Actinium-225 (Ac-225) is a radioactive isotope of actinium with a half-life of about 9.9 days. It decays by alpha emission through a chain of short-lived daughter isotopes, ultimately reaching stable bismuth-209. The decay chain releases several alpha particles and also produces gamma radiation, including a characteristic emission around 440 keV from its daughter Bi-213.
Ac-225 is primarily used and studied for targeted alpha therapy (TAT), an emerging form of nuclear medicine. It can be attached to molecules designed to selectively bind to cancer cells, delivering highly energetic alpha radiation over a very short distance. Ac-225-based radiopharmaceuticals are being investigated for several cancers, including prostate cancer, leukemia, and neuroendocrine tumors.
Ac-225 does not occur naturally in significant quantities. It is produced artificially for medical and research applications using several methods, including the decay of Th-229 and accelerator-based production from Ra-226. In practice, Ac-225 is mainly encountered at specialized isotope-production facilities, radiopharmaceutical laboratories, and nuclear medicine research or treatment centers.
Ac-225
Actinium-225
Half-life: 9.9 days Main emission lines: 10.376 - 18.623 keV
Decay mode Alpha
Alpha
| Energy, keV | Intensity, % |
| 5830 | 50.7 |
| 5792.5 | 18.1 |
| 5790.6 | 8.6 |
| 5732 | 8 |
| 5637 | 4.4 |
| 5724 | 3.1 |
| 5732 | 1.32 |
| 5682 | 1.3 |
| 5580 | 1.2 |
| 5609 | 1.1 |
| 5731 | 0.87 |
Gamma
| Energy, keV | Intensity, % |
| 99.8 | 1 |
| 99.6 | 0.7 |
| 150.1 | 0.6 |
| 188 | 0.45 |
| 62.9 | 0.43 |
| 157.3 | 0.32 |
| 216.9 | 0.27 |
| 73.9 | 0.26 |
| 111.5 | 0.26 |
| 87.4 | 0.23 |
| 108.4 | 0.22 |
| 153.9 | 0.18 |
| 145.2 | 0.13 |
| 195.8 | 0.12 |
| 253.5 | 0.12 |
| 224.7 | 0.1 |
| 452.4 | 0.09 |
| 94.9 | 0.08 |
| 100.8 | 0.08 |
| 123.8 | 0.07 |
| 119.9 | 0.07 |
| 64.3 | 0.04 |
| 526.1 | 0.03 |
| 197.9 | 0.03 |
| 481.1 | 0.03 |
| 96.7 | 0.03 |
| 134.9 | 0.03 |
| 279.3 | 0.03 |
| 73.5 | 0.03 |
| 124.8 | 0.02 |
| 197.4 | 0.02 |
| 74.6 | 0.02 |
| 152.6 | 0.02 |
| 515.3 | 0.02 |
| 133.6 | 0.02 |
| 198.4 | 0.02 |
| 170.7 | 0.02 |
| 36.7 | 0.02 |
| 517.9 | 0.02 |
| 178.3 | 0.01 |
| 71.4 | 0.01 |
| 249.6 | 0.01 |
| 169.9 | 0.01 |
| 186.1 | 0.01 |
| 78.8 | 0.01 |
| 240.7 | 0.01 |
| 173.4 | 0.01 |
X-rays
| Energy, keV | Intensity, % |
| 10.376 - 18.623 | 13.3 |
| 86.102 | 1.27 |
| 83.227 | 0.77 |
| 96.806 - 100.558 | 0.58 |
| 96.806 - 98.132 | 0.44 |
| 100.127 - 100.291 | 0.14 |