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[1] Decay
abun­dance half-life (t1/2) mode pro­duct
241Am synth 432.2 y α 237Np
SF –
242m1Am synth 141 y IT 242Am
α 238Np
SF –
243Am synth 7350 y α 239Np
SF –

Eighteen radioisotopes of americium, ranging from 230Am to 247Am with the exception of 231Am, have been characterized; another isotope, 223Am, has also been reported but is unconfirmed. The most stable isotopes are 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
Nuclide
[n 1]		 Z N Isotopic mass (Da)
[n 2][n 3]		 Half-life[1]
 Decay
mode[1]
[n 4]		 Daughter
isotope
 Spin and
parity[1]
[n 5][n 6]
Excitation energy[n 6]
223Am[n 7] 95 128 223.04584(32)# 10(9) ms α 219Np 9/2–#
229Am 95 134 229.04528(11) 1.8(15) s α 225Np 5/2–#
230Am 95 135 230.04603(15)# 40(9) s β+ (<70%) 230Pu 1–#
β+SF (>30%) (various)
232Am 95 137 232.04661(32)# 1.31(4) min β+ (97%) 232Pu 1–#
α (3%) 228Np
β+SF (0.069%) (various)
233Am 95 138 233.04647(12)# 3.2(8) min β+ (95.5%) 233Pu 5/2–#
α (4.5%) 229Np
234Am 95 139 234.04773(17)# 2.32(8) min β+ (99.95%) 234Pu 0–#
α (0.039%) 230Np
β+, SF (0.0066%) (various)
235Am 95 140 235.047906(57) 10.3(6) min β+ (99.60%) 235Pu 5/2−#
α (0.40%) 231Np
236Am 95 141 236.04943(13)# 3.6(1) min β+ 236Pu 5−
α (4×10−3%) 232Np
236mAm 50(50)# keV 2.9(2) min β+ 236Pu (1−)
α ? 232Np
237Am 95 142 237.049995(64)# 73.6(8) min β+ (99.975%) 237Pu 5/2−
α (.025%) 233Np
238Am 95 143 238.051983(63) 98(3) min β+ 238Pu 1+
α (1.0×10−4%) 234Np
238mAm 2500(200)# keV 35(18) μs SF (various)
IT ? 238Am
239Am 95 144 239.0530227(21) 11.9(1) h EC (99.99%) 239Pu 5/2−
α (0.01%) 235Np
239mAm 2500(200) keV 163(12) ns SF (various) (7/2+)
IT ? 239Am
240Am 95 145 240.055298(15) 50.8(3) h β+ 240Pu (3−)
α (1.9×10−4%) 236Np
240mAm 3000(200) keV 940(40) μs SF (various)
IT ? 240Am
241Am 95 146 241.0568273(12) 432.6(6) y α 237Np 5/2−
SF (3.6×10−10%) (various)
241mAm 2200(200) keV 1.2(3) μs SF (various)
242Am 95 147 242.0595474(12) 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 ? (various)
242m2Am 2200(80) keV 14.0(10) ms SF (various) (2+, 3−)
IT ? 242Am
243Am 95 148 243.0613799(15) 7,350(9) y α 239Np 5/2−
SF (3.7×10−9%) (various)
243mAm 2300(200) keV 5.5(5) μs SF (various)
IT ? 243Am
244Am 95 149 244.0642829(16) 10.01(3) h β− 244Cm (6−)
244m1Am 89.3(16) keV 26.13(43) min β− (99.96%) 244Cm 1+
EC (0.0364%) 244Pu
244m2Am 2000(200)# 900(150) μs SF (various)
IT ? 244Am
244m3Am 2200(200)# ~6.5 μs SF (various)
IT ? 244Am
245Am 95 150 245.0664528(20) 2.05(1) h β− 245Cm 5/2+
245mAm 2400(400)# 640(60) ns SF (various)
IT ? 245Am
246Am 95 151 246.069774(19)# 39(3) min β− 246Cm (7−)
246m1Am 30(10)# keV 25.0(2) min β− 246Cm 2(−)
IT ? 246Am
246m2Am 2000(800)# keV 73(10) μs SF (various)
IT ? 246Am
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. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  7. The discovery of this isotope is uncertain due to disagreements between theoretical predictions and reported experimental data.[2]

Actinides vs fission products
Actinides and fission products by half-life
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 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac№ 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[5] 249Cfƒ 242mAmƒ 141–351 a

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

241Amƒ 251Cfƒ[6] 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[7]

232Th№ 238U№ 235Uƒ№ 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
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.

ParameterValue
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
242mAm decay modes (half-life: 141 years)
ProbabilityDecay modeDecay energyDecay product
99.54%isomeric transition0.05 MeV242Am
  0.46%alpha decay5.64 MeV238Np
(1.5±0.6) × 10−10 [10]spontaneous fission~200 MeVfission products

Americium-242m has a mass of 242.0595492 g/mol. It is one of the rare cases, like 108mAg, 166mHo, 180mTa, 186mRe, 192mIr, 210mBi, 212mPo and others, where a higher-energy nuclear isomer is more stable than the ground state, 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 is quickly destroyed if it is produced in a nuclear reactor. 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)
ProbabilityDecay modeDecay energyDecay product
82.70%beta decay0.665 MeV242Cm
17.30%electron capture0.751 MeV242Pu

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.[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 rarely, 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. 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. Sun, M. D.; et al. (2017). "New short-lived isotope 223Np and the absence of the Z = 92 subshell closure near N = 126". Physics Letters B. 771: 303–308. Bibcode:2017PhLB..771..303S. doi:10.1016/j.physletb.2017.03.074.
  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 eight quadrillion years.
  8. "Americium" Archived 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 Am242m". 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 Archived 2011-07-19 at the Wayback Machine
  12. "Critical Mass Calculations for 241Am, 242mAm and 243Am" (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" Archived 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:
  • Half-life, spin, and isomer data selected from the following sources.
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