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From Neutrons to Neutron Stars

UT physicists play a major role in two of three of new Department of Energy SciDAC awards in computational nuclear physics. The combined $2.4M awarded to the university is part of a nationwide initiative to answer fundamental questions about the atomic nucleus and the origins of heavy nuclei.

DOE launched SciDAC (Scientific Discovery through Advanced Computing) in 2001 with the goal of using supercomputers to advance scientific discovery. The program supports basic energy sciences, biological and environmental research, fusion energy, high energy physics, and nuclear physics. For 2017 three SciDAC grants were awarded for computational nuclear physics, with UT faculty heavily involved in two of them. The first is the Nuclear Computational Low Energy Initiative (NUCLEI).

The Sequel is Even Better

The NUCLEI project is dedicated to using math, computer science, and physics to describe all aspects of the atomic nucleus. Nuclei may be small, but they’re sophisticated systems with strong interactions and fascinating properties. UT Physics Professor Thomas Papenbrock is the NUCLEI co-director for physics. The total funding is $10M over five years and involves 13 U.S. universities and national labs, including Oak Ridge National Laboratory. UT’s part includes $1.2M to cover funding for a postdoctoral assistant and sub-grants to the University of North Carolina at Chapel Hill and Iowa State University. Bringing together numerous collaborators from across the country can be challenging, but NUCLEI scientists are old hands at making it work.

The NUCLEI project is in its third act, so to speak, having been funded for nearly 10 years in previous cycles as UNEDF and NUCLEI projects. Papenbrock explained that at the heart of the project is a core team of leaders in their respective scientific communities who have worked well together from the beginning. Their research has led to numerous publications crossing the fields of physics, math, and computer science, with their annual meetings attracting scientists from outside the project.

“The team has complementary skills, and various individuals and sub-teams took joint responsibilities for science deliverables,” Papenbrock said. “We tackle ‘big’ problems that require collaboration not only between physicists but also between physicists and applied mathematicians (and) computer scientists.”

That combined expertise will come in handy in this NUCLEI sequel, where the collaboration will take on the challenge of the nuclear many-body problem for short-lived nuclei that will be studied at the Facility for Rare Isotope Beams (FRIB). Under construction at Michigan State University, FRIB will be the world’s most powerful radioactive ion beam facility. The NUCLEI team will work toward solving the vexing problem of pinning down the interactions of a nucleus’ components. The more components there are—as in complex, neutron-rich nuclei—the more difficult it can be to pin down their properties and interactions. Such rare isotopes live fleeting lives, but studying them is fundamental to understanding the behavior of atomic nuclei. In nature they’re found in neutron stars or supernovae, and FRIB will create most of the rare isotopes created in the cosmos. FRIB is scheduled to come online in 2022, so until then scientists researching nuclear structure and astrophysics will work with facilities at Argonne National Laboratory, the National Superconducting Cyclotron Laboratory, and sites involved with the Association for Research at University Nuclear Accelerators.

As physics lead on the project, Papenbrock will work with an executive council to coordinate efforts and document progress. ORNL’s Gaute Hagen, an adjunct assistant professor with UT Physics, is also involved with the project. Beyond the many-body problem, another pillar of the work is to use nuclear density functional (DFT) approaches to study the full mass range of nuclei. Ultimately, NUCLEI seeks to develop a comprehensive description of nuclei, nuclear reactions, and nucleonic matter.

The When, Where, and How of Heavy Elements

While Papenbrock is helping lead research that ties nuclear theory to both terrestrial and astrophysical environments, UT physicists Andrew W. Steiner, assistant professor, and Anthony Mezzacappa (professor and Newton W. and Wilma C. Thomas Endowed Chair) play key roles in a related SciDAC grant. TEAMS (Toward Exascale Astrophysics Simulations of Mergers and Supernovae), is aimed at furthering our understanding of the rapid neutron capture process (r-process) responsible for most elements heavier than iron. The project involves 12 institutions, with Raph Hix of Oak Ridge National Lab (also a joint faculty professor at UT) serving as principal investigator of the $7.25M, five-year grant. Steiner is UT’s principal investigator, with Mezzacappa as co-principal investigator. UT will receive $1.2M, with funding for graduate students and a computer scientist as well as subcontracts to the University of Washington and the University of Notre Dame.

Within minutes of the Big Bang, newly formed neutrons and protons found one another to fuse into the lighter elements like helium. The evolution of heavier elements still provides research opportunities for scientists, however, and the TEAMS collaboration is going beyond how they came to be—the r-process—to investigate when and where that process takes place. The r-process is carried out over short timescales so that an original nucleus doesn’t decay before it can capture additional neutrons. This requires a neutron-rich environment, indicating a link with core-collapse supernovae or the mergers of their remnant neutron stars. A supernova is the final and spectacular end to the life of massive stars—those with an iron core and mass 10 times that of our sun. As they accumulate layers of lighter elements, they eventually collapse, resulting in neutron stars. These events send multiple signals that scientists want to calculate to learn more about where and how the r-process works. UT’s physicists have much to offer here.

As Steiner explained, “the overarching research goal is to resolve the origin of the r-process, the process by which heavy nuclei are made in the universe. All gold and platinum nuclei on earth, for example, were originally created either in core-collapse supernovae or in neutron star mergers.”

core-collapse simulation
A UT core-collapse simulation. This is the code on which part of the new work funded by the SciDAC grant will be based.

UT’s expertise lies in the multi-scale physics and the commensurate modeling required to simulate supernovae and mergers. Steiner said they’re “building a modern soup-to-nuts description of core-collapse supernovae, from scales of 10-15 meters to tens of thousands of kilometers (107 m). In addition, some of the components will be an important part of the TEAMS infrastructure for simulating neutron star mergers. The last point is particularly important if LIGO detects the gravitational wave signal from a neutron star merger.”

(In 2016, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, first reported the observation of gravitational waves—a ripple in spacetime signaling a cataclysmic event in the distant universe and a major step toward confirming Albert Einstein’s 1915 General Theory of Relativity.)

As with NUCLEI, TEAMS research will support scientific efforts at FRIB. Both projects will also take advantage of high-performance computing locally available—including Titan and (in 2018) Summit at ORNL. Access to Beacon and EVEREST computing power is available through the UT-ORNL Joint Institute for Computational Sciences, where Mezzacappa serves as director.

The TEAMS proposal will likely support graduate students Spencer Beloin and Xingfu Du with the potential to hire another student over the course of the grant. Other UT-related personnel with primary appointments at ORNL include Christian Cardall (adjunct research assistant professor), Eirik Endeve (joint faculty assistant professor), and physics alumnus Bronson Messer, a joint faculty associate professor.

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