Protactinium

Protactinium is a dense, silvery, radioactive metal that is both rare and highly toxic. A short-lived form was first identified in 1913 by Kasimir Fajans and Oswald Göhring. In 1917–18, Otto Hahn and Lise Meitner found the longer-lived protactinium-231, which has a half-life of about 33,000 years. The name means before actinium, because it decays into actinium.

It occurs in tiny amounts in uranium ores. One of the largest stocks ever, about 125 grams, was extracted from some 60 tons of nuclear waste in Britain in 1961. Protactinium has no uses outside research, but geologists compare it with thorium in seafloor mud to work out the age of sediments and past ocean currents.