(Gr. aktis,
aktinos, beam or ray). Discovered by Andre
Debierne in 1899 and independently by F. Giesel
in 1902. Occurs naturally in association with
uranium minerals. Actinium-227, a decay product
of uranium-235, is a beta emitter with a
21.6-year half-life. Its principal decay products
are thorium-227 (18.5-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-degrees 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
21.6-year half-life. It is about 150 times as
active as radium, making it of value in the
production of neutrons.
