Isotopes of americium

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. Despite being an order of magnitude longer lived than, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.
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 7,350 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. 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

Actinides vs fission products

Americium-241

Americium-241 is the most common isotope of americium in nuclear waste. 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 or the alternative strontium-90. Its decay heat is 0.114 W/g; its rate of spontaneous fission 1.2/g/s.
The alpha decay of Am is accompanied by a significant emission of gamma rays. Its presence in plutonium is determined by the original concentration of Pu and the sample age. Older samples of plutonium containing plutonium-241 build up Am, and chemical separation of americium from such plutonium may be required.

Americium-242m

Americium-242m is one of the rare cases, like Ag, Ho, Ta, Re, Ir, Bi, Po and others, where a higher-energy nuclear isomer is more stable than its ground state. While that ground state, Am, 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 or emits an alpha particle.
Am is fissile with a low critical mass, comparable to that of Pu. 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.

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. Production increases exponentially with increasing burnup as a total of 5 neutron captures on U are required. If MOX-fuel is used, particularly MOX-fuel high in and, more americium overall and more will be produced.
It decays by either emitting an alpha particle to become Np, which then quickly goes to Pu, or, very rarely, spontaneous fission. The fission rate is about 60% that of americium-241 or about 0.7/g/s.
As for the other americium isotopes, and more generally for all alpha emitters, Am is carcinogenic in case of internal contamination after being inhaled or ingested. Am also presents a risk of external irradiation associated with the gamma ray emitted by its short-lived decay product Np. The external irradiation risk for the other two americium isotopes is less than 10% of that for americium-243.