Americium (₉₅Am) is anartificial element, and thus astandard atomic weight cannot be given. Like all artificial elements, it has no knownstable isotopes. The firstisotope to 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
Am being 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.

Eighteenradioisotopes of 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-life of 7,350 years and²⁴¹Am with a half-life of 432.6 years. All of the remainingradioactive isotopes have half-lives that are less than seven days, the majority of which are shorter than two hours. This element also has fourteenmeta 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.

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 normalionization smoke detectors, which work as anionization chamber. It is a potential fuel for long-lifetimeradioisotope thermoelectric generators, with a half-life longer than that of the standardplutonium-238 (87.7 years) or the alternativestrontium-90 (28.91 years). Itsdecay heat is 0.114 W/g; its rate ofspontaneous fission 1.2/g/s.

The alpha decay of²⁴¹Am is accompanied by a significant emission ofgamma rays. Its presence in plutonium is determined by the original concentration of²⁴¹Pu (which decays to it) and the sample age. Older samples of plutonium containing plutonium-241 build up²⁴¹Am, and chemical separation of americium from such plutonium (e.g. during reworking ofplutonium pits) may be required.

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-energynuclear isomer is more stable than its ground state. While that ground state,²⁴²Am, decays with half-life 16.02 hours bybeta emission orelectron 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 analpha particle (0.45%, partial half-life 31 ky).

^242mAm isfissile with a lowcritical mass, comparable to that of²³⁹Pu.^[11] 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.^[12][13]

Americium-243, an alpha emitter, has ahalf-life of 7350 years^[1], the longest of all americium isotopes. It is formed in thenuclear fuel cycle mainly byneutron capture onplutonium-242 followed bybeta decay.^[14] Production increases exponentially with increasingburnup as a total of 5 neutron captures on²³⁸U are required. IfMOX-fuel is used, particularly MOX-fuel high in²⁴¹
Pu and²⁴²
Pu, more americium overall and more²⁴³
Am will be produced.

It decays by either emitting analpha particle (decay energy 5.439 MeV)^[15] to become²³⁹Np, which then quickly goes to²³⁹Pu, 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 iscarcinogenic 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 product²³⁹Np. The external irradiation risk for the other two americium isotopes (²⁴¹Am and^242mAm) is less than 10% of that for americium-243.^[9]

• Isotope masses from:
  • National Nuclear Data Center."NuDat 3.0 database".Brookhaven National Laboratory.
• Half-life, spin, and isomer data selected from the following sources.
  • National Nuclear Data Center."NuDat 3.0 database".Brookhaven National Laboratory.
  • IAEA - Nuclear Data Section.Live Chart of Nuclides. Vienna International Centre.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.).CRC Handbook of Chemistry and Physics (85th ed.).Boca Raton, Florida:CRC Press.ISBN 978-0-8493-0485-9.

• Isotopes of americium
• Americium
• Lists of isotopes by element

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