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(Gr. protos, first) The first isotope of element 91 to be discovered was 234Pa, alsoknown as UX2, a short-lived member of the naturally occurring 238U decay series. It wasidentified by K. Fajans and O.H. Gohring in 1913 and the named the new element brevium.When the longer-lived isotope 231-Pa was identified by Hahn and Meitner in 1918, the nameprotoactinium was adopted as being more consistent with the characteristics of the mostabundant isotope. Sody, Cranson, and Fleck were also active in this work. The nameprotoactinium was shortened to protactinium in 1949. In 1927, Grosse prepared 2 mg of awhite powder, which was shown to be Pa2O5. Later, in 1934, from 0.1 g of pure Pa2O5 heisolated the element by two methods, one of which was by converting the oxide to an iodideand "cracking" it in a high vacuum by an electrically heated filament by thereaction: 2PaI5 --> 2Pa+ 5I2. Protactinium has abright metallic luster which it retains for some time in air. The element occurs inpitchblende to the extent of about 1 part 231Pa to 10 million of ore. Ores from Zaire haveabout 3 ppm. Protactinium has 20 isotopes, the most common of which is 231Pa with ahalf-life of 32,700 years. A number of protactinium compounds are known, some of which arecolored. The element is superconductive below 1.4K. The element is a dangerous materialand requires precautions similar to those used when handling plutonium. In 1959 and 1961,it was announced that the Great Britain Atomic Energy Authority extracted by a 12-stageprocess 125 g of 99.9% protactinium, the world's only stock of the metal for many years tocome. The extraction was made from 60 tons of waste material at a cost of about $500,000.Protactinium is one of the rarest and most expensive naturally occurring elements.O.R.N.L. supplies promethium-231 at a cost of about $280/g. The elements is an alphaemitter (5.0 MeV) and is a radiological hazard similar to polonium.