Pursuing the Exotic

Theorists from Oak Ridge and Japan will collaborate on understanding the behavior of exotic nuclei.

As a child, David Dean thought all he needed to get to China was a shovel. None too happy about having his son dig a massive hole in the backyard, Dean's father diverted his attention by taking him to the library. "I read a book about the middle of the Earth, and I decided then that I wanted to be a scientist," says Dean, now a nuclear physicist at ORNL.

Dean's once child-like curiosity has remained, transformed into the search for exotic nuclei. These non-stable groupings of protons and electrons exist only for a few seconds, at most, but play an important role in astrophysical processes and the production of the elements. The exotic behavior of these nuclei is both puzzling and fascinating, says Dean, citing a lithium isotope. The lithium-11 nucleus has three protons and eight neutrons and the same size radius as the nucleus of lead-208, which has almost 20 times more nuclear particles. "That's why these nuclei are called exotic," he explains.

Lithium-11 is one of 2000 exotic nuclei that scientists have recorded at facilities such as Holifield Radioactive Ion Beam Facility in ORNL's Physics Division. Of the roughly 6000 nuclei known to exist, Dean says only 300 are stable. "We know very little about the nuclei that lie beyond the stable ones," he notes, adding that many of the nuclei scientists could uncover are too elusive even for facilities like Holifield.

New experimental facilities worldwide, including two in Japan, are making the next step toward understanding nuclei. Theoretical centers at the University of Tokyo and Rikagaku Kenkyusho, RIKEN: the Institute of Chemical and Physical Sciences of Japan will support a newly funded Japan-U.S. Theory Institute for Physics with Exotic Nuclei, known as JUSTIPEN. In March the U.S. Department of Energy announced funding for JUSTIPEN, which resides at RIKEN.

In 2008 RIKEN will open the Rare Isotope Beam Facility, placing nuclear theorists and experimentalists side by side at one of the world's most powerful nuclear research facilities. The new facility will accelerate a beam of normal nuclei to high energy to smash into a target, creating exotic nuclei. RIKEN's RadioIsotope Beam Factory will allow scientists to study exotic nuclei that exist for only milliseconds, enabling a qualitative leap in our understanding of nuclear properties.

David Dean's focus lies in the theory institute. He was in Japan on July 10 when the institute opened, assuming the position of JUSTIPEN's associate director. "We want to establish international collaboration among theorists. We are trying to get U.S. scientists to spend time in Japan and have Japanese scientists come to Oak Ridge. I think in time such an exchange will enhance our understanding of nuclei."

Dean began working on collaboration plans for the institute two years ago when he was chair of the Rare Isotope Accelerator Theory Group Executive Committee. He and his ORNL colleagues, including Witold Nazarewicz, a professor of physics at the University of Tennessee, wanted to submit a proposal for the theory institute's recruitment of American scientists. The proposal called for JUSTIPEN to host American scientists in Japan each year to do their research alongside Japanese theorists. The institute hopes to host 12 visitors in 2006 for one to four weeks. In the future, the theory institute hopes to double that number and encourage long-term stays.

If Japanese theorists win funding, they will visit the United States. Oak Ridge will host many of them at the Joint Institute for Heavy Ion Research, which supports collaborative nuclear research at Holifield.

The collaborators hope to help each other better understand features of exotic nuclei that will be studied experimentally, and to provide a unified, predictive theory for those nuclei and nuclear properties that are difficult or impossible to measure. If they can obtain a better idea about the behavior of these elusive proton-neutron clusters and their fate as they interact with the environment, the scientists will be able to help pin down how nuclei are created in the universe.—Ashley Yeager, ORNL science writing intern from the University of Tennessee

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