Most of the isotope data on this site has been obtained from the National Nuclear Data Center.
Naturally Occurring Isotopes
Actinium has no naturally occurring isotopes.
Known Isotopes
| Mass Number | Half-life | Decay Mode | Branching Percentage |
| 206 | 22 milliseconds | Alpha Decay | 100.00% |
| 206m | 33 milliseconds | Alpha Decay | 100.00% |
| 207 | 27 milliseconds | Alpha Decay | 100.00% |
| 208 | 95 milliseconds | Alpha Decay | ~ 99.00% |
| Electron Capture | ~ 1.00% | ||
| 208m | 25 milliseconds | Electron Capture | ~ 10.00% |
| Alpha Decay | ~ 90.00% | ||
| 209 | 0.10 seconds | Alpha Decay | ~ 99.00% |
| Electron Capture | ~ 1.00% | ||
| 210 | 0.35 seconds | Alpha Decay | 91.00% |
| Electron Capture | ~ 9.00% | ||
| 211 | 0.21 seconds | Alpha Decay | 100.00% |
| 212 | 0.93 seconds | Alpha Decay | ~ 57.00% |
| Electron Capture | ~ 43.00% | ||
| 213 | 738 milliseconds | Alpha Decay | <= 100.00% |
| 214 | 8.2 seconds | Electron Capture | <= 11.00% |
| Alpha Decay | >= 89.00% | ||
| 215 | 0.17 seconds | Alpha Decay | 99.91% |
| Electron Capture | 0.09% | ||
| 216 | 440 microseconds | Alpha Decay | 100.00% |
| 216m | 441 microseconds | Alpha Decay | 100.00% |
| 217 | 69 nanoseconds | Alpha Decay | 100.00% |
| Electron Capture | <= 2.00% | ||
| 218 | 1.08 microseconds | Alpha Decay | 100.00% |
| 219 | 11.8 microseconds | Alpha Decay | 100.00% |
| 220 | 26.4 milliseconds | Alpha Decay | 100.00% |
| Electron Capture | 5.0×10-4 % | ||
| 221 | 52 milliseconds | Alpha Decay | 100.00% |
| 222 | 5.0 seconds | Alpha Decay | 99.00% |
| Electron Capture | 1.00% | ||
| 222m | 63 seconds | Alpha Decay | >= 88.00% |
| Isomeric Transition | <= 10.00% | ||
| Electron Capture | >= 0.70% | ||
| 223 | 2.10 minutes | Alpha Decay | 99.00% |
| Electron Capture | 1.00% | ||
| 224 | 2.78 hours | Electron Capture | 90.90% |
| Alpha Decay | 9.10% | ||
| Beta-minus Decay | < 1.60% | ||
| 225 | 10.0 days | Alpha Decay | 100.00% |
| Emission of a carbon-14 nucleus | 4×10-12 % | ||
| 226 | 29.37 hours | Beta-minus Decay | 83.00% |
| Electron Capture | 17.00% | ||
| Alpha Decay | 6.0×10-3 % | ||
| 227 | 21.772 years | Beta-minus Decay | 98.62% |
| Alpha Decay | 1.38% | ||
| 228 | 6.15 hours | Beta-minus Decay | 100.00% |
| 229 | 62.7 minutes | Beta-minus Decay | 100.00% |
| 230 | 122 seconds | Spontaneous Fission | 1.2×10-6 % |
| Beta-minus Decay | 100.00% | ||
| 231 | 7.5 minutes | Beta-minus Decay | 100.00% |
| 232 | 119 seconds | Beta-minus Decay | 100.00% |
| 233 | 145 seconds | Beta-minus Decay | 100.00% |
| 234 | 44 seconds | Beta-minus Decay | 100.00% |
| 235 | 60 seconds | Beta-minus Decay | 100.00% |
| 236 | No Data Available | Beta-minus Decay (suspected) | No Data Available |
| 237 | No Data Available | No Data Available | No Data Available |
This page is maintained by Steve Gagnon.