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).
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 242mAm (t1/2 = 141 years).
List of isotopes
|Z||N||Isotopic mass (Da)
[n 2][n 3]
[n 5][n 6]
|Excitation energy[n 6]|
|230Am||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|
|β+, SF (.069%)||(various)|
|234Am||95||139||234.04781(22)#||2.32(8) min||β+ (99.95%)||234Pu|
|β+, SF (.0066%)||(various)|
|237Am||95||142||237.05000(6)#||73.0(10) min||β+ (99.97%)||237Pu||5/2(−)|
|238mAm||2500(200)# keV||35(10) μs|
|239Am||95||144||239.0530245(26)||11.9(1) h||EC (99.99%)||239Pu||(5/2)−|
|239mAm||2500(200) keV||163(12) ns||(7/2+)|
|241Am[n 7]||95||146||241.0568291(20)||432.2(7) y||α||237Np||5/2−|
|CD (7.4×10−10%)||207Tl, 34Si|
|241mAm||2200(100) keV||1.2(3) μs||SF|
|242Am||95||147||242.0595492(20)||16.02(2) h||β− (82.7%)||242Cm||1−|
|242m1Am||48.60(5) keV||141(2) y||IT (99.54%)||242Am||5−|
|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−|
|244mAm||86.1(10) keV||26(1) min||β− (99.96%)||244Cm||1+|
|246m1Am||30(10) keV||25.0(2) min||β− (99.99%)||246Cm||2(−)|
|246m2Am||~2000 keV||73(10) μs|
|This table header & footer:|
- 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.
- # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
- Most common isotopes
Actinides vs fission products
Actinides and fission products by half-life
|Actinides by decay chain||Half-life
|Fission products of 235U by yield|
No fission products
... nor beyond 15.7 Ma
Legend for superscript symbols
Americium-241 is the most prevalent isotope of americium in nuclear waste. 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.
|Atomic mass||241.056829 u|
|Mass excess||52930 keV|
|Beta decay energy||-767 keV|
|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.
|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 ||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.
242mAm is fissile (because it has an odd number of neutrons) and has a low critical mass, comparable to that of 239Pu. 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.
|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 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. Production increases exponentially with increasing burnup as a total of 5 neutron captures on 238U are required.
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.
- "Observation of new neutron-deficient isotopes with Z ≥ 92 in multinucleon transfer reactions" http://inspirehep.net/record/1383747/files/scoap3-fulltext.pdf
- 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.
- 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 U-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.
- "Americium" Archived 2012-07-29 at WebCite. 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 Am242m". 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.
- "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.
- Isotope masses from:
- 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. Lay summary.
- 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.