Protactinium

Protactinium was first identified in a short-lived form in 1913 by Kasimir Fajans and Oswald Gohring, who called it brevium, and the long-lived isotope was characterised in 1917 and 1918 by Otto Hahn and Lise Meitner in Germany and, independently, by Frederick Soddy and John Cranston in Britain. Its name means parent of actinium, from the Greek protos, first. Protactinium is a dense, silvery, highly radioactive metal, one of the rarest and most expensive natural elements. Extracted only in tiny amounts from spent reactor fuel or uranium ore, protactinium has no uses outside research, where its isotopes help to date marine sediments and reconstruct past climate; its scarcity and radioactivity make it one of the least studied of the elements.

Almost the entire world's usable stock of protactinium, roughly 125 grams, was extracted in 1961 by the United Kingdom Atomic Energy Authority from some 60 tonnes of radioactive waste material left over from uranium ore processing, at a cost reported at around half a million US dollars; that single, expensive extraction effort supplied most of the protactinium used in research laboratories worldwide for decades afterward, since no comparable large-scale purification has been repeated since.

Protactinium's name records its place in the radioactive decay chains that so preoccupied the physics of the early twentieth century: it decays into actinium, and so was named its parent, protos-actinium, the element that comes before. The original short-lived form found by Fajans and Gohring had been called brevium for its brevity, but when the far longer-lived isotope was discovered it took the more meaningful genealogical name.

The discovery came in the midst of the First World War, and Lise Meitner, one of its finders, would go on to help explain nuclear fission two decades later. Protactinium has no ancient, mythological or alchemical tradition, being a product of the new science of radioactivity; its name is a small piece of the family tree of decay by which the heavy elements were being mapped. Protactinium-231 dating is most powerful used alongside the related thorium-230 method, since the two isotopes derive from different parents, uranium-235 and uranium-234 respectively, and decay at different rates; when both clocks agree on the age of the same layer of ocean sediment or fossil coral, researchers gain much greater confidence in reconstructions of past ocean circulation and climate than either isotope could provide alone.