The half-life of the unstable, exotic nucleus Ni-78 was established at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. Previous to this experiment, Ni-78 had only existed in two places: exploding stars, and Darmstadt Germany, where three nuclei of this exotic species were produced at the GSI laboratory but their properties could not be determined. At the NSCL a group of scientists from the US and Germany was now able to produce eleven Ni-78 nuclei, enough to finally measure its half-life, which describes how long the nucleus exists before it decays. The half-life was found to be only 110 ms, or about a tenth of a second.
Ni-78 is doubly magic, which means that it has a number of both protons and neutrons that completely fills certain shells in the nucleus, very much like electrons fill shells in noble gas atoms. Based on the classical nuclear shells there are only ten such nuclei in nature and Ni-78 is the one with the largest neutron excess. Magic nuclei are of particular interest to nuclear theorists.
In some models, the decay of Ni-78 is a key link in the chain of reactions called the rapid neutron capture process (r-process) that is thought to occur in exploding stars. This process is responsible for producing about half of all the heavy elements found today in nature, including among many others gold, silver, platinum, and uranium. Where this process takes place is still a mystery and one of the most important open questions in science. Ni-78 acts as kind of a valve in the process, partly due to its “magic” property. The half-life of Ni-78 was found to be substantially shorter than expected, which means that nature can produce heavy elements faster than previously thought.
This work was supported by the National Science Foundation through grants PHY 0110253 and PHY 0216783.
P. Hosmer et al, Phys Rev. Letters 94 (2005) 112501.
schatz at nscl.msu.edu, 517-333-6397.