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
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

...
Wikipedia