Americium-243

Most stable isotopes of americium

iso NA half-life DM DE (MeV) DP 241Am syn 432.2 y SF – – α 5.486 237Np 242m1Am syn 141 y IT 0.049 242Am α 5.637 238Np SF – – 243Am syn 7370 y SF – – α 5.275 239Np

Americium (Am) 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 into alpha particles. Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom).

Nineteen radioisotopes of americium have been characterized, with the most stable being 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 ^242mAm (t_1/2 141 years). The isotopes of americium range in atomic weight from 231.046 u (²³¹Am) to 249.078 u (²⁴⁹Am).

No fission products
have a half-life
in the range of
100–210 k years ...

... nor beyond 15.7 M years

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
m  metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 y: Medium-lived fission product
‡  over 200,000 y: Long-lived fission product

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Wikipedia