Isotopes of americium (₉₅Am)

Main isotopes^[1]			Decay
	abundance	half-life (t_1/2)	mode	product
²⁴¹Am	synth	432.2 y	α	²³⁷Np
²⁴¹Am	synth	432.2 y	SF	–
^242m1Am	synth	141 y	IT	²⁴²Am
			α	²³⁸Np
			SF	–
²⁴³Am	synth	7350 y	α	²³⁹Np
²⁴³Am	synth	7350 y	SF	–

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.

Nineteen radioisotopes of americium—²²³Am, ²²⁹Am, ²³⁰Am, and those ranging from ²³²Am to ²⁴⁷Am—have been characterized, with the most stable being 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.

List of isotopes

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	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) m	β⁺ (99.60%)	²³⁵Pu	5/2−#
²³⁵Am	95	140	235.047906(57)	10.3(6) m	α (0.40%)	²³¹Np	5/2−#
²³⁶Am	95	141	236.04943(13)#	3.6(1) m	β⁺	²³⁶Pu	5−
²³⁶Am	95	141	236.04943(13)#	3.6(1) m	α (4×10⁻³%)	²³²Np	5−
^236mAm	50(50)# keV			2.9(2) m	β⁺	²³⁶Pu	(1−)
^236mAm	50(50)# keV			2.9(2) m	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) m	β⁺	²³⁸Pu	1+
²³⁸Am	95	143	238.051983(63)	98(3) m	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

^ ^mAm – 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).
^ Modes of decay:

CD: Cluster decay

EC: Electron capture

IT: Isomeric transition

SF: Spontaneous fission
^ ( ) spin value – Indicates spin with weak assignment arguments.
^ ^a ^b # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).

Actinides vs fission products

Actinides and fission products by half-life v t e
Actinides^[2] by decay chain				Half-life range (a)	Fission products of ²³⁵U by yield^[3]
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
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
²⁴⁸Bk^[4]	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^[5]	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^₡
				1.53 Ma	⁹³Zr
	²³⁷Np^ƒ			2.1–6.5 Ma	¹³⁵Cs^₡	¹⁰⁷Pd
²³⁶U			²⁴⁷Cm^ƒ	15–24 Ma		¹²⁹I^₡
²⁴⁴Pu				80 Ma	... nor beyond 15.7 Ma^[6]
²³²Th^№		²³⁸U^№	²³⁵U^ƒ№	0.7–14.1 Ga	... nor beyond 15.7 Ma^[6]
₡, 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)

Notable isotopes

Americium-241

Main article: Americium-241

Americium-241 is the most prevalent isotope of americium in nuclear waste.^[7] 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.

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

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

Americium-241 decays by alpha emission, with a by-product of gamma rays. Its presence in plutonium is determined by the original concentration of plutonium-241 and the sample age. Because of the low penetration of alpha radiation, Americium-241 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

Transmutation flow between ²³⁸Pu and ²⁴⁴Cm in LWR.^[8]
Fission percentage is 100 minus shown percentages.
Total rate of transmutation varies greatly by nuclide.
²⁴⁵Cm–²⁴⁸Cm are long-lived with negligible decay.

^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 ^[9]	spontaneous fission	~200 MeV	fission products

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.^[10]

^242mAm is fissile and has a low critical mass, comparable to that of ²³⁹Pu.^[11] It has a very high cross section for fission, and is quickly destroyed if it is produced in a nuclear reactor. Work has been done investigating if this isotope could be used for a novel type of nuclear rocket.^[12]^[13]

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

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.^[14] 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)^[14] to become ²³⁹Np, which then quickly decays to ²³⁹Pu, or rarely, by spontaneous fission.^[15]

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.^[7]

References

^ ^a ^b ^c ^d 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.
^ 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.
^ Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
^ 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]."
^ This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
^ 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 nearly eight quadrillion years.
^ ^a ^b "Americium" Archived 2012-07-30 at the Wayback Machine. Argonne National Laboratory, EVS. Retrieved 25 December 2009.
^ 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.
^ 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)
^ 95-Am-242 Archived 2011-07-19 at the Wayback Machine
^ "Critical Mass Calculations for ²⁴¹Am, ^242mAm and ²⁴³Am" (PDF). Archived from the original (PDF) on July 22, 2011. Retrieved February 3, 2011.
^ "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, 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.
^ ^a ^b "Americium-243" Archived 2011-02-25 at the Wayback Machine. Oak Ridge National Laboratory. Retrieved 25 December 2009.
^ "Isotopes of the Element Americium". Jefferson Lab Science Education. Retrieved 25 December 2009.

Sources

Isotope masses from:
- National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
Isotopic compositions and standard atomic masses from:
- de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
- Wieser, Michael E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051.
"News & Notices: Standard Atomic Weights Revised". International Union of Pure and Applied Chemistry. 19 October 2005.
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.

CD:	Cluster decay
EC:	Electron capture
IT:	Isomeric transition
SF:	Spontaneous fission