ORNL team reaches into atoms’ depths to look at particle interactions driving nuclear stability
Where do elements come from? How does the strong force bind subatomic particles into nuclei? What can scientists understand from nuclei with unusual proton–neutron ratios? Nuclear physicists at the US Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL) are seeking answers to questions like these.
One element is of particular interest to ORNL. In 2010, ORNL researchers discovered that the nucleus of a tin isotope, tin-132, was doubly “magic.” Isotopes are deemed magic when they have nucleons (positively-charged proton particles or neutrally-charged neutron particles) that complete a shell within the nucleus, making the magic isotopes much more strongly bound than those that are not magic. Isotopes with 2, 8, 20, 28, 50, 82, or 126 neutrons or protons are considered magic. A doubly magic isotope has two of these special numbers—one that describes its number of protons and one that describes its number of neutrons. Tin-132, for example, has 50 protons and 82 neutrons.
Now a team of nuclear physicists at ORNL and collaborators have simulated tin-100, an isotope that researchers have long sought to understand. Tin-100 is not only doubly magic but also possesses the same number of protons and neutrons (50 each). On the chart of nuclides—a table that orders the radioactive behaviors of isotopes—tin-100 exists in a region where, if a proton is added, a proton is ejected from the nucleus in the same way that someone might be eliminated from a game of musical chairs.
Using the Cray XK7 Titan supercomputer at the Oak Ridge Leadership Computing Facility (OLCF), a DOE Office of Science User Facility at ORNL, the team computed the structure of the tin-100 nucleus (and its neighbors), a configuration consisting of 100 strongly interacting particles, and determined that the isotope does, in fact, have a doubly magic nature. The team collaborated with researchers at TRIUMF in Canada; the Institut fur Kernphysik in Germany; and Reed College in Oregon to complete the simulations.
“We are the first to provide a realistic solution of a nuclear 100-body problem starting from forces that describe how two and three nucleons interact with each other,” said Titus Morris, a postdoctoral researcher in ORNL’s Quantum Information Group. “There were so many strongly interacting particles, we really needed a supercomputer so we could describe them as exactly as possible.”
Heavy elements tend to have more neutrons to keep the charged protons apart and lower the energy of the nucleus. Therefore, although tin-100’s equal number of protons and neutrons makes it neutron-deficient and weakly bound, the nuclear force is strong enough to bind it. The results led scientists closer to an understanding of astrophysical phenomena and the elements’ origins. They also provided a look into some of the most fundamental aspects of subatomic particles, which could inform larger simulations of even heavier nuclei.
Heavy nuclei, heavy computations
Based on early calculations used to anticipate particle behavior, the team modeled the tin-100 nucleus on Titan to find out what is really happening inside this heavy exotic isotope.
“We already know how the nuclear force works in light systems,” said Gaute Hagen, a researcher at ORNL. “But we were able to link this much heavier, 100-body system to nuclear forces that were only known well enough in very light nuclei. So it’s a tremendous extrapolation and works surprisingly well at this scale.”
The findings show how scientists can use high-performance computing (HPC) resources to uncover crucial details about atomic nuclei.
“Our simulations of tin-100 can give us a base for studying interactions in even heavier nuclei,” said Thomas Papenbrock, a researcher at the University of Tennessee and ORNL. Because the simulations consisted of 100 strongly interacting nucleons, the project demanded HPC resources. Each of the team’s many runs used up to 3,000 of Titan’s nodes, and the project consumed tens of millions of core hours.
The ORNL team used the NUCCOR code, a nuclear physics application that allows scientists to glimpse into the structure and reactions of atomic nuclei, to complete the simulations. NUCCOR is one of 13 codes selected for the OLCF’s Center for Accelerated Application Readiness (CAAR) project. With the approaching arrival of the OLCF’s next leadership-class supercomputer, the IBM AC922 Summit, CAAR teams are optimizing their codes for the projected 200-petaflop system.
A new magic number
Hagen, Morris, and Gustav Jansen, a computational scientist in the Scientific Computing Group at the OLCF, have also recently used NUCCOR on Titan to decipher how well a given particle interaction predicts the energy and the radii of a system.
The team gathered observable data—such as the charge radius, the amount of energy required to remove nucleons, or the effect on nuclei when more nucleons are added—for carbon isotopes, discovering that the proton number 6 is also magic. This magicity, the quality of carrying a magic number of protons, prevailed through all the neutron-rich carbon isotopes, confirming the results of previous experiments.
“There is a big family of interactions that describes how we think neutrons and protons interact,” Morris said. “They are a little bit different from each other, and we aren’t sure how to make the process of describing these completely robust. Using resources like Titan, we can gain new insights into these interactions.”
Related publications: T. D. Morris, J. Simonis, S. R. Stroberg, C. Stumpf, G. Hagen, J. D. Holt, G. R. Jansen, T. Papenbrock, R. Roth, and A. Schwenk, “Structure of the Lightest Tin Isotopes.” Physical Review Letters 120 (2018): 152503.
D. T. Tran, et al., “Evidence for Z=6 ‘Magic Number’ in Neutron-Rich Carbon Isotopes.” Nature Communications 9 (2018): 1594. doi:10.1038/s41467-018-04024-y.
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