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Isotopes of americium

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Isotopesofamericium(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 –

Americium(95Am) is anartificial element,and thus astandard atomic weightcannot be given. Like all artificial elements, it has no knownstable isotopes.The firstisotopeto be synthesized was241Am in 1944. The artificial element decays by ejectingAlpha particles.Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom). Despite243
Ambeing an order of magnitude longer lived than241
Am,the former is harder to obtain than the latter as more of it is present inspent nuclear fuel.

Eighteenradioisotopesof americium, ranging from229Am to247Am with the exception of231Am, have been characterized; another isotope,223Am, has also been reported but is unconfirmed. The most stable isotopes are243Am with ahalf-lifeof 7,370 years and241Am with a half-life of 432.2 years. All of the remainingradioactiveisotopes 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 8meta states,with the most stable being242m1Am (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

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Nuclide
[n 1] 		Z N Isotopic mass(Da)
[n 2][n 3] 		Half-life[1]
 Decay
mode[1]
[n 4] 		Daughter
isotope
 Spinand
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 – Excitednuclear 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. ^ab# – 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

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Actinides and fission products by half-life
Actinides[3]bydecay chain Half-life
range (a) 		Fission productsof235Ubyyield[4]
4n 4n+ 1 4n+ 2 4n+ 3 4.5–7% 0.04–1.25% <0.001%
228Ra№ 4–6 a 155Euþ
248Bk[5] > 9 a
244Cmƒ 241Puƒ 250Cf 227Ac№ 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
249Cfƒ 242mAmƒ 141–351 a

No fission products have ahalf-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₡
1.33 Ma 135Cs₡
237Npƒ 1.61–6.5 Ma 93Zr 107Pd
236U 247Cmƒ 15–24 Ma 129I₡
244Pu 80 Ma

... nor beyond 15.7 Ma[7]

232Th№ 238U№ 235Uƒ№ 0.7–14.1 Ga

Notable isotopes

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Americium-241

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Americium-241 is used in ionizationsmoke detectors.
Main article:Americium-241

Americium-241is the most common isotope of americium in nuclear waste.[8]It is the isotope used in anamericium smoke detectorbased on anionization chamber.It is a potential fuel for long-lifetimeradioisotope thermoelectric generators.

Parameter Value
Atomic mass 241.056829u
Mass excess 52930keV
Beta decayenergy −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 from241Pu,electron capture from241Cm, Alpha from245Bk.

241AmAlpha decays,with a by-product ofgamma rays.Its presence inplutoniumis determined by the original concentration of241Pu and the sample age. Due to the low penetration of Alpha radiation,241Am only poses a health risk when ingested or inhaled. Older samples of plutonium containing plutonium-241 contain a buildup of241Am. A chemical removal of americium from reworked plutonium (e.g. during reworking ofplutonium pits) may be required.

Americium-242m

[edit]
Transmutation flow between238Pu and244Cm in LWR.[9]
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 0.05MeV 242Am
0.46% Alpha decay 5.64 MeV 238Np
(1.5±0.6) × 10−10 [10] spontaneous fission ~200 MeV fission products

Americium-242mhas a mass of 242.0595492 g/mol. It is one of the rare cases, like108mAg,166mHo,180mTa,186mRe,192mIr,210mBi,212mPoand others, where a higher-energynuclear isomeris more stable than the ground state,americium-242.[11]

242mAm isfissilewith a lowcritical mass,comparable to that of239Pu.[12]It has a very high fissioncross 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 ofnuclear rocket.[13][14]

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

Americium-243

[edit]
A sample of Am-243

Americium-243has a mass of 243.06138 g/mol and ahalf-lifeof 7,370 years, the longest lasting of all americium isotopes. It is formed in thenuclear fuel cyclebyneutron captureonplutonium-242followed bybeta decay.[15]Production increases exponentially with increasingburnupas a total of 5 neutron captures on238Uare required. IfMOX-fuelis used, particularly MOX-fuel high in241
Puand242
Pu,more americium overall and more243
Amwill be produced.

It decays by either emitting anAlpha particle(with a decay energy of 5.27 MeV)[15]to become239Np, which then quickly decays to239Pu,or rarely, byspontaneous fission.[16]

As for the other americium isotopes, and more generally for all Alpha emitters,243Am iscarcinogenicin 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 product239Np.The external irradiation risk for the other two americium isotopes (241Am and242mAm) is less than 10% of that for americium-243.[8]

References

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  1. ^abcdKondev, 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 isotope223Np 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 afterpolonium(84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap isradon-222with a half life of less than fourdays). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
  4. ^Specifically fromthermal neutronfission of uranium-235, e.g. in a typicalnuclear 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 Bk248with a half-life greater than 9 [years]. No growth of Cf248was 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 of232Th; e.g., while113mCd has a half-life of only fourteen years, that of113Cd is eightquadrillionyears.
  8. ^ab"Americium"Archived2012-07-30 at theWayback 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)×1011years)
  11. ^95-Am-242Archived2011-07-19 at theWayback Machine
  12. ^"Critical Mass Calculations for241Am,242mAm and243Am "(PDF).Archived fromthe original(PDF)on July 22, 2011.RetrievedFebruary 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. ^ab"Americium-243"Archived2011-02-25 at theWayback Machine.Oak Ridge National Laboratory. Retrieved 25 December 2009.
  16. ^"Isotopes of the Element Americium".Jefferson Lab Science Education. Retrieved 25 December 2009.

Sources

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