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 243Am being an order of magnitude longer lived than 241Am, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.

Nineteen radioisotopes of americium, ranging from 229Am to 247Am 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

|-id=Americium-223 | 223Am[n 1] | style="text-align:right" | 95 | style="text-align:right" | 128 | 223.04584(32)# | style="text-align:center" | 2015 | 10 ms | α | 219Np | 9/2–# |-id=Americium-229 | 229Am | style="text-align:right" | 95 | style="text-align:right" | 134 | 229.04528(11) | style="text-align:center" | 2015 | 1.8 s | α | 225Np | 5/2–# |-id=Americium-230 | rowspan=2 | 230Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 135 | rowspan=2 | 230.04603(15)# | rowspan=2 style="text-align:center" | 2016 | rowspan=2 | 40 s | β+ (<70%) | 230Pu | rowspan=2 | 1–# |- | β+, SF (>30%) | (various) |-id=Americium-231 | rowspan=2 | 231Am[1] | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 136 | rowspan=2 | 231.04553(32)# | rowspan=2 style="text-align:center" | 2026 | rowspan=2 | 75 ± 137 s | β+ (~83%) | 231Pu | rowspan=2 | 5/2–# |- | α (~17%) | 227Np |-id=Americium-232 | rowspan=3 | 232Am | rowspan=3 style="text-align:right" | 95 | rowspan=3 style="text-align:right" | 137 | rowspan=3 | 232.04661(32)# | rowspan=3 style="text-align:center" | 1967 | rowspan=3 | 1.31 min | β+ (97%) | 232Pu | rowspan=3 | 1–# |- | α? (3%) | 228Np |- | β+, SF (0.069%) | (various) |-id=Americium-233 | rowspan=2 | 233Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 138 | rowspan=2 | 233.04647(12)# | rowspan=2 style="text-align:center" | 2000 | rowspan=2 | 3.2 min | β+? (95.5%) | 233Pu | rowspan=2 | 5/2–# |- | α (4.5%) | 229Np |-id=Americium-234 | rowspan=3 | 234Am | rowspan=3 style="text-align:right" | 95 | rowspan=3 style="text-align:right" | 139 | rowspan=3 | 234.04773(17)# | rowspan=3 style="text-align:center" | 1967 | rowspan=3 | 2.32 min | β+ (99.95%) | 234Pu | rowspan=3 | 0–# |- | α (0.039%) | 230Np |- | β+, SF (0.0066%) | (various) |-id=Americium-235 | rowspan=2 | 235Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 140 | rowspan=2 | 235.04791(6) | rowspan=2 style="text-align:center" | 1996 | rowspan=2 | 10.3 min | β+ (99.60%) | 235Pu | rowspan=2 | 5/2−# |- | α (0.40%) | 231Np |-id=Americium-236 | rowspan=2 | 236Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 141 | rowspan=2 | 236.04943(13)# | rowspan=2 style="text-align:center" | 1998 | rowspan=2 | 3.6 min | β+ | 236Pu | rowspan=2 | 5− |- | α (4×10−3%) | 232Np |-id=Americium-236m | style="text-indent:1em" | 236mAm | colspan=3 style="text-indent:2em" | 50(50)# keV | style="text-align:center" | 2004 | 2.9 min | β+ | 236Pu | (1−) |-id=Americium-237 | rowspan=2 | 237Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 142 | rowspan=2 | 237.05000(6)# | rowspan=2 style="text-align:center" | 1970 | rowspan=2 | 73.6 min | β+ (99.975%) | 237Pu | rowspan=2 | 5/2− |- | α (0.025%) | 233Np |-id=Americium-238 | rowspan=2 | 238Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 143 | rowspan=2 | 238.05198(6) | rowspan=2 style="text-align:center" | 1950 | rowspan=2 | 98 min | β+ | 238Pu | rowspan=2 | 1+ |- | α (1.0×10−4%) | 234Np |-id=Americium-238m | style="text-indent:1em" | 238mAm | colspan="3" style="text-indent:2em" | 2500(200)# keV | style="text-align:center" | 1967 | 35 us | SF | (various) | |-id=Americium-239 | rowspan=2 | 239Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 144 | rowspan=2 | 239.0530227(21) | rowspan=2 style="text-align:center" | 1949 | rowspan=2 | 11.9 h | EC (99.990%) | 239Pu | rowspan=2 | 5/2− |- | α (0.010%) | 235Np |-id=Americium-239m | style="text-indent:1em" | 239mAm | colspan="3" style="text-indent:2em" | 2500(200) keV | style="text-align:center" | 1969 | 163 ns | SF | (various) | (7/2+) |-id=Americium-240 | rowspan=2 | 240Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 145 | rowspan=2 | 240.055298(15) | rowspan=2 style="text-align:center" | 1949 | rowspan=2 | 50.8 h | β+ | 240Pu | rowspan=2 | (3−) |- | α (1.9×10−4%) | 236Np |-id=Americium-240m | style="text-indent:1em" | 240mAm | colspan="3" style="text-indent:2em" | 3000(200) keV | style="text-align:center" | 1967 | 940 us | SF | (various) | |-id=Americium-241 | rowspan=2 | 241Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 146 | rowspan=2 | 241.0568273(12) | rowspan=2 style="text-align:center" | 1949 | rowspan=2 | 432.6 y | α | 237Np | rowspan=2 | 5/2− |- | SF (3.6×10−10%) | (various) |-id=Americium-241m | style="text-indent:1em" | 241mAm | colspan="3" style="text-indent:2em" | 2200(200) keV | style="text-align:center" | 1969 | 1.2 us | SF | (various) | |-id=Americium-242 | rowspan=2 | 242Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 147 | rowspan=2 | 242.0595474(12) | rowspan=2 style="text-align:center" | 1949 | rowspan=2 | 16.02 h | β− (82.7%) | 242Cm | rowspan=2 | 1− |- | EC (17.3%) | 242Pu |-id=Americium-242m1 | rowspan=3 style="text-indent:1em" | 242m1Am | rowspan=3 colspan="3" style="text-indent:2em" | 48.60(5) keV | rowspan=3 style="text-align:center" | 1959 | rowspan=3 | 141 y | IT (99.55%) | 242Am | rowspan=3 | 5− |- | α (0.45%) | 238Np |- | SF (<4.7×10−9%) | (various) |-id=Americium-242m2 | rowspan=2 style="text-indent:1em" | 242m2Am | rowspan=2 colspan="3" style="text-indent:2em" | 2200(80) keV | rowspan=2 style="text-align:center" | 1962 | rowspan=2 | 14.0 ms | SF | (various) | rowspan=2 | (2+, 3−) |- | IT | 242Am |-id=Americium-243 | rowspan=2 | 243Am | rowspan=2 style="text-align:right" | 95 | rowspan=2 style="text-align:right" | 148 | rowspan=2 | 243.0613799(15) | rowspan=2 style="text-align:center" | 1950 | rowspan=2 | 7,350 y | α | 239Np | rowspan=2 | 5/2− |- | SF (3.7×10−9%) | (various) |-id=Americium-243m | style="text-indent:1em" | 243mAm | colspan="3" style="text-indent:2em" | 2300(200) keV | style="text-align:center" | 1970 | 5.5 us | SF | (various) | |-id=Americium-244 | 244Am | style="text-align:right" | 95 | style="text-align:right" | 149 | 244.0642829(16) | style="text-align:center" | 1950 | 10.01 h | β− | 244Cm | (6−) |-id=Americium-244m1 | rowspan=2 style="text-indent:1em" | 244m1Am | rowspan=2 colspan="3" style="text-indent:2em" | 89.3(16) keV | rowspan=2 style="text-align:center" | 1962 | rowspan=2 | 26.13 min | β− (99.96%) | 244Cm | rowspan=2 | 1+ |- | EC (0.0364%) | 244Pu |-id=Americium-244m2 | style="text-indent:1em" | 244m2Am | colspan="3" style="text-indent:2em" | 2000(200)# keV | style="text-align:center" | 1967 | 0.90 ms | SF | (various) | |-xxid=Americium-244m3 | style="text-indent:1em" | 244m3Am | colspan="3" style="text-indent:2em" | 2200(200)# xxkeV | style="text-align:center" | (1967)[n 2] | ~6.5 us | SF | (various) | |-id=Americium-245 | 245Am | style="text-align:right" | 95 | style="text-align:right" | 150 | 245.0664528(20) | style="text-align:center" | 1955 | 2.05 h | β− | 245Cm | 5/2+ |-id=Americium-245m | style="text-indent:1em" | 245mAm | colspan="3" style="text-indent:2em" | 2400(400)# keV | style="text-align:center" | 1972 | 0.64 us | SF | (various) | |-id=Americium-246 | 246Am | style="text-align:right" | 95 | style="text-align:right" | 151 | 246.069774(19)# | style="text-align:center" | 1955 | 39 min | β− | 246Cm | 7− |-id=Americium-246m1 | style="text-indent:1em" | 246m1Am | colspan="3" style="text-indent:2em" | 30(10)# keV | style="text-align:center" | 1967 | 25.0 min | β− | 246Cm | 2(−) |-id=Americium-246m2 | style="text-indent:1em" | 246m2Am | colspan="3" style="text-indent:2em" | 2000(800)# keV | style="text-align:center" | 1972 | 73 us | SF | (various) | |-id=Americium-247 | 247Am | style="text-align:right" | 95 | style="text-align:right" | 152 | 247.07209(11)# | style="text-align:center" | 1967 | 23.0 min | β− | 247Cm | 5/2#

Americium-241

Main article: Americium-241

Americium-241 (alpha emitter, half-life 432.6 years) is the most common isotope of americium in nuclear waste.[2] 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 than 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

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.[3] 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.[4][5]

Americium-243

Americium-243, an alpha emitter, has a half-life of 7350 years, 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.[6] 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 241Pu and 242Pu, more americium overall and more 243Am will be produced.

It decays by either emitting an alpha particle (decay energy 5.439 MeV)[7] 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.[8]

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.[2]

References

  1. ^ Wang, J. G.; Li, Z. C.; Zhang, Z. Y. (2026). "Decay properties of new isotopes 235Bk and 231Am". Physics Letters B. 875 (140365). doi:10.1016/j.physletb.2026.140365
  2. ^ "Americium" Archived 2012-07-30 at the Wayback Machine. Argonne National Laboratory, EVS. Retrieved 25 December 2009.
  3. ^ "Critical Mass Calculations for 241Am, 242mAm and 243Am". Archived from the original on July 22, 2011. Retrieved February 3, 2011.
  4. ^ "Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks". Ben-Gurion University Of The Negev. December 28, 2000.
  5. ^ 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
  6. ^ "Americium-243" Archived 2011-02-25 at the Wayback Machine. Oak Ridge National Laboratory. Retrieved 25 December 2009.
  7. ^
  8. ^ Calculated from Nubase data.

Sources

  • Isotope masses from:
  • Half-life, spin, and isomer data selected from the following sources.
  1. ^ The discovery of this isotope is uncertain due to disagreements between theoretical predictions and reported experimental data.[9]
  2. ^ The discovery of this isotope was only reported in a conference proceeding and not included in the discovery database.