History of Transuranium Element Discovery

The first scientific attempts to prepare the elements beyond uranium were performed by Enrico Fermi, Emilio Segre, and co-workers in Rome in 1934, shortly after the existence of the neutron was discovered. This group of investigators irradiated uranium with slow neutrons and found several radioactive products, which were thought to be due to new elements. However, detailed chemical studies by Otto Hahn and Fritz Strassman in Berlin showed these species were isotopes of the known elements created by the fission of uranium into two approximately equal parts (see Chap. 11). This discovery of nuclear fission in December of 1938 was thus a by-product of man s quest for the transuranium elements. 

Neptunium, the element beyond uranium, was named after the planet Neptune because this planet is beyond the planet Uranus for which uranium is named. 

Early in 1941, 239Pu, the most important isotope of plutonium was discovered by Kennedy, Segre, Wahl, and Seaborg. 239Pu was produced by the decay of 239Np, which in turn was produced by the irradiation of 238U by neutrons, using the reaction discovered by McMillan 

This isotope, 239Pu, was shown to have a cross section for thermal neutron-induced fission that exceeded that of 235U, a property that made it important for nuclear weapons, considering that it could be prepared by chemical separation as compared to isotopic separation that was necessary for 235U. 

The next transuranium elements to be discovered, americium and curium (Am and Cm Z = 95 and 96, respectively), represent an important milestone in 

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The discovery of curium ends the first breakthrough period in the history of transuranium elements. The discoveries of neptunium, plutonium, americium, and curium were of great significance for science. It was for the first time that scientists artificially extended the boundaries of the periodic system. The properties of these elements proved to be quite different from those expected and chemists had to start seriously thinking how best to fit them into the periodic system. 

The history of the discovery of transuranium elements is a fascinating story that has been described in several articles (e.g., Seaborg and Loveland 1990 Morss and Fuger 1992 Seaborg 1995 Hoffinan et al. 2000). 

It has been more than fifty years since the discovery of the transuranium elements. The initial activities in this field established the fundamental solution and solid-state chemistry of the first two of these elements and their compounds under the auspices of the Manhattan Project. New separation methods including solvent extraction techniques and uranium isotope separation played a leading role in these programs. Tracer techniques were widely used to determine solubilities (or solubility liinits) of transuranium compounds as well as to obtain information about the coorination chemistry in aqueous solution. A little later, special solvent extraction and ion-exchange techniques were developed to isolate pure transplutonium elements on the milligram and smaller scale. The second edition of The Chemistry of the Actinide Elements, published in 1986 (i), covers most of these topics. A detailed overview of the history of transuranium chemistry is given in Transuranium Elements A Half Century (2). 

The discovery of fission was a complete surprise and also a great shock, because it shattered fundamental ideas of nuclear behavior that had guided the investigation. The surprise was evident in the events of December 1938. On December 10, Enrico Fermi was awarded the Nobel Prize in physics. He and his group in Rome had been the first to irradiate uranium with neutrons and to propose that transuranium elements had been formed in the process. In his Nobel lecture, Fermi was so confident of the first two, elements 93 and 94, that he referred to them by name ausonium and hesperium. But at that very moment, the Berlin team of Otto Hahn, Lise Meitner, and Fritz Strafimann was on the verge of identifying barium among the uranium products. By the end of the year, they understood that uranium had split, explained the fission process, and concluded that the transuranium elements were false. When Fermi published his Nobel lecture, he added a footnote to that effect, but by then ausonium and hesperium were themselves footnotes (if that) in the history of science. [1]... 

The history of syntheses saw its periods of breakthroughs and slack periods. The first breakthrough period was from 1940 to 1945 when four transuranium elements were synthesized, namely, neptunium (Z = 93), plutonium (Z = 94), americium (Z = 95), and curium (Z = 96). The period till 1949 was a slack time and no new elements were discovered. In the next breakthrough period from 1949 to 1952 four more transuranium elements were added to the periodic system, namely berklium (Z = 97), californium (Z = 98), einsteinium (Z = 99), and fermium (Z = 100). In 1955, fifteen years after the synthesis of the first transuranium element, one more element, mendelevium (Z = 101), was synthesized. The next 25 years saw much less syntheses and only six new elements appeared in the periodic system. Here scientists encountered an entirely new situation and many former criteria for evaluating discoveries of elements proved inapplicable. 

Another major turning point in the history of nuclear science came with the discovery of fission by Otto Hahn and Fritz Strassmann in December 1938 (Hahn and Strassmann 1939a, b). In several laboratories in Rome, Berlin, and Paris, a complex series of P-decay chains resulting from neutron irradiation of uranium had been investigated since 1934, and these chains had been assigned to putative transuranium elements formed by neutron capture in uranium with subsequent P" transitions increasing the atomic numbers (see Sect. 1.2.3). But then evidence appeared that known elements in the vicinity of uranium, such as radium, were produced as well. When Hahn and Strassmaim attempted to prove this by a classical fractional crystallization separation of radium from barium serving as its carrier, the radioactivity turned out to be barium, not radium hence, new and totally unexpected type of nuclear reaction had to be invoked. 

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