Isotopes
Naturally occurring actinium is composed of one radioactive isotope; 227Ac. Thirty-six radioisotopes have been identified, the most stable being 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 remaining radioactive isotopes have half-lives that are less than 10 hours and the majority of them have half-lives shorter than one minute. The shortest-lived known isotope of actinium is 217Ac (half-life of 69 nanoseconds) which decays through alpha decay and electron capture. Actinium also has two meta states.
Purified 227Ac comes into equilibrium with its decay products at the end of 185 days. It decays according to its 21.773-year half-life emitting mostly beta (98.8%) and some alpha particles (1.2%); the successive decay products are part of the actinium series. Owing to the low available amounts, low energy of its beta particles (46 keV) and low intensity of alpha radiation, 227Ac is difficult to detect directly by its emission and it is therefore traced via its decay products. The isotopes of actinium range in atomic weight from 206 u (206Ac) to 236 u (236Ac).
| Isotope | Production | Decay | Half-life |
|---|---|---|---|
| 221Ac | 232Th(d,9n)225Pa(α)→221Ac | α | 52 ms |
| 222Ac | 232Th(d,8n)226Pa(α)→222Ac | α | 5.0 s |
| 223Ac | 232Th(d,7n)227Pa(α)→223Ac | α | 2.1 min |
| 224Ac | 232Th(d,6n)228Pa(α)→224Ac | α | 2.78 hours |
| 225Ac | 232Th(n,γ)233Th(β−)→233Pa(β−)→233U(α)→229Th(α)→225Ra(β−)225Ac | α | 10 days |
| 226Ac | 226Ra(d,2n)226Ac | α, β− electron capture |
29.37 hours |
| 227Ac | 235U(α)→231Th(β−)→231Pa(α)→227Ac | α, β− | 21.77 years |
| 228Ac | 232Th(α)→228Ra(β−)→228Ac | β− | 6.15 hours |
| 229Ac | 228Ra(n,γ)229Ra(β−)→229Ac | β− | 62.7 min |
| 230Ac | 232Th(d,α)230Ac | β− | 122 s |
| 231Ac | 232Th(γ,p)231Ac | β− | 7.5 min |
| 232Ac | 232Th(n,p)232Ac | β− | 119 s |
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