In search for “magic” nuclei, theory catches up to experiments

The bundle of 78 nucleons in a single nickel-78 nucleus are infinitesimal. And yet, calculating that nucleus’s structure entailed an enormous computing effort: 5 million CPU hours on the most powerful supercomputer in the United States. The results could offer key insights into the potential existence of the long-sought “island of stability”: ultraheavy but unusually stable nuclei. The most powerful supercomputer in the United States, Titan at Oak Ridge National Laboratory, needed 5 million CPU hours to calculate the nickel nucleus's structure. Image courtesy of the Oak Ridge National Laboratory and US Department of Energy. In October, three scientists at Oak Ridge National Laboratory and the University of Tennessee in Knoxville published the results of this nickel-78 calculation in Physical Review Letters (1). The researchers demonstrated that the nucleus’s combination of protons and neutrons formed a very stable structure, what physicists call a doubly magic nucleus. It was no small feat: the finding involved solving a quantum mechanical system, including the strong nuclear force acting between 28 protons and 50 neutrons. “The immensity of these calculations are way beyond anything you can think about doing on a laptop or desktop or small cluster,” says Gaute Hagen of Oak Ridge, the study’s first author. It was the latest example of a supercomputing trend that is shaking up the field of nuclear physics. For decades, scientists have sought to find the “magic numbers” of protons and neutrons that form these especially stable nuclei. But although experimentalists have forged heavier and heavier such magic nuclei in the beams of their accelerators, theorists have lacked the computational power to keep up. Although theoretical calculations of the structure and stability of nuclei with small numbers of nucleons have been tractable, there has simply been no way to perform the calculations necessary to accurately predict and …