Number 729 #2, April 27, 2005 by Phil Schewe and Ben Stein
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
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 (firstname.lastname@example.org, 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 Hosmeret al., Physical Review Letters, 25 March 2005)