Isotopes of americium

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Isotopes of americium (95Am)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
241Am synth 432.2 y α 237Np
242m1Am synth 141 y IT 242Am
α 238Np
243Am synth 7350 y α 239Np

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
being an order of magnitude longer lived than 241
, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.

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 (t1/2 = 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]

[n 1]
Z N Isotopic mass (Da)
[n 2][n 3]

[n 4]

Spin and
[n 5][n 6]
Excitation energy[n 6]
223Am[2] 95 128 5.2(+12.0-4.4) ms α 219Np
229Am[2] 95 134 229.04525(9) 1.8(1.5) s α 225Np
230Am[3] 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:
  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. ^ a b # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  7. ^ a b Most common isotope

Actinides vs fission products[edit]

Actinides[4] by decay chain Half-life
range (a)
Fission products of 235U by yield[5]
4n 4n + 1 4n + 2 4n + 3 4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 a 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[6] 249Cfƒ 242mAmƒ 141–351 a

No fission products have a half-life in the range of 100 a–210 ka ...

241Amƒ 251Cfƒ[7] 430–900 a
226Ra 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th 231Pa 32–76 ka
236Npƒ 233Uƒ 234U 150–250 ka 99Tc 126Sn
248Cm 242Pu 327–375 ka 79Se
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs 107Pd
236U 247Cmƒ 15–24 Ma 129I
244Pu 80 Ma

... nor beyond 15.7 Ma[8]

232Th 238U 235Uƒ№ 0.7–14.1 Ga

Notable isotopes[edit]


Americium-241 is used in ionization smoke detectors.

Americium-241 is the most prevalent isotope of americium in nuclear waste.[9] 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.


Transmutation flow between 238Pu and 244Cm in LWR.[10]
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[11] 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.[12]

242mAm is fissile and has a low critical mass, comparable to that of 239Pu.[13] 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.[14][15]

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


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.[16] 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
and 242
, more Americium overall and more 243
will be produced.

It decays by either emitting an alpha particle (with a decay energy of 5.27 MeV)[16] to become 239Np, which then quickly decays to 239Pu, or infrequently, by spontaneous fission.[17]

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.[9]


  1. ^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  2. ^ a b "Observation of new neutron-deficient isotopes with Z ≥ 92 in multinucleon transfer reactions"
  3. ^ 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. Bibcode:2016JPSJ...85a5002K. doi:10.7566/JPSJ.85.015002.
  4. ^ 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.
  5. ^ Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
  6. ^ 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]."
  7. ^ This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
  8. ^ 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.
  9. ^ a b "Americium" Archived 2012-07-30 at the Wayback Machine. Argonne National Laboratory, EVS. Retrieved 25 December 2009.
  10. ^ 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.
  11. ^ 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)
  12. ^ 95-Am-242 Archived 2011-07-19 at the Wayback Machine
  13. ^ "Critical Mass Calculations for 241Am, 242mAm and 243Am" (PDF). Archived from the original (PDF) on July 22, 2011. Retrieved February 3, 2011.
  14. ^ "Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks" (Press release). Ben-Gurion University Of The Negev. December 28, 2000.
  15. ^ 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.
  16. ^ a b "Americium-243" Archived 2011-02-25 at the Wayback Machine. Oak Ridge National Laboratory. Retrieved 25 December 2009.
  17. ^ "Isotopes of the Element Americium". Jefferson Lab Science Education. Retrieved 25 December 2009.