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December 21, 2006


Hassium-270 Is Long-Lived

Experiment confirms predictions of relatively stable superheavy nucleus having 108 protons and 162 neutrons

Mitch Jacoby

Radioactive nuclei that hang around for a mere half-minute before falling apart hardly seem stable. Yet compared with the fleeting lifetimes of their superheavy atomic neighbors, the roughly 30-second period that transpired from creation to disintegration of four atoms of a newly discovered isotope of element 108 qualifies those atoms as rock solid.

Courtesy of Jan Dvorak
Dvorak prepares the liquid-nitrogen-cooled detector for transactinide experiments.

Theoretical physicists predicted years ago that some nuclei of elements much more massive than uranium should survive for a relatively long time—possibly long enough to probe their chemical properties—if they could be synthesized. On the chart of nuclides, theoreticians pinpointed a region with coordinates corresponding to 114 protons and 184 neutrons and indicated that nuclei with those "magic" numbers of subatomic particles should lie at the center of an island of stability. The nuclear longevity, according to the models, is due to the closing of proton and neutron shells, which renders the particles stable against spontaneous fission much the same way that a filled outer electron shell endows noble gases with chemical inertness. Experimentalists, though, haven't yet found a route to reach the center of the island.

Other theoreticians calculated the effects of subshell closings in other superheavy nuclei. They concluded that an isotope of hassium containing 108 protons and 162 neutrons (270Hs) should survive a long time—much longer than the millisecond or shorter lifetimes typical of most of the heaviest nuclides.

Now, an international team of experimentalists has detected four of those atoms and probed some of their chemical properties during the roughly 30 seconds the nuclei survive (Phys. Rev. Lett. 2006, 97, 242501). The findings confirm the predictions and provide new statistical data with which such theoretical models can be refined. The team includes 24 scientists from 10 research institutions, including the Technical University of Munich (TUM) and the Institute for Heavy-Ion Research (GSI), both in Germany, as well as institutions in Russia, the U.S., Switzerland, Japan, China, and Poland.

As TUM graduate student Jan Dvorak explains, the hassium nuclei were formed by firing a high-energy beam of 26Mg projectiles into a target enriched in 248Cm. The target was also doped with a small amount of gadolinium to produce isotopes of hassium's lighter homolog, osmium. Upon formation, nuclear products were exposed to a stream of oxygen. From earlier studies of 269Hs, scientists learned that hassium and osmium—but not other heavy elements—form volatile tetroxides, thereby providing a method for filtering unwanted products.

In the latest experiments, the volatile oxides were swept into a multistage chromatographic detector, which was cooled along its length in a gradient from room temperature at one end to -150 °C. On the basis of the two sets of experiments, 269Hs and 270Hs exhibit distinct nuclear properties but similar chemical properties, as expected.

The study paints a very consistent picture of that region of the chart of the nuclides and makes clever use of chemistry to sort out an assignment of atomic number, says Kenton J. Moody, a heavy-element research group leader at Lawrence Livermore National Laboratory. Moody adds that the observations support theoretical calculations that scientists have been using to predict transactinide properties and plan superheavy element experiments.

Chemical & Engineering News
ISSN 0009-2347
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