Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

TANTALUM AT A GLANCE Name: Named after the Greek mythological figure Tantalus. Atomic mass: 180.95. History: Discovered in 1802 by Swedish chemist Anders Gustav Ekeberg. Occurrence: Primarily obtained from columbite, tantalite, and euxenite. Appearance: Grayish silver metal. Behavior: It is very similar chemically to niobium, and the two freely replace each other in minerals. Uses: The biggest use of Ta is for producing capacitors. Tantalum carbide is used in cutting tools.

DUBNIUM

IVO J. ZVARA, JOINT INSTITUTE FOR NUCLEAR RESEARCH, DUBNA

In the late 1950s, Georgy Flerov, the discoverer of spontaneous fission of uranium (in 1940) and key "coauthor" of the Soviet atomic bomb, was fully back to basic research. He felt that the interactions of accelerated "heavy ions" (boron, carbon, oxygen, and so on) might open great prospects in nuclear physics and chemistry. At the top of the list would be the production of exotic nuclides. He pushed forward the project for a dedicated 300-cm heavy-ion cyclotron. The machine was built at the international Joint Institute for Nuclear Research (JINR) established in 1956 at Dubna, Moscow. Flerov's international research team consisted mostly of young scientists and engineers. The nuclear sciences then were very prestigious, and such a famous leader could choose from a number of good to excellent graduates applying for the positions. Most of them soon became well-known experts.

Of the broad research program at the new JINR Laboratory of Nuclear Reactions (today named after Flerov), the most ambitious goal was synthesis of new chemical elements, starting with number 102. Such research had been systematically conducted only by Glenn T. Seaborg and his collaborators at the University of California, Berkeley, and resulted in the discovery of elements 94 to 101. With the heavy ions, the simple basic idea is to fuse the projectiles with suitable heavy targets. But even if formed, the compound very seldom survives prompt fission, and the bombardment produces a tremendous amount of radioactive "by-products." With the higher atomic number of the new element, the effective cross-section for its production drops steeply, and it is extremely technically difficult to develop more and more intense accelerator beams and targets that can withstand them. But even more serious problems arise in isolation, measurement of decay properties, and conclusive identification of the atomic and mass numbers of the wanted atoms. One could attempt purely physical methods or combine these with radiochemical isolation and identification; the latter had been the case with the relatively long-lived elements 93 to 101.

At Dubna, from the very beginning, Flerov insisted on the development, however difficult, of new fast radiochemical methods for the transactinides, elements 104 and beyond. Their first isotopes in sight were expected to live seconds or less, and traditional techniques could not cope with such lifetimes and the minute yields.

The young newcomers to the field at Dubna were very enthusiastic; they learned fast and worked hard. The cyclotron U-300 was put in operation in 1960, and just three years later--after development of a number of original techniques, methods, and approaches--Dubna was able to claim production of element 102. During the subsequent decade, the claims could be extended up to element 106 and included radiochemical identification of the first transactinides by a new, fast "gas phase" method.

The research was paralleled by the work at Berkeley with competing claims of discoveries. After years of disputes, the two laboratories failed to agree on priorities and names for the new elements. In 1985, the International Union of Pure & Applied Chemistry (IUPAC) and the International Union of Pure & Applied Physics established the Transfermium Working Group, consisting of renowned impartial nuclear scientists. After five years of work, the experts concluded that element 102 (nobelium) was discovered at Dubna; that element 103 (lawrencium)

APPARATUS The chemistry of volatile compounds of elements 104 to 106 (with half-lives of only a few seconds) were studied using this machine.