Nickel-78, The Most Neutron-Rich Of The Doubly-Magic Nuclei

Nickel-78, the most neutron-rich of the doubly-magic nuclei, has had its lifetime measured for the first time, which will help us better understand how heavy elements are made. Indeed, where do gold atoms come from? Physicists believe gold and other heavy elements (beyond iron) were built from lighter atoms inside star explosions billions of years ago. In the “r-process” (r standing for rapid) unfolding inside the explosion, a succession of nuclei bulk up on the many available neutrons.

This evolutionary buildup is nicely captured in a movie simulation showing all the species in the chart of the nuclides being made one after the other (http://www.jinaweb.org/html/movies.html). In some models the buildup can slow down at certain strategic bottlenecks. Nickel-78 is one such roadblock. This is because Ni-78 is a “doubly magic” nucleus. It has both closed neutron and proton shells; it is “noble” in a nuclear sense in the way that a noble gas atom is noble in the chemical sense owing to its completely filled electron shell.

Knowing more about this crucial nuclide is made difficult by the fact that it is, in our modern era, very rare, and hard to make artificially. Nevertheless, scientists at the National Superconducting Cyclotron (NCSL) at Michigan State University have now culled 11 specimens of Ni-78 from among billions of high-energy collision events recorded. In effect, the NCSL is a factory for reproducing supernova conditions here on Earth. Hendrik Schatz (schatz@ncsl.msu.edu, 517-333-6397), speaking at last week’s American Physical Society meeting in Tampa, reported that from the available Ni-78 decays recorded, a lifetime of 110 milliseconds could be deduced.

This is some 4 times shorter than previous theoretical estimates, meaning that the bottleneck nucleus lived shorter than was thought, which in turn means that the obstacle to making heavier elements was that much less. So far the exact conditions and site for the r-process are still unknown. With the new measurement model conditions have to be readjusted to produce the observed amounts of precious metals in the universe. This will provide a better idea of what to look for when searching for the site of the r-process. (See also Hosmer et al., Physical Review Letters, 25 March 2005)