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Title: Americium-243  
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Subject: Americium, Actinide
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Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (a)
Fission products by yield[2]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 155Euþ
244Cm 241Puƒ 250Cf 227Ac 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Pu 243Cmƒ 29–97 137Cs 151Smþ 121mSn
249Cfƒ 242mAmƒ 141–351

No fission products
have a half-life
in the range of
100–210k years…

241Am 251Cfƒ[3] 430–900
226Ra 247Bk 1.3k–1.6k
240Pu 229Th 246Cm 243Am 4.7k–7.4k
245Cmƒ 250Cm 8.3k–8.5k
239Puƒ 24.1k
230Th 231Pa 32k–76k
236Npƒ 233Uƒ 234U 150k–250k 99Tc 126Sn
248Cm 242Pu 327k–375k 79Se
1.53M 93Zr
237Np 2.1M–6.5M 135Cs 107Pd
236U 247Cmƒ 15M–24M 129I
244Pu 80M

...nor beyond 15.7M[4]

232Th 238U 235Uƒ№ 0.7G–14G

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4a–97a: Medium-lived fission product
‡  over 200ka: 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 stable isotopes. The first isotope to be synthesized was 241Am in 1944.

Trace quantities are found in nature from neutron capture by uranium atoms.[5]

19 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 (t½ 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 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.


242mAm decay modes (halflife: 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]

242Am decay modes (halflife: 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 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.[12] 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 5.27MeV)[12] to become 239Np, which then quickly decays to 239Pu, or infrequently, by spontaneous fission.[13]

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]


Z(p) N(n)  
isotopic mass (u)
half-life decay
mode(s)[14][n 1]
excitation energy
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) a 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) a 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) a 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


  • 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

Template:World Heritage Encyclopedia-Books


  • Isotope masses from:
  • Isotopic compositions and standard atomic masses from:
  • Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.

Isotopes of plutonium Isotopes of americium Isotopes of curium
Table of nuclides
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