Actinium-(Gr. aktis, aktinos, beam or ray),Ac; at.wt. (227); at.no. 89; m.p.1051 deg C, b.p. 3200+/-300 deg C (est.); sp.gr. 10.07(calc.). Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals. Thirty two isotopes and isomers are now recognized. All are radioactive. Actinium-227,a decay product of uranium-235 is an alpha and beta emitter with a 2l.77-year half- life. Its principal decay products are thorium-227 (18.72-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha rays. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300 deg C. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.77-year half-life. It is about 150 times as active fas radium, making it of value in the production of neutrons.