Nickel nuclei yield magic finding
Apr 12, 2005
Physicists in the US and Germany have measured the half-life of the "doubly-magic" nucleus nickel-78 for the first time and have found that it is shorter than expected (Phys. Rev. Lett. 94 112501). Nickel-78 is thought to produce about half the elements heavier than iron in the universe. The finding, obtained by Paul Hosmer of Michigan State University and colleagues, could mean that supernova explosions produce gold and other heavy elements much faster than previously thought.
With 50 neutrons and 28 protons nickel-78 has an extremely large neutron excess compared to naturally occuring nickel isotopes. Nickel-78 is said to be a "doubly-magic" nucleus because it has closed shells of both protons and neutrons. It is one of only ten such nucleides that can be formed in nature. It is also very difficult to produce experimentally. Although physicists at the GSI lab in Darmstadt, Germany, had produced three nickel-78 nuclei before, they were not able to measure their properties.
Working at the National Superconducting Cyclotron Laboratory at Michigan State University, Hosmer and colleagues first fired a beam of stable krypton into a beryllium target. The krypton fragments to produce many exotic neutron-rich isotopes. About twice a day, one out of ten billion attempts per second produces a nickel-78 nucleus. This isotope was separated out and its decay half-life measured to be about 110 milliseconds, which is about four times shorter than nuclear theories predict.
According to some models, the decay of nickel-78 is part of the so-called "rapid neutron capture process" or r-process, which is thought to produce about half the elements heavier than iron in the universe. The r-process is the main source of elements such as gold, platinum and uranium and may take place in supernova explosions. More importantly, nickel-78 is one of the main bottlenecks in this process and acts like a valve for the build-up of heavier elements. The new shorter half-life for the isotope means that the r-process could be creating gold and other heavy elements much faster than previously thought. The finding could force existing models for the synthesis of heavy elements in the universe to be modified.
"Our result provides an important benchmark for nuclear theories attempting to extend our knowledge into the unknown domain of exotic neutron-rich nuclei," says team member Hendrik Schatz. "It is also crucial for the ongoing quest of finding the origin of the heavy elements in nature -- one of the most important unanswered questions in nuclear astrophysics today."
About the author
Belle Dumé is Science Writer at PhysicsWeb