Dubnium

Dubnium is a synthetic chemical element with the symbol Db and atomic number 105. Dubnium is highly radioactive: the most stable known isotope, dubnium-268, has a half-life of about 28 hours. This greatly limits the extent of research on dubnium. Dubnium does not occur naturally on Earth and is produced artificially. The Soviet Joint Institute for Nuclear Research (JINR) claimed the first discovery of the element in 1968, followed by the American Lawrence Berkeley Laboratory in 1970. Both teams proposed their names for the new element and used them without formal approval. The long-standing dispute was resolved in 1993 by an official investigation of the discovery claims by the IUPAC/IUPAP Joint Working Party, resulting in credit for the discovery being officially shared between both teams. The element was formally named dubnium in 1997 after the town of Dubna, the site of the JINR. Theoretical research establishes dubnium as a member of group 5 in the 6d series of transition metals, placing it under vanadium, niobium, and tantalum. Dubnium should share most properties, such as its valence electron configuration and having a dominant +5 oxidation state, with the other group 5 elements, with a few anomalies due to relativistic effects. A limited investigation of dubnium chemistry has confirmed this. Solution chemistry experiments have revealed that dubnium often behaves more like niobium rather than tantalum, breaking periodic trends. Uranium, element 92, is the heaviest element to occur in significant quantity in nature; heavier elements can only be produced practically by synthesis. The first synthesis of a new element—neptunium, element 93—was achieved in 1940 by a team of researchers in the United States. In the following years, American scientists synthesized the elements up to mendelevium, element 101, which was synthesized in 1955. From element 102, the priority of discoveries was contested between American and Soviet physicists. Their rivalry resulted in a race for new elements and credit for their discoveries, later named the Transfermium Wars. The first report of the discovery of element 105 came from the Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Russian SFSR, Soviet Union, in April 1968. The scientists bombarded 243Am with a beam of 22Ne ions, and reported 9.4 MeV (with a half-life of 0.1–3 seconds) and 9.7 MeV (t1/2 > 0.05 s) alpha activities followed by alpha activities similar to those of either 256103 or 257103. Based on prior theoretical predictions, the two activity lines were assigned to 261105 and 260105, respectively. After observing the alpha decays of element 105, the researchers aimed to observe spontaneous fission (SF) of the element and study the resulting fission fragments. They published a paper in February 1970, reporting multiple examples of two such activities, with half-lives of 14 ms and 2.2±0.5 s. They assigned the former activity to 242mfAm and ascribed the latter activity to an isotope of element 105. They suggested that it was unlikely that this activity could come from a transfer reaction instead of element 105, because the yield ratio for this reaction was significantly lower than that of the 242mfAm-producing transfer reaction, in accordance with theoretical predictions. To establish that this activity was not from a (22Ne,xn) reaction, the researchers bombarded a 243Am target with 18O ions; reactions producing 256103 and 257103 showed very little SF activity (matching the established data), and the reaction producing heavier 258103 and 259103 produced no SF activity at all, in line with theoretical data. The researchers concluded that the activities observed came from SF of element 105. In April 1970, a team at Lawrence Berkeley Laboratory (LBL), in Berkeley, California, United States, claimed to have synthesized element 105 by bombarding californium-249 with nitrogen-15 ions, with an alpha activity of 9.1 MeV. To ensure this activity was not from a different reaction, the team attempted other reactions: bombarding 249Cf with 14N, Pb with 15N, and Hg with 15N. They stated no such activity was found in those reactions. The characteristics of the daughter nuclei matched those of 256103, implying that the parent nuclei were of 260105.