Nothing Special   »   [go: up one dir, main page]

Remove ads

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.

Quick Facts Main isotopes, Decay ...
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
Close

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 7,370 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.

Remove ads

List of isotopes


More information Nuclide, Z ...
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:
Close
  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. () spin value  Indicates spin with weak assignment arguments.
  5. #  Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  6. The discovery of this isotope is uncertain due to disagreements between theoretical predictions and reported experimental data.[2]
Remove ads

Actinides vs fission products

More information Half-life range (a), 4n ...
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
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 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
1.33 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
Close
Remove ads

Notable isotopes

Americium-241

Thumb
Americium-241 is used in ionization smoke detectors.

Americium-241 is the most common 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.

More information Parameter, Value ...
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
Close

Possible parent nuclides: beta from 241Pu, electron capture from 241Cm, alpha from 245Bk.

241Am alpha decays, with a by-product of gamma rays. Its presence in plutonium is determined by the original concentration of 241Pu 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 of 241Am. A chemical removal of americium from reworked plutonium (e.g. during reworking of plutonium pits) may be required.

Americium-242m

Thumb
Transmutation flow between 238Pu and 244Cm 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.
More information Probability, Decay mode ...
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
Close

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 with a low critical mass, comparable to that of 239Pu.[12] 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.[13][14]

More information Probability, Decay mode ...
242Am decay modes (half-life: 16 hours)
ProbabilityDecay modeDecay energyDecay product
82.70%beta decay0.665 MeV242Cm
17.30%electron capture0.751 MeV242Pu
Close

Americium-243

Thumb
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.[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]

Remove ads

References

Sources

Wikiwand in your browser!

Seamless Wikipedia browsing. On steroids.

Every time you click a link to Wikipedia, Wiktionary or Wikiquote in your browser's search results, it will show the modern Wikiwand interface.

Wikiwand extension is a five stars, simple, with minimum permission required to keep your browsing private, safe and transparent.

Remove ads