BERKELEY, CA — An international collaboration of radiochemists has used the PHILIPS cyclotron at the Paul Scherrer Institute (PSI) in Bern, Switzerland, to determine the volatility of bohrium, element 107 -- the heaviest element yet whose chemistry has been successfully investigated.
Crucial to the research was the use of an isotope of bohrium with a relatively long half-life of about 15 seconds, detected earlier this year by researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley. The team worked at Berkeley Lab's 88-Inch Cyclotron with visitors from the PSI and the University of Bern.
Although several elements heavier than bohrium have been identified, the correct placement of the heaviest elements in the periodic table is under study.
"In the discovery experiments of new elements, only the existence of a new, very heavy atomic nucleus is demonstrated," says Heinz Gäggeler, leader of the PSI team, "but no information about its chemical properties is obtained." To date, the heaviest element whose chemical properties have been widely investigated by experiment is seaborgium, element 106. "Thus, in the view of a chemist," Gäggeler says, "the periodic table currently ends at seaborgium."
"Elements beyond 100 are made an atom at a time, with very low production rates, and have very short half lives," says Darleane C. Hoffman, a longtime collaborator with Gäggeler's team and coleader of the group which identified the relatively long-lived isotope, bohrium 267, at the 88-Inch Cyclotron. A member of the Nuclear Science Division at Berkeley Lab, she is a professor of chemistry at UC Berkeley. Hoffman says, "The chemistry of the heavy elements requires separations that come to equilibrium very rapidly, and these must be valid on an atom-by-atom basis."
Such atoms are created in the laboratory by bombarding heavy target nuclei with an accelerated beam of projectile ions. The nuclei of interest, which are created by the evaporation of a few neutrons, are only a very small fraction of the huge number of reaction products produced. At PSI, the PHILIPS cyclotron yielded about three atoms of bohrium during a day of beam time, but only four bohrium nuclei were detected in the first two weeks of the volatility experiment.
The PSI researchers used a beam of neon 22 to bombard a target of berkelium 249, which has a half-life of 320 days. The targets were prepared at Berkeley Lab from material furnished by the Department of Energy through its Transplutonium Element Production Program at Oak Ridge National Laboratory.
Immediately after bombardment, the reaction products were swept into an automated isothermal system called the On-Line Automated Gas Analyzer (OLGA) developed by Gäggeler and his colleagues. There, reaction products formed molecules in oxygen-containing hydrogen chloride gas. These oxychlorides were then passed through a chromatography column, in which the more volatile species pass through at lower temperatures. In this system, bohrium 267 compound was shown to be volatile at 180 degrees Celsius.
The four bohrium atoms were found only after they had passed through the chromatography column, when the oxychloride molecules containing them were deposited on a rotating detector that carried each small sample under a set of radiation detectors. Bohrium 267 was unambiguously identified by the pattern of its alpha decay, first to dubnium 263, then to lawrencium 259, and subsequently to mendelevium 255.
Because the positive charge of a heavy nucleus is so great, the electronic structure of the atom is distorted. These so-called "relativistic effects" can produce unexpected deviations from chemical properties extrapolated from the element's lighter homologues in the periodic table.
Bohrium may also prove to deviate in this way. The oxychloride of bohrium was shown to be volatile at 180 C, similar to its lighter homologues in group VII of the periodic table, such as rhenium and technetium. Continuing experiments will determine whether bohrium is also volatile at lower temperatures. Technetium, for example, is volatile at 50 C and rhenium at 75 C under the same conditions.
The need to develop techniques for understanding the chemistry of the heaviest elements is partly driven by the search for the "island of stability," a group of superheavy elements whose nuclear shell structure is predicted to make them stable for hundreds or thousands of years or longer, instead of for mere seconds or milliseconds. Isotopes with the number of neutrons required to reach the island of stability have not yet been created.
Meanwhile, however, there is a region of relative stability due to "deformed shells" at lower neutron and proton numbers, which includes bohrium 267. Thus chemical studies of bohrium are not only intrinsically interesting, but aid in what Darleane Hoffman calls "the long march up the periodic table toward the island of stability."
Besides Hoffman, the collaborating team at Berkeley included Berkeley Lab senior scientists Kenneth Gregorich and Heino Nitsche, who is also a professor of chemistry at UC Berkeley, postdoctoral fellows Uwe Kirbach and Carola Laue, and graduate students Joshua Patin, Dan Strellis, and Philip Wilk.
In addition to PSI, Berkeley Lab, and UC Berkeley, collaborating institutions included the University of Bern in Switzerland, the Flerov Laboratory in Russia, the Forschungzentrum Rossendorf, Gesellschaft für Schwerionenforschung (GSI), and Technical University of Dresden in Germany, and the Japan Atomic Energy Research Institute in Japan.
The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.