| Actinium was discovered in 1899 in pitchblende residues by Andre-Louis Debierne, Curie& |
| 10 actinium isotopes with mass numbers from 221 to 230 are known. The most long-lived of them is 227Ac (half-life T1/2 = 21.8 years) which emits β-particles (98.8%) and α-particles (1.2%). Isotopes 227Ac and 228Ac (also called mesothorium, MsThll T1/2 = 6.13 hours) occur in uranium ores as members of natural radioactive series). Earth's crust surface layer 1.6 km deep contains 11 300 metric tons of Actinum-227. However, comparing with other elements actinium abundance is extremely small, 6x10-10% by mass. |