decayed into protactinium. It remained unsolved until the mother isotope, uranium-235, was discovered in 1929. For their discovery Hahn and Meitner were repeatedly nominated for the Nobel Prize in Chemistry in the 1920s by several scientists, among them Max Planck, Heinrich Goldschmidt, and Fajans himself. In 1949, the International Union of Pure and Applied Chemistry (IUPAC) named the new element definitively protactinium, and confirmed Hahn and Meitner as discoverers. Discovery of nuclear isomerism With the discovery of protactinium, most of the decay chains of uranium had been mapped. When Hahn returned to his work after the war, he looked back over his 1914 results, and considered some anomalies that had been dismissed or overlooked. He dissolved uranium salts in a hydrofluoric acid solution with tantalic acid. First the tantalum in the ore was precipitated, then the protactinium. In addition to the uranium X1 (thorium-234) and uranium X2 (protactinium-234), Hahn detected traces of a radioactive substance with a half life of between 6 and 7 hours. There was one isotope known to have a half life of 6.2 hours, mesothorium II (actinium-228). This was not in any probable decay chain, but it could have been contamination, as the Kaiser Wilhelm Institute for Chemistry had experimented with it. Hahn and Meitner demonstrated in 1919 that when actinium is treated with hydrofluoric acid, it remains in the insoluble residue. Since mesothorium II was an isotope of actinium, the substance was not mesothorium II; it was protactinium. Hahn was now confident enough that he had found something that he named his new isotope "uranium Z", and in February 1921, he published the first report on his discovery. Hahn determined that uranium Z had a half life of around 6.7 hours (with a two per cent margin of error) and that when uranium X1 decayed, it became uranium X2 about 99.75 per cent of the time, and uranium Z around 0.25 per cent of the time. He found that the proportion of uranium X to uranium Z extracted from several kilograms of uranyl nitrate remained constant over time, strongly indicating that uranium X was the mother of uranium Z. To prove this, Hahn obtained a hundred kilograms of uranyl nitrate; separating the uranium X from it took weeks. He found that the half life of the parent of uranium Z differed from the known 24 day half life of uranium X1 by no more than