Aiming for superheavy elements.

When a light atomic nucleus smashes into a much heavier one, the two particles occasionally stick together to create a new element. Using such a strategy, researchers have synthesized more than two dozen elements having higher atomic numbers than that of uranium, which has 92 protons in its nucleus. Typically, these new elements survive for just fractions of a second before they radioactively decay or fission into isotopes of other elements.

Now, a team of physicists has discovered two new isotopes of element 106, seaborgium (SN: 3/19/94, p.180). One isotope has an atomic mass of 265 (having 159 neutrons in its nucleus), and the other has an atomic mass of 266 (having 160 neutrons in its nucleus). The isotopes take longer to decay than expected, suggesting that these particular combinations of protons and neutrons in a nucleus have an enhanced stability.

Yuri A. Lazarev of the Joint Institute for Nuclear Research in Dubna, Russia, Ronald W. Lougheed of the Lawrence Livermore National Laboratory in Livermore, Calif., and their collaborators report the discovery in the Aug. 1 PHYSICAL REVIEW LETTERS.

The researchers created these rare isotopes by hurling neon-22 nuclei into curium-248. One product, seaborgium-265, has a half-life (the time it takes for half of a sample to decay) between 2 and 30 seconds; seaborgium-266 has a half-life between 10 and 30 seconds. These half-lives are at least 10 times longer than those previously observed for a seaborgium isotope.

Theorists attribute the enhanced stability of these isotopes to a slight deformation of the nuclei from a spherical to an oval shape. They predict a stronger deformation and even greater stability for a nucleus with atomic number 108 and mass number 270.

"The remarkable influence seen here of small shape changes on nuclear properties will undoubtedly stimulate modeling of new nuclear shapes," Arthur N. James of the University of Liverpool in England comments in the Sept. 8 NATURE.

Meanwhile Lazarev and his coworkers are bombarding uranium with sulfur nuclei to create what they hope will be new, longer-lived isotopes of element 108, hassium.