Isotopes of americium

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 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 (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, 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]