Isotopes of americium

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.

Isotopes of americium (₉₅Am)

Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct ^241Am syn 432.2 y α ^237Np SF – ^242m1Am syn 141 y IT ^242Am α ^238Np SF – ^243Am syn 7350 y α ^239Np SF –

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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	Decay mode ^[n 4]	Daughter isotope	Spin and parity ^[n 5]^[n 6]
	Excitation energy^[n 6]
^223Am^[1]	95	128		5.2(+12.0-4.4) ms	α	^219Np
^229Am^[1]	95	134	229.04525(9)	1.8(1.5) s	α	^225Np
^230Am^[2]	95	135	230.04609(14)#	32(+22-9) s	β^+ (64.7%)	^230Pu
					β^+, SF (35.3%)	(various)
^232Am	95	137	232.04659(32)#	79(2) s	β^+ (98%)	^232Pu
					α (2%)	^228Np
					β^+, SF (.069%)	(various)
^233Am	95	138	233.04635(11)#	3.2(8) min	β^+	^233Pu
					α	^229Np
^234Am	95	139	234.04781(22)#	2.32(8) min	β^+ (99.95%)	^234Pu
					α (.04%)	^230Np
					β^+, SF (.0066%)	(various)
^235Am	95	140	235.04795(13)#	9.9(5) min	β^+	^235Pu	5/2−#
					α (rare)	^231Np
^236Am	95	141	236.04958(11)#	3.6(1) min	β^+	^236Pu
					α	^232Np
^237Am	95	142	237.05000(6)#	73.0(10) min	β^+ (99.97%)	^237Pu	5/2(−)
					α (.025%)	^233Np
^238Am	95	143	238.05198(5)	98(2) min	β^+	^238Pu	1+
					α (10^−4%)	^234Np
^238mAm	2500(200)# keV			35(10) μs
^239Am	95	144	239.0530245(26)	11.9(1) h	EC (99.99%)	^239Pu	(5/2)−
					α (.01%)	^235Np
^239mAm	2500(200) keV			163(12) ns			(7/2+)
^240Am	95	145	240.055300(15)	50.8(3) h	β^+	^240Pu	(3−)
					α (1.9×10^−4%)	^236Np
^241Am^[n 7]	95	146	241.0568291(20)	432.2(7) y	α	^237Np	5/2−
					CD (7.4×10^−10%)	^207Tl, ^34Si
					SF (4.3×10^−10%)	(various)
^241mAm	2200(100) keV			1.2(3) μs	SF
^242Am	95	147	242.0595492(20)	16.02(2) h	β^− (82.7%)	^242Cm	1−
					EC (17.3%)	^242Pu
^242m1Am	48.60(5) keV			141(2) y	IT (99.54%)	^242Am	5−
					α (.46%)	^238Np
					SF (1.5×10^−8%)	(various)
^242m2Am	2200(80) keV			14.0(10) ms			(2+, 3−)
^243Am^[n 7]	95	148	243.0613811(25)	7,370(40) y	α	^239Np	5/2−
					SF (3.7×10^−9%)	(various)
^244Am	95	149	244.0642848(22)	10.1(1) h	β^−	^244Cm	(6−)#
^244mAm	86.1(10) keV			26(1) min	β^− (99.96%)	^244Cm	1+
					EC (.0361%)	^244Pu
^245Am	95	150	245.066452(4)	2.05(1) h	β^−	^245Cm	(5/2)+
^246Am	95	151	246.069775(20)	39(3) min	β^−	^246Cm	(7−)
^246m1Am	30(10) keV			25.0(2) min	β^− (99.99%)	^246Cm	2(−)
					IT (.01%)	^246Am
^246m2Am	~2000 keV			73(10) μs
^247Am	95	152	247.07209(11)#	23.0(13) min	β^−	^247Cm	(5/2)#
This table header & footer: view

1. ^mAm – 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. Modes of decay:

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

5. ( ) spin value – Indicates spin with weak assignment arguments.
6. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
7. Most common isotope

Actinides vs fission products

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		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
₡,  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

 

Americium-241 is used in ionization smoke detectors.

Main article: Americium-241

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

 

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 ^180mTa, ^210mBi and multiple holmium isomers, where a higher-energy nuclear isomer is more stable than the lower-energy one, 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 if in a nuclear reactor is destroyed relatively quickly. 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

 

A sample of Am-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 infrequently, 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

1. "Observation of new neutron-deficient isotopes with Z ≥ 92 in multinucleon transfer reactions" http://inspirehep.net/record/1383747/files/scoap3-fulltext.pdf
2. Kaji, D.; Morimoto, K.; Haba, H.; Ideguchi, E.; Koura, H.; Morita, K. (2016). "Decay Properties of New Isotopes ²³⁴Bk and ²³⁰Am, and Even–Even Nuclides ²³⁴Cm and ²³⁰Pu" (PDF). Journal of the Physical Society of Japan. 84 (15002): 015002. doi:10.7566/JPSJ.85.015002.
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 nearly eight quadrillion years.
8. "Americium" 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 2011-07-19 at the Wayback Machine
12. (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" 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.

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.