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Calculating the Heavy from the Light

May 7, 2018

UT’s nuclear theorists and their colleagues have calculated the properties of tin-100 (100Sn), a nucleus that serves as a cornerstone of an entire region of the nuclear chart. Importantly, their predictions were based on interactions that had only been applied to much lighter nuclei, opening the door to calculating heavier nuclei where very little data is available.

Near the Edge, at a Magic Intersection

The Chart of the Nuclides is a kind of colorful, narrow map describing the nuclear landscape—what’s stable, what’s unstable, and a frontier that’s still unknown. Nuclides are a type of atom with a specific number of protons and neutrons. The chart is laid out with protons (Z) along the vertical axis and neutrons (N) along the horizontal access. At certain points, N and Z collide at what scientists call "magic numbers." A nucleus that has either protons or neutrons in specific numbers (2, 8, 20, 28, 50, 82 and 126) has a more stable structure. A nucleus with both protons and neutrons in magic numbers is said to be "doubly magic."

Going into the study, the research team had already deemed tin-100 a "nucleus of superlatives." Nestled near the upper edge of the nuclide chart, it’s near the end of a region with enhanced alpha decays—a process where an atom sheds Helium nuclei to become more stable. It’s also close to the proton dripline—the boundary where a nucleus fails to bind an additional proton. A better understanding of tin-100 means a better understanding of its neighbors in this interesting nuclear territory.

Availing themselves of the Cray XK7 Titan supercomputer at the Oak Ridge Leadership Computing Facility (OLCF), scientists were able to determine the structure of the tin-100 nucleus—no trivial matter considering it includes 100 strongly-interacting protons and neutrons. They found that among its other attributes, tin-100 is also doubly-magic, with 50 neutrons and 50 protons.

More Data for the Map

Physics Professor Thomas Papenbrock is among the scientists involved in the research. He explained that an exciting feature of the work was that the calculations for tin-100 were based on interactions that have only been adjusted to nuclei with only two or three protons and neutrons.

"We were thrilled that an interaction that had only been adjusted to light nuclei did so well compared to the sparse data that exist on tin-100, such as its binding energy and the structure of its neighbor tin-101. This allowed us to trust our predictions for the spectrum of tin-100," he said.

Papenbrock added this is the most microscopic approach to date in outlining the structure of tin-100, which determines the properties of an entire region of the nuclear chart.

"We can think of tin-101 as simply one neutron added to the almost inert core of tin-100," he explained. "This is why doubly magic nuclei are so important. If we understand them and their immediate neighbors, we can make predictions for dozens of other nuclei in the region ‘northwest’ of tin-100 on the nuclear chart."

While the superheavy elements (like tennessine) are the frontier of the nuclear landscape, Papenbrock is optimistic that with better resources and innovative thinking it could also be possible to map out their properties. This collaboration between scientists from UT, Oak Ridge National Laboratory, Reed College (Oregon), TRIUMF (Canada) and the TU Darmstadt (Germany) shows what’s possible when pooling the resources of engaged researchers and powerful user facilities like OLCF, which is part of the U.S. Department of Energy.

The research is explained in the paper "Structure of the Lightest Tin Isotopes," which was published in Physical Review Letters and selected as an Editor’s Suggestion: a designation reserved for papers that not only share interesting results in a given field but also communicate those findings in a way that interests readers from different disciplines.

Along with Papenbrock, authors with UT Physics affiliations include Titus Morris (former postdoc) and Gaute Hagen (adjunct assistant professor). More information on the research is available on OLCF’s website in the article "Nuclear Physicists Wield HPC to Uncover Magic Isotopes."

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