dubnium (Db), an artificially produced radioactivetransuranium element in Group Vb of theperiodic table,atomic number 105. The discovery of dubnium (element 105), like that ofrutherfordium (element 104), has been a matter of dispute between Soviet and American scientists. The Soviets may have synthesized a few atoms of element 105 in 1967 at theJoint Institute for Nuclear Research inDubna, Russia,U.S.S.R., by bombarding americium-243 with neon-22 ions, producing isotopes of element 105 having mass numbers of 260 and 261 and half-lives of 0.1 second and 3 seconds, respectively. Because the Dubnagroup did not propose a name for the element at the time they announced their preliminary data—a practice that has been customary following the discovery of a new element—it was surmised by American scientists that the Soviets did not have strong experimental evidence tosubstantiate their claims. Soviet scientists contended, however, that they did not propose a name in 1967 because they preferred to accumulate more data about the chemical and physical properties of the element before doing so. After completing further experiments, they proposed the name nielsbohrium.

In 1970 a group of investigators at theLawrence Radiation Laboratory of the University of California at Berkeley announced that they had synthesizedisotope 260 of element 105, whereupon they proposed the name hahnium for the element, in honour ofOtto Hahn, the discoverer ofnuclear fission. The American team could not duplicate the Soviet experiment; but, when its members bombarded californium-249 with the nuclei of nitrogen-15 atoms, they produced “hahnium-260,” which had a half-life of about 1.6 seconds. As further evidence of their discovery, the scientists atBerkeley measured the amount of energyemitted by “hahnium-260” as it decayed, as well as the elements produced in the process; these characteristics were quite different from those of previously known elements in the periodic system. The International Union of Pure and Applied Chemistry ultimately determined that the element be named dubnium.