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ORNL discovers superdeformed light nuclei
OAK RIDGE, Tenn.,
June 11, 1996
Physicists from the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) and their collaborators are the first to have detected short-lived, rapidly rotating, football-shaped nuclei of four relatively light elements-strontium, yttrium, zirconium, and niobium.
The team making this discovery of the fastest-spinning nuclei ever observed was led by Cyrus Baktash of ORNL's Physics Division. Nuclear physicists from around the world including Canada, Denmark, England, France, Germany, Italy, Japan, and the United States have been searching for "superdeformed" nuclei that are twice as long as they are wide. These atomic cores are different from normal baseball-shaped nuclei because, for a split second, they look like footballs.
In the 1960s, it was discovered experimentally that nuclei of very heavy atoms called actinides, such as plutonium-240, assume elongated shapes and have a good chance to break up, or fission spontaneously, into two fragments. So theorists proposed that rapidly rotating nuclei of certain groups of lighter elements could take on the same football shape. Subsequent calculations identified groups most likely to show this effect-several elements with atomic masses of 190-210, 150-160, and 80-90. Atomic mass refers to the number of protons and neutrons making up the nucleus of a particular atom.
In 1986 the first experimental evidence of a superdeformed shape in a nucleus outside the actinide group was observed in dysprosium-152. In 1989 a similar shape was seen in mercury-192. But until the discovery by the Oak Ridge group, the race by many research groups to catch a glimpse of nuclear footballs in the mass 80-90 range went on without success.
"Our first hunting ground was a tandem accelerator at Daresbury Laboratory in England," Baktash said. "There, in 1993, we discovered the first light-mass superdeformed nucleus in strontium-83. Since then, we have continued our search at DOE's Lawrence Berkeley National Laboratory (LBNL), where we take advantage of the Gammasphere, the world's most sensitive gamma-ray detector system.
"Working with our collaborating groups from Washington University, Lawrence Berkeley National Laboratory, the University of Pittsburgh, and Florida State University, we have detected 10 cases of superdeformed nuclei that have masses in the range of 80 to 90. Our discoveries span four different elements: strontium, yttrium, zirconium, and niobium. They are the fastest-spinning nuclei yet observed."
The nuclei of interest are typically synthesized by bombarding a nickel-58 target with beams of silicon-28 or sulfur-32 ions (electrically charged atoms). Occasionally, some ions strike the target nucleus nearly head-on and fuse with it with little spin. But many ions hit the target nucleus off center and produce fused compound nuclei that rotate very rapidly. As a spinning nucleus drops down from its excited state, it releases its excess energy, first by emitting particles (neutrons, protons, alpha particles) and then gamma rays. The gamma rays are particularly effective in carrying away the excess energy of rotation.
In superdeformed nuclei, the gamma-ray energies drop with decreasing spin in a very regular fashion. The regularity of the energies of these gamma rays and the time it takes for the nucleus to emit them are used by physicists to determine how deformed these nuclei are. Measurements of lifetimes of these superdeformed states indicate that, unlike toy gyroscopes, these fast-rotating nuclear tops last for a mere millionth of a billionth of a second before they jump to their next lower state.
The measured properties of these nuclei are in reasonable agreement with the predictions of theory, but some puzzles remain. The race is on to find the solutions.
The research is supported by DOE's Office of Energy Research, Basic Energy Sciences.
ORNL, one of the Department of Energy's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Research Corp.