Actinium-223 is a radioactive isotope of the element actinium, which is part of the actinide series in the periodic table. It has a half-life of about 11.4 days and decays by emitting alpha particles. This property makes it useful in targeted alpha therapy for certain types of cancer, particularly prostate cancer. In medical applications, actinium-223 is used to deliver radiation directly to cancer cells while minimizing damage to surrounding healthy tissue. It is often administered in the form of a radiopharmaceutical, allowing for precise treatment of tumors that express specific markers, enhancing the effectiveness of cancer therapies.