Actinium

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.

Actinium's name, from the Greek for ray, was chosen to mark the powerful radiation it emits, so strongly that the pure element visibly glows in darkness from the ionisation of the surrounding air. Beyond its own properties, actinium lends its name to a whole family: the actinides, the fifteen elements from actinium through lawrencium, which include the naturally important thorium and uranium and the entirely synthetic transuranium elements, and which occupy the lower of the two detached rows beneath the main body of the periodic table.

Actinium has no ancient or alchemical tradition, being a product of the study of radioactivity at the turn of the twentieth century; its significance is that of a namesake and a marker, the ray-giving head of one of the great series of the modern table. The actinide series divides in practice into two very different halves: the first few members, actinium through uranium, occur naturally on Earth in workable if often minute quantities, while everything past uranium, the transuranium elements from neptunium onward, exists only because twentieth-century physicists learned to manufacture new nuclei in reactors and accelerators, a split that makes actinium's own header position a hinge between elements found in the ground and elements built entirely by human hands.