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Actinium-228

Main isotopes of actinium
Iso­tope Decay
abun­dance half-life mode energy (MeV) pro­duct
225Ac trace 10 d α 5.935 221Fr
226Ac syn 29.37 h β 1.117 226Th
ε 0.640 226Ra
α 5.536 222Fr
227Ac trace 21.772 y β 0.045 227Th
α 5.042 223Fr

Actinium (89Ac) has no stable isotopes and no characteristic terrestrial isotopic composition, thus a standard atomic weight cannot be given. There are 31 known isotopes, from 206Ac to 236Ac, and 2 isomers. Two isotopes are found in nature, 227Ac and 228Ac, as intermediate decay products of, respectively, 235U and 232Th. 228Ac is extremely rare, and almost all natural actinium is 227Ac.

The most stable isotopes are 227Ac with a half-life of 21.772 years, 225Ac with a half-life of 10.0 days, and 226Ac with a half-life of 29.37 hours. All other isotopes have half-lives under 10 hours, and most under a minute. The shortest-lived known isotope is 217Ac with a half-life of 69 ns. Actinium also has two meta states.

Purified 227Ac comes into equilibrium with its decay products (227Th and 223Fr) after 185 days.

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
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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