A chemical element, Ac, atomic number 89, and atomic weight 227.0. Actinium was discovered by A. Debierne in 1899. Milligram quantities of the element are available by irradiation of radium in a nuclear reactor. Actinium-227 is a beta-emitting element whose half-life is 22 years. Six other radioisotopes with half-lives ranging from 10 days to less than 1 minute have been identified.