Isotopes of americium

Isotopes of americium (₉₅Am)
SF	–
α	238Np
SF	–
SF	–
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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.

Eighteen radioisotopes of americium, ranging from ²³⁰Am 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.

List of isotopes edit

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

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

Actinides vs fission products edit

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
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^[5]	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^₡
				1.53 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^[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)

Notable isotopes edit

Americium-241 edit

Americium-241 is the most prevalent 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.

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 edit

Transmutation flow between ²³⁸Pu and ²⁴⁴Cm in LWR.^[9]
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 ^[10]	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.^[11]

^242mAm is fissile and has a low critical mass, comparable to that of ²³⁹Pu.^[12] 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.^[13]^[14]

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 edit

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]

References edit

Sources edit

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] Am – Excited nuclear isomer.

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

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

[5] Modes of decay:
CD: Cluster decay
EC: Electron capture
IT: Isomeric transition
SF: Spontaneous fission

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

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

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

[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]

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