Actinium was discovered in 1899 by the French chemist Andre-Louis Debierne, a colleague of the Curies, in the residues of pitchblende, and independently by Friedrich Oskar Giesel in 1902. Its name comes from the Greek aktis, aktinos, a ray or beam, after its radioactivity. Actinium is a soft, silvery, intensely radioactive metal that glows with a pale blue light in the dark, and it gives its name to the actinide series of elements that follows it. So intensely radioactive that it glows and gives off heat, actinium has few uses beyond research, though a heavy isotope is studied as a source of alpha particles for targeted cancer therapy, and it once served in neutron sources and radioactivity standards.
Nearly all actinium available today, a few grams worldwide at most, is manufactured by irradiating radium-226 with neutrons in a small number of specialised reactors, since natural pitchblende yields only trace amounts too minute for practical extraction. Demand for actinium-225 has grown quickly in recent years as a component of targeted alpha-particle cancer therapies, creating a persistent global supply shortage that has pushed several countries and private companies to invest in new dedicated production routes.