Isotopes of americium

Actinides and fission products by half-life
Actinides[1] by decay chain				Half-life range (y)	Fission products of ^235U by yield[2]
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
^228Ra^№				4–6	†		^155Eu^þ
^244Cm	^241Pu^ƒ	^250Cf	^227Ac^№	10–29		^90Sr	^85Kr	^113mCd^þ
^232U^ƒ		^238Pu^№	^243Cm^ƒ	29–97		^137Cs	^151Sm^þ	^121mSn
^248Bk[3]	^249Cf^ƒ	^242mAm^ƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	^241Am^ƒ		^251Cf^ƒ[4]	430–900
		^226Ra^№	^247Bk	1.3 k – 1.6 k
^240Pu	^229Th^№	^246Cm	^243Am^ƒ	4.7 k – 7.4 k
	^245Cm^ƒ	^250Cm		8.3 k – 8.5 k
			^239Pu^ƒ№	24.1 k
		^230Th^№	^231Pa^№	32 k – 76 k
^236Np^ƒ	^233U^ƒ№	^234U^№		150 k – 250 k	‡	^99Tc^₡	^126Sn
^248Cm		^242Pu		327 k – 375 k			^79Se^₡
				1.53 M		^93Zr
	^237Np^№			2.1 M – 6.5 M		^135Cs^₡	^107Pd
^236U^№			^247Cm^ƒ	15 M – 24 M			^129I^₡
^244Pu^№				80 M	... nor beyond 15.7 M years[5]
^232Th^№		^238U^№	^235U^ƒ№	0.7 G – 14.1 G
Legend for superscript symbols ₡  has thermal neutron capture cross section in the range of 8–50 barns ƒ  fissile m  metastable isomer №  naturally occurring radioactive material (NORM) þ  neutron poison (thermal neutron capture cross section greater than 3k barns) †  range 4–97 y: Medium-lived fission product ‡  over 200,000 y: Long-lived fission product

Americium (Am) is an artificial element, and thus a standard atomic mass 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 into alpha particles. Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom).

Nineteen radioisotopes of americium 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 ^242mAm (t_1/2 141 years). The isotopes of americium range in atomic weight from 231.046 u (²³¹Am) to 249.078 u (²⁴⁹Am).

Some notable isotopes

Americium-241

Americium-241 used in a smoke detector

Americium-241 is the most prevalent isotope of americium in nuclear waste.^[6] It is the americium 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.^[7]
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[8]	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, where a higher-energy nuclear isomer is more stable than the lower-energy one, Americium-242.^[9]

^242mAm is fissile (because it has an odd number of neutrons) and has a low critical mass, comparable to that of ²³⁹Pu.^[10] It has a very high cross section for fission, and if in a nuclear reactor is destroyed relatively quickly. Another report claims that ^242mAm has a much lower critical mass, can sustain a chain reaction even as a thin film, and could be used for a novel type of nuclear rocket.^[11][12]

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.^[13] Production increases exponentially with increasing burnup as a total of 5 neutron captures on ²³⁸U are required.

It decays by either emitting an alpha particle (with a decay energy of 5.27 MeV)^[13] to become ²³⁹Np, which then quickly decays to ²³⁹Pu, or infrequently, by spontaneous fission.^[14]

²⁴³Am is a hazardous substance, because it can cause cancer. ²³⁹Np, which is formed from ²⁴³Am, emits dangerous gamma rays, making ²⁴³Am the most dangerous isotope of Americium.^[6]

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)[15][n 1]	daughter isotope(s)	nuclear spin
	excitation energy
^229Am[16]	95	134		3.7 s	α	^225Np
^231Am	95	136	231.04556(32)#	30# s	β^+	^231Pu
					α (rare)	^227Np
^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 2]	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
^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 2]	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)#
^248Am	95	153	248.07575(22)#	3# min	β^−	^248Cm
^249Am	95	154	249.07848(32)#	1# min	β^−	^249Cm

1. Abbreviations:
   CD: Cluster decay
   EC: Electron capture
   IT: Isomeric transition
   SF: Spontaneous fission
2. Most common isotopes

Notes

• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses – ( ).
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.

See also

References

1. 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 isotopes have half-lives of at least four years (the longest-lived isotope 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.
2. Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
3. 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. 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 y. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
4. This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
5. Excluding those "classically stable" isotopes 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.
6. "Americium". Argonne National Laboratory, EVS. Retrieved 25 December 2009.
7. 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.
8. J. T. Caldwell; S. C. Fultz; C. D. Bowman; R. W. Hoff (March 1967). "Spontaneous Fission Half-Life of Am242m^242m". Physical Review 155. doi:10.1103/PhysRev.155.1309. (halflife (9.5±3.5)×10¹¹
9. 95-Am-242
10. (PDF) http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/6.5_022.pdf. Retrieved February 3, 2011. Missing or empty |title= (help)
11. "Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks" (Press release). Ben-Gurion University Of The Negev. December 28, 2000.
12. Ronen, Yigal; Shwageraus, E. (2000). "Ultra-thin 241mAm fuel elements in nuclear reactors". Nuclear Instruments and Methods in Physics Research A 455: 442–451.
13. "Americium-243". Oak Ridge National Laboratory. Retrieved 25 December 2009.
14. "Isotopes of the Element Americium". Jefferson Lab Science Education. Retrieved 25 December 2009.
15. "Universal Nuclide Chart". nucleonica. (registration required (help)).
16. "Observation of new neutron-deficient isotopes with Z ≥ 92 in multinucleon transfer reactions" http://inspirehep.net/record/1383747/files/scoap3-fulltext.pdf

Sources

• Isotope masses from:
  • Audi, G.; Wapstra, A. H.; Thibault, C.; Blachot, J.; Bersillon, O. (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
• Isotopic compositions and standard atomic masses from:
  • de Laeter, J. R.; Böhlke, J. K.; De Bièvre, P.; Hidaka, H.; Peiser, H. S.; Rosman, K. J. R.; Taylor, P. 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, M. E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry 78 (11): 2051–2066. doi:10.1351/pac200678112051. Lay summary.
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, G.; Wapstra, A. H.; Thibault, C.; Blachot, J.; Bersillon, O. (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
  • "NuDat 2.1 database". NNDC.BNL.gov. Brookhaven National Laboratory. Retrieved September 2005.
  • Holden, N. E. (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9.