Superheavy Nuclei: Taking Shape in Theory
ORNL and UT researchers help predict the structure and stability of superheavy nuclei.
The Feb. 17, 2004, issue of Nature magazine featured collaborative work on these cores of superheavy atoms by researchers at ORNL, the University of Tennessee, and universities in Poland and Belgium. The authors described the behavior of superheavy nuclei that are loaded almost to their limits with protons and neutrons, posing a major challenge to the physical forces that hold them together.
The research was funded by DOE's Office of Nuclear Physics in the Office of Science and the National Nuclear Security Administration.
"Predicting the stabilities of extremely heavy nuclei has been a long-term goal of nuclear scientists," says Witold Nazarewicz, a researcher in ORNL's Physics Division and UT's Department of Physics and Astronomy. "This research represents the very best we can do at predicting the structure of these species."
The paper illustrates how protons and neutrons of extremely heavy nuclei arrange into shapes that can be either oblong or flat. The shape can help determine the stability or life of the nucleus, which is, in turn, a factor in determining if the atomic species can even exist or be synthetically created.
Because of the strong electrostatic repulsion that drives protons apart, some of these superheavy nuclei may have extremely short lifetimes.
"A typical lifetime of a nucleus in the extremely heavy range is a millisecond," Nazarewicz says. However, in some cases, certain isotopes may be much more stable, or long-lived. This stability may depend on the nuclear shape.
Experiments performed in Germany, Japan, and Russia have bolstered theories that the lives of nuclei become longer as certain configurations of protons and neutrons are achieved. Computationally intense theoretical modeling indicates that a significant difference in the shape of a "parent" nucleus that decays by emitting an alpha particle and the shape of its daughter isotope will hinder the rate of decay from parent to daughter.
"It takes time for a nucleus to decay from a flat, oblate shape to a well-deformed, elongated shape," Nazarewicz says. "Because these protons and neutrons must rearrange themselves, this shape change causes difficulty."
Some experiments indicate that the addition of neutrons to a nucleus can extend the life of an isotope of a superheavy element, such as the as-yet unnamed element 112, from a fraction of a second to more than 30 seconds. In terms of existence for extremely heavy nuclei, a half-minute is an eternity.
Nuclei in the particularly well-bound isotopes find arrangements that physicists regard as "magic." Such nuclei are reminiscent of noble gases—for instance, helium, argon, and neon—that, because of their closed electron shells, are so stable that they are known as inert gases.
Nuclei also can have closed shells of protons and neutrons. Lead-208 is the heaviest "doubly magic" nucleus with closed shells of 82 protons and 126 neutrons. "We do not really know what is the next doubly magic nucleus beyond lead-208," Nazarewicz says.
Theorists such as Nazarewicz and his Nature co-authors, the late S. Cwiok of the Warsaw University of Technology and P.-H. Heenen of the Free University of Brussels, believe that in the extremely heavy regions, the interplay of nuclear shapes and proton and neutron arrangements eventually will approach relatively stable, "near-magic" states.
"These theories are supported by large-scale, state-of-the-art calculations, but at the same time, lab experimenters are trying to understand the mechanisms of nuclear collisions," says Nazarewicz, who is scientific director of the Department of Energy's Holifield Radioactive Ion Beam Facility at ORNL. "Experiments with beams of radioactive, neutron-rich nuclei such as doubly magic tin-132 may teach us how to pump more neutrons into the nuclei of these superheavy elements."—Bill Cabage
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