137	232.04661(32)#	1.31(4) min	β⁺ (97%)	²³²Pu	1–#
					α (3%)	²²⁸Np
					β⁺SF (0.069%)	(various)
²³³Am	95	138	233.04647(12)#	3.2(8) min	β⁺ (95.5%)	²³³Pu	5/2–#
²³³Am	95	138	233.04647(12)#	3.2(8) min	α (4.5%)	²²⁹Np	5/2–#
²³⁴Am	95	139	234.04773(17)#	2.32(8) min	β⁺ (99.95%)	²³⁴Pu	0–#
					α (0.039%)	²³⁰Np
					β⁺, SF (0.0066%)	(various)
²³⁵Am	95	140	235.047906(57)	10.3(6) min	β⁺ (99.60%)	²³⁵Pu	5/2−#
²³⁵Am	95	140	235.047906(57)	10.3(6) min	α (0.40%)	²³¹Np	5/2−#
²³⁶Am	95	141	236.04943(13)#	3.6(1) min	β⁺	²³⁶Pu	5−
²³⁶Am	95	141	236.04943(13)#	3.6(1) min	α (4×10⁻³%)	²³²Np	5−
^236mAm	50(50)# keV			2.9(2) min	β⁺	²³⁶Pu	(1−)
^236mAm	50(50)# keV			2.9(2) min	α ?	²³²Np	(1−)
²³⁷Am	95	142	237.049995(64)#	73.6(8) min	β⁺ (99.975%)	²³⁷Pu	5/2−
²³⁷Am	95	142	237.049995(64)#	73.6(8) min	α (.025%)	²³³Np	5/2−
²³⁸Am	95	143	238.051983(63)	98(3) min	β⁺	²³⁸Pu	1+
²³⁸Am	95	143	238.051983(63)	98(3) min	α (1.0×10⁻⁴%)	²³⁴Np	1+
^238mAm	2500(200)# keV			35(18) μs	SF	(various)
^238mAm	2500(200)# keV			35(18) μs	IT ?	²³⁸Am
²³⁹Am	95	144	239.0530227(21)	11.9(1) h	EC (99.99%)	²³⁹Pu	5/2−
²³⁹Am	95	144	239.0530227(21)	11.9(1) h	α (0.01%)	²³⁵Np	5/2−
^239mAm	2500(200) keV			163(12) ns	SF	(various)	(7/2+)
^239mAm	2500(200) keV			163(12) ns	IT ?	²³⁹Am	(7/2+)
²⁴⁰Am	95	145	240.055298(15)	50.8(3) h	β⁺	²⁴⁰Pu	(3−)
²⁴⁰Am	95	145	240.055298(15)	50.8(3) h	α (1.9×10⁻⁴%)	²³⁶Np	(3−)
^240mAm	3000(200) keV			940(40) μs	SF	(various)
^240mAm	3000(200) keV			940(40) μs	IT ?	²⁴⁰Am
²⁴¹Am	95	146	241.0568273(12)	432.6(6) y	α	²³⁷Np	5/2−
²⁴¹Am	95	146	241.0568273(12)	432.6(6) y	SF (3.6×10⁻¹⁰%)	(various)	5/2−
^241mAm	2200(200) keV			1.2(3) μs	SF	(various)
²⁴²Am	95	147	242.0595474(12)	16.02(2) h	β⁻ (82.7%)	²⁴²Cm	1−
²⁴²Am	95	147	242.0595474(12)	16.02(2) h	EC (17.3%)	²⁴²Pu	1−
^242m1Am	48.60(5) keV			141(2) y	IT (99.54%)	²⁴²Am	5−
					α (.46%)	²³⁸Np
					SF ?	(various)
^242m2Am	2200(80) keV			14.0(10) ms	SF	(various)	(2+, 3−)
^242m2Am	2200(80) keV			14.0(10) ms	IT ?	²⁴²Am	(2+, 3−)
²⁴³Am	95	148	243.0613799(15)	7,350(9) y	α	²³⁹Np	5/2−
²⁴³Am	95	148	243.0613799(15)	7,350(9) y	SF (3.7×10⁻⁹%)	(various)	5/2−
^243mAm	2300(200) keV			5.5(5) μs	SF	(various)
^243mAm	2300(200) keV			5.5(5) μs	IT ?	²⁴³Am
²⁴⁴Am	95	149	244.0642829(16)	10.01(3) h	β⁻	²⁴⁴Cm	(6−)
^244m1Am	89.3(16) keV			26.13(43) min	β⁻ (99.96%)	²⁴⁴Cm	1+
^244m1Am	89.3(16) keV			26.13(43) min	EC (0.0364%)	²⁴⁴Pu	1+
^244m2Am	2000(200)#			900(150) μs	SF	(various)
^244m2Am	2000(200)#			900(150) μs	IT ?	²⁴⁴Am
^244m3Am	2200(200)#			~6.5 μs	SF	(various)
^244m3Am	2200(200)#			~6.5 μs	IT ?	²⁴⁴Am
²⁴⁵Am	95	150	245.0664528(20)	2.05(1) h	β⁻	²⁴⁵Cm	5/2+
^245mAm	2400(400)#			640(60) ns	SF	(various)
^245mAm	2400(400)#			640(60) ns	IT ?	²⁴⁵Am
²⁴⁶Am	95	151	246.069774(19)#	39(3) min	β⁻	²⁴⁶Cm	(7−)
^246m1Am	30(10)# keV			25.0(2) min	β⁻	²⁴⁶Cm	2(−)
^246m1Am	30(10)# keV			25.0(2) min	IT ?	²⁴⁶Am	2(−)
^246m2Am	2000(800)# keV			73(10) μs	SF	(various)
^246m2Am	2000(800)# keV			73(10) μs	IT ?	²⁴⁶Am
²⁴⁷Am	95	152	247.07209(11)#	23.0(13) min	β⁻	²⁴⁷Cm	5/2#
This table header & footer: view

Close

[n 1]
^mAm – Excited nuclear isomer.
[n 2]
( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
[n 3]
# – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
[n 4]
Modes of decay:

CD: Cluster decay

EC: Electron capture

IT: Isomeric transition

SF: Spontaneous fission
[n 5]
( ) spin value – Indicates spin with weak assignment arguments.
[n 6]
# – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
[n 7]
The discovery of this isotope is uncertain due to disagreements between theoretical predictions and reported experimental data.^[2]

More information Half-life range (a), 4n ...

Actinides and fission products by half-life v t e
Actinides^[3] by decay chain				Half-life range (a)	Fission products of ²³⁵U by yield^[4]
4n	4n + 1	4n + 2	4n + 3	Half-life range (a)	4.5–7%	0.04–1.25%	<0.001%
²²⁸Ra^№				4–6 a		¹⁵⁵Eu^þ
²⁴⁸Bk^[5]				> 9 a
²⁴⁴Cm^ƒ	²⁴¹Pu^ƒ	²⁵⁰Cf	²²⁷Ac^№	10–29 a	⁹⁰Sr	⁸⁵Kr	^113mCd^þ
²³²U^ƒ		²³⁸Pu^ƒ	²⁴³Cm^ƒ	29–97 a	¹³⁷Cs	¹⁵¹Sm^þ	^121mSn
	²⁴⁹Cf^ƒ	^242mAm^ƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	²⁴¹Am^ƒ		²⁵¹Cf^ƒ^[6]	430–900 a
		²²⁶Ra^№	²⁴⁷Bk	1.3–1.6 ka
²⁴⁰Pu	²²⁹Th	²⁴⁶Cm^ƒ	²⁴³Am^ƒ	4.7–7.4 ka
	²⁴⁵Cm^ƒ	²⁵⁰Cm		8.3–8.5 ka
			²³⁹Pu^ƒ	24.1 ka
		²³⁰Th^№	²³¹Pa^№	32–76 ka
²³⁶Np^ƒ	²³³U^ƒ	²³⁴U^№		150–250 ka	⁹⁹Tc^₡	¹²⁶Sn
²⁴⁸Cm		²⁴²Pu		327–375 ka		⁷⁹Se^₡
				1.33 Ma	¹³⁵Cs^₡
	²³⁷Np^ƒ			1.61–6.5 Ma	⁹³Zr	¹⁰⁷Pd
²³⁶U			²⁴⁷Cm^ƒ	15–24 Ma		¹²⁹I^₡
²⁴⁴Pu				80 Ma	... nor beyond 15.7 Ma^[7]
²³²Th^№		²³⁸U^№	²³⁵U^ƒ№	0.7–14.1 Ga	... nor beyond 15.7 Ma^[7]
₡, has thermal neutron capture cross section in the range of 8–50 barns ƒ, fissile №, primarily a naturally occurring radioactive material (NORM) þ, neutron poison (thermal neutron capture cross section greater than 3k barns)

Close

Americium-241

Americium-241 is the most common isotope of americium in nuclear waste.^[8] It is the isotope used in an americium smoke detector based on an ionization chamber. It is a potential fuel for long-lifetime radioisotope thermoelectric generators.

More information Parameter, Value ...

Parameter	Value
Atomic mass	241.056829 u
Mass excess	52930 keV
Beta decay energy	−767 keV
Spin	5/2−
Half-life	432.6 years
Spontaneous fissions	1200 per kg s
Decay heat	114 watts/kg

Close

Possible parent nuclides: beta from ²⁴¹Pu, electron capture from ²⁴¹Cm, alpha from ²⁴⁵Bk.

²⁴¹Am alpha decays, with a by-product of gamma rays. Its presence in plutonium is determined by the original concentration of ²⁴¹Pu and the sample age. Due to the low penetration of alpha radiation, ²⁴¹Am only poses a health risk when ingested or inhaled. Older samples of plutonium containing plutonium-241 contain a buildup of ²⁴¹Am. A chemical removal of americium from reworked plutonium (e.g. during reworking of plutonium pits) may be required.

Americium-242m

More information Probability, Decay mode ...

^242mAm decay modes (half-life: 141 years)
Probability	Decay mode	Decay energy	Decay product
99.54%	isomeric transition	0.05 MeV	²⁴²Am
0.46%	alpha decay	5.64 MeV	²³⁸Np
(1.5±0.6) × 10⁻¹⁰ ^[10]	spontaneous fission	~200 MeV	fission products

Close

Americium-242m has a mass of 242.0595492 g/mol. It is one of the rare cases, like ^108mAg, ^166mHo, ^180mTa, ^186mRe, ^192mIr, ^210mBi, ^212mPo and others, where a higher-energy nuclear isomer is more stable than the ground state, americium-242.^[11]

^242mAm is fissile with a low critical mass, comparable to that of ²³⁹Pu.^[12] It has a very high fission cross section, and is quickly destroyed if it is produced in a nuclear reactor. It has been investigated whether this isotope could be used for a novel type of nuclear rocket.^[13]^[14]

More information Probability, Decay mode ...

²⁴²Am decay modes (half-life: 16 hours)
Probability	Decay mode	Decay energy	Decay product
82.70%	beta decay	0.665 MeV	²⁴²Cm
17.30%	electron capture	0.751 MeV	²⁴²Pu

Close

Americium-243

Americium-243 has a mass of 243.06138 g/mol and a half-life of 7,370 years, the longest lasting of all americium isotopes. It is formed in the nuclear fuel cycle by neutron capture on plutonium-242 followed by beta decay.^[15] Production increases exponentially with increasing burnup as a total of 5 neutron captures on ²³⁸U are required. If MOX-fuel is used, particularly MOX-fuel high in ²⁴¹
Pu and ²⁴²
Pu, more americium overall and more ²⁴³
Am will be produced.

It decays by either emitting an alpha particle (with a decay energy of 5.27 MeV)^[15] to become ²³⁹Np, which then quickly decays to ²³⁹Pu, or rarely, by spontaneous fission.^[16]

As for the other americium isotopes, and more generally for all alpha emitters, ²⁴³Am is carcinogenic in case of internal contamination after being inhaled or ingested. ²⁴³Am also presents a risk of external irradiation associated with the gamma ray emitted by its short-lived decay product ²³⁹Np. The external irradiation risk for the other two americium isotopes (²⁴¹Am and ^242mAm) is less than 10% of that for americium-243.^[8]

[1]
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.
[2]
Sun, M. D.; et al. (2017). "New short-lived isotope ²²³Np and the absence of the Z = 92 subshell closure near N = 126". Physics Letters B. 771: 303–308. Bibcode:2017PhLB..771..303S. doi:10.1016/j.physletb.2017.03.074.
[3]
Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
[4]
Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
[5]
Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
"The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk²⁴⁸ with a half-life greater than 9 [years]. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
[6]
This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
[7]
Excluding those "classically stable" nuclides with half-lives significantly in excess of ²³²Th; e.g., while ^113mCd has a half-life of only fourteen years, that of ¹¹³Cd is eight quadrillion years.
[8]
"Americium" Archived 2012-07-30 at the Wayback Machine. Argonne National Laboratory, EVS. Retrieved 25 December 2009.
[9]
Sasahara, Akihiro; Matsumura, Tetsuo; Nicolaou, Giorgos; Papaioannou, Dimitri (April 2004). "Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels". Journal of Nuclear Science and Technology. 41 (4): 448–456. doi:10.3327/jnst.41.448.
[10]
J. T. Caldwell; S. C. Fultz; C. D. Bowman; R. W. Hoff (March 1967). "Spontaneous Fission Half-Life of Am^242m". Physical Review. 155 (4): 1309–1313. Bibcode:1967PhRv..155.1309C. doi:10.1103/PhysRev.155.1309. (halflife (9.5±3.5)×10¹¹ years)
[11]
95-Am-242 Archived 2011-07-19 at the Wayback Machine
[12]
"Critical Mass Calculations for ²⁴¹Am, ^242mAm and ²⁴³Am" (PDF). Archived from the original (PDF) on July 22, 2011. Retrieved February 3, 2011.
[13]
"Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks" (Press release). Ben-Gurion University Of The Negev. December 28, 2000.
[14]
Ronen, Yigal; Shwageraus, E. (2000). "Ultra-thin 241mAm fuel elements in nuclear reactors". Nuclear Instruments and Methods in Physics Research A. 455 (2): 442–451. Bibcode:2000NIMPA.455..442R. doi:10.1016/s0168-9002(00)00506-4.
[15]
"Americium-243" Archived 2011-02-25 at the Wayback Machine. Oak Ridge National Laboratory. Retrieved 25 December 2009.
[16]
"Isotopes of the Element Americium". Jefferson Lab Science Education. Retrieved 25 December 2009.

Isotope masses from:
- National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
Half-life, spin, and isomer data selected from the following sources.
- National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
- IAEA - Nuclear Data Section. Live Chart of Nuclides. Vienna International Centre.
- Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.

[2] [n 1]
^mAm – Excited nuclear isomer.

[3] [n 2]
( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.

[TMS-4] [n 3]
# – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).

[5] [n 4]
Modes of decay:

CD: Cluster decay

EC: Electron capture

IT: Isomeric transition

SF: Spontaneous fission

[6] [n 5]
( ) spin value – Indicates spin with weak assignment arguments.

[TNN-7] [n 6]
# – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).

[9] [n 7]
The discovery of this isotope is uncertain due to disagreements between theoretical predictions and reported experimental data.^[2]

[NUBASE2020-1] [1]
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.

[Np223-8] [2]
Sun, M. D.; et al. (2017). "New short-lived isotope ²²³Np and the absence of the Z = 92 subshell closure near N = 126". Physics Letters B. 771: 303–308. Bibcode:2017PhLB..771..303S. doi:10.1016/j.physletb.2017.03.074.

[10] [3]
Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.

[11] [4]
Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.

[12] [5]
Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
"The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk²⁴⁸ with a half-life greater than 9 [years]. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."

[13] [6]
This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".

[14] [7]
Excluding those "classically stable" nuclides with half-lives significantly in excess of ²³²Th; e.g., while ^113mCd has a half-life of only fourteen years, that of ¹¹³Cd is eight quadrillion years.

[anl-15] [8]
"Americium" Archived 2012-07-30 at the Wayback Machine. Argonne National Laboratory, EVS. Retrieved 25 December 2009.

[Am242-Sasahara-16] [9]
Sasahara, Akihiro; Matsumura, Tetsuo; Nicolaou, Giorgos; Papaioannou, Dimitri (April 2004). "Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels". Journal of Nuclear Science and Technology. 41 (4): 448–456. doi:10.3327/jnst.41.448.

[242Am-Calswell-17] [10]
J. T. Caldwell; S. C. Fultz; C. D. Bowman; R. W. Hoff (March 1967). "Spontaneous Fission Half-Life of Am^242m". Physical Review. 155 (4): 1309–1313. Bibcode:1967PhRv..155.1309C. doi:10.1103/PhysRev.155.1309. (halflife (9.5±3.5)×10¹¹ years)

[242Am-matpack-18] [11]
95-Am-242 Archived 2011-07-19 at the Wayback Machine

[242Am-typhoon-19] [12]
"Critical Mass Calculations for ²⁴¹Am, ^242mAm and ²⁴³Am" (PDF). Archived from the original (PDF) on July 22, 2011. Retrieved February 3, 2011.

[Am242-sciencedaily-20] [13]
"Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks" (Press release). Ben-Gurion University Of The Negev. December 28, 2000.

[Ronen-21] [14]
Ronen, Yigal; Shwageraus, E. (2000). "Ultra-thin 241mAm fuel elements in nuclear reactors". Nuclear Instruments and Methods in Physics Research A. 455 (2): 442–451. Bibcode:2000NIMPA.455..442R. doi:10.1016/s0168-9002(00)00506-4.

[243Am-ornl-22] [15]
"Americium-243" Archived 2011-02-25 at the Wayback Machine. Oak Ridge National Laboratory. Retrieved 25 December 2009.

[243Am-jlab-23] [16]
"Isotopes of the Element Americium". Jefferson Lab Science Education. Retrieved 25 December 2009.

[1]

[n 1]

[n 2]

[n 3]

[n 4]

[n 5]

[n 6]

[n 7]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]