Isotopes of americium

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Isotopes of americium  (95Am)
Main isotopes [1] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
241Am synth 432.6 y α 237Np
SF
242Amsynth16.02 h β 242Cm
ε 242Pu
242m1Amsynth141 y IT 242Am
α 238Np
243Amsynth7350 yα 239Np
SF

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.

Contents

Eighteen radioisotopes of americium, ranging from 229Am 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 7,350 years and 241Am with a half-life of 432.6 years. All of the remaining radioactive isotopes have half-lives that are less than seven days, the majority of which are shorter than two hours. This element also has fourteen meta states, with the most stable being 242m1Am (half-life 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) [2]
[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] 95128223.04584(32)#10(9) ms α 219Np9/2–#
229Am95134229.04528(11)1.8(15) sα225Np5/2–#
230Am95135230.04603(15)#40(9) s β+ (<70%)230Pu1–#
β+, SF (>30%)(various)
232Am95137232.04661(32)#1.31(4) minβ+ (97%)232Pu1–#
α? (3%)228Np
β+, SF (0.069%)(various)
233Am95138233.04647(12)#3.2(8) minβ+? (95.5%)233Pu5/2–#
α (4.5%)229Np
234Am95139234.04773(17)#2.32(8) minβ+ (99.95%)234Pu0–#
α (0.039%)230Np
β+, SF (0.0066%)(various)
235Am95140235.04791(6)10.3(6) minβ+ (99.60%)235Pu5/2−#
α (0.40%)231Np
236Am95141236.04943(13)#3.6(1) minβ+236Pu5−
α (4×10−3%)232Np
236mAm50(50)# keV2.9(2) minβ+236Pu(1−)
237Am95142237.05000(6)#73.6(8) minβ+ (99.975%)237Pu5/2−
α (0.025%)233Np
238Am95143238.05198(6)98(3) minβ+238Pu1+
α (1.0×10−4%)234Np
238mAm2500(200)# keV35(18) μsSF(various)
239Am95144239.0530227(21)11.9(1) h EC (99.990%)239Pu5/2−
α (0.010%)235Np
239mAm2500(200) keV163(12) nsSF(various)(7/2+)
240Am95145240.055298(15)50.8(3) hβ+240Pu(3−)
α (1.9×10−4%)236Np
240mAm3000(200) keV940(40) μsSF(various)
241Am 95146241.0568273(12)432.6(6) yα237Np5/2−
SF (3.6×10−10%)(various)
241mAm2200(200) keV1.2(3) μsSF(various)
242Am95147242.0595474(12)16.02(2) hβ (82.7%)242Cm1−
EC (17.3%)242Pu
242m1Am48.60(5) keV141(2) y IT (99.55%)242Am5−
α (0.45%)238Np
SF (<4.7×10−9%)(various)
242m2Am2200(80) keV14.0(10) msSF(various)(2+, 3−)
IT242Am
243Am95148243.0613799(15)7350(9) yα239Np5/2−
SF (3.7×10−9%)(various)
243mAm2300(200) keV5.5(5) μsSF(various)
244Am95149244.0642829(16)10.01(3) hβ244Cm(6−)
244m1Am89.3(16) keV26.13(43) minβ (99.96%)244Cm1+
EC (0.0364%)244Pu
244m2Am2000(200)# keV0.90(15) msSF(various)
244m3Am2200(200)# keV~6.5 μsSF(various)
245Am95150245.0664528(20)2.05(1) hβ245Cm5/2+
245mAm2400(400)# keV0.64(6) μsSF(various)
246Am95151246.069774(19)#39(3) minβ246Cm7−
246m1Am30(10)# keV25.0(2) minβ246Cm2(−)
246m2Am2000(800)# keV73(10) μsSF(various)
247Am95152247.07209(11)#23.0(13) minβ247Cm5/2#
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:
    EC: Electron capture
    CD: Cluster decay
    IT: Isomeric transition
    SF: Spontaneous fission
  5. () spin value  Indicates spin with weak assignment arguments.
  6. 1 2 #  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. [3]

Actinides vs fission products

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%
228 Ra 4–6 a 155 Euþ
248 Bk [6] > 9 a
244 Cmƒ 241 Puƒ 250 Cf 227 Ac 10–29 a 90 Sr 85 Kr 113m Cdþ
232 Uƒ 238 Puƒ 243 Cmƒ 29–97 a 137 Cs 151 Smþ 121m Sn
249 Cfƒ 242m Amƒ141–351 a

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

241 Amƒ 251 Cfƒ [7] 430–900 a
226 Ra 247 Bk1.3–1.6 ka
240 Pu 229 Th 246 Cmƒ 243 Amƒ4.7–7.4 ka
245 Cmƒ 250 Cm8.3–8.5 ka
239 Puƒ24.1 ka
230 Th 231 Pa32–76 ka
236 Npƒ 233 Uƒ 234 U 150–250 ka 99 Tc 126 Sn
248 Cm 242 Pu 327–375 ka 79 Se
1.33 Ma 135 Cs
237 Npƒ 1.61–6.5 Ma 93 Zr 107 Pd
236 U 247 Cmƒ 15–24 Ma 129 I
244 Pu80 Ma

... nor beyond 15.7 Ma [8]

232 Th 238 U 235 Uƒ№0.7–14.1 Ga

Americium-241

Americium-241, as used in ionization smoke detectors. Americium button hd.jpg
Americium-241, as used in ionization smoke detectors.

Americium-241 (alpha emitter, half-life 432.6 years) is the most common isotope of americium in nuclear waste. [9] It is the isotope used in normal ionization smoke detectors, which work as an ionization chamber. It is a potential fuel for long-lifetime radioisotope thermoelectric generators, with a half-life longer then that of the standard plutonium-238 (87.7 years) or the alternative strontium-90 (28.91 years). Its decay heat is 0.114 W/g; its rate of spontaneous fission 1.2/g/s.

The alpha decay of 241Am is accompanied by a significant emission of gamma rays. Its presence in plutonium is determined by the original concentration of 241Pu (which decays to it) and the sample age. Older samples of plutonium containing plutonium-241 build up 241Am, and chemical separation of americium from such plutonium (e.g. during reworking of plutonium pits) may be required.

Americium-242m

Transmutation flow between Pu and Cm in LWR.
Fission percentage is 100 minus shown percentages.
Total rate of transmutation varies greatly by nuclide.
Cm- Cm are long-lived with negligible decay. Sasahara.svg
Transmutation flow between Pu and Cm in LWR.
Fission percentage is 100 minus shown percentages.
Total rate of transmutation varies greatly by nuclide.
Cm– Cm are long-lived with negligible decay.

Americium-242m (half-life 141 years) 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 its ground state. While that ground state, 242Am, decays with half-life 16.02 hours by beta emission or electron capture, in a typical example of spin-forbiddenness the isomer does not decay by those modes, but falls to the ground state very slowly (99.55% of decays) or emits an alpha particle (0.45%, partial half-life 31 ky).

242mAm is fissile with a low critical mass, comparable to that of 239Pu. [11] It has a very high fission cross section, and is quickly destroyed if it is produced in a nuclear reactor. It has been investigated whether this isotope could be used for a novel type of nuclear rocket. [12] [13]

Americium-243

A sample of Am-243 Am243.png
A sample of Am-243

Americium-243, an alpha emitter, has a half-life of 7350 years [1] , the longest of all americium isotopes. It is formed in the nuclear fuel cycle mainly by neutron capture on plutonium-242 followed by beta decay. [14] 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 (decay energy 5.439 MeV) [15] to become 239Np, which then quickly goes to 239Pu, or, very rarely, spontaneous fission. The fission rate is about 60% that of americium-241 or about 0.7/g/s. [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. [9]

References

  1. 1 2 3 4 5 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. Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3) 030003. doi:10.1088/1674-1137/abddaf.
  3. 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 .
  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 eight quadrillion years.
  9. 1 2 "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. "Critical Mass Calculations for 241Am, 242mAm and 243Am" (PDF). Archived from the original (PDF) on July 22, 2011. Retrieved February 3, 2011.
  12. "Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks" (Press release). Ben-Gurion University Of The Negev. December 28, 2000.
  13. Ronen, Yigal; Shwageraus, E. (2000). "Ultra-thin 242mAm 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.
  14. "Americium-243" Archived 2011-02-25 at the Wayback Machine . Oak Ridge National Laboratory. Retrieved 25 December 2009.
  15. National Nuclear Data Center. "NuDat 3.0 database". Brookhaven National Laboratory.
  16. Calculated from Nubase data.

Sources