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Down to the Core

December 29, 2013

A neutron star is the dense, collapsed core of a massive star that exploded as a supernova. The neutron star contains about a Sun's worth of mass packed in a sphere the size of a large city. Credit: NASA/Dana Berry.

By Catherine Longmire

Most nuclear physics theorists like Witek Nazarewicz will tell you that they don’t typically go out of their way looking for company. They prefer to stick together and keep their roster fairly short—until now.

Nuclear physicist Witek Nazarewicz

Nuclear physicist Witek Nazarewicz

Thanks to the advent of supercomputers, they’ve begun taking advantage of the common bonds they share with computer scientists and applied mathematicians to broaden their research horizons.

A result of this new partnership is NUCLEI: the Nuclear Computational Low-Energy Initiative, comprised of scientists from fifteen universities and national labs. Its objective is to facilitate new and interesting science with practical benefits for energy production and national security.

Nazarewicz, the James McConnell Distinguished Professor of Physics, co-directs the project sponsored by the US Department of Energy and the National Nuclear Security Administration. He and physics professor Thomas Papenbrock are the stewards of a five-year, $1 million grant funding UT’s participation in the program.

Painting the Picture

With access to the most powerful supercomputers available for open scientific research, NUCLEI scientists perform large-scale calculations to help paint a complete picture of nuclear structure, properties, and dynamics.

They employ sophisticated computer models to investigate the high-energy reactions associated with fusion (when two light nuclei fuse to form a heavier nucleus); fission (when a heavy nucleus splits into two smaller nuclei); and stellar burning (the dramatic aging process of stars that creates almost all natural elements).

One of the benefits of this kind of computing power, Nazarewicz says, is that it provides answers to questions that experiments or theory can’t address, essentially becoming a “third pillar” of nuclear physics.

“It’s a profound statement,” he explains, “because it shows that high-performance computing takes us into the areas where you cannot do experiments, like exploding supernovae. Or we can calculate properties of nuclei created deep in reactor cores. It’s a unique tool.”

Models that improve the understanding of how a nucleus holds together, falls apart, or reacts with other nuclei are critical to predicting the behavior and performance of aging nuclear weapons without resorting to nuclear testing. They may also provide the key to recreating the powerful fusion process found inside our sun, giving rise to clean, safe energy.

Expanding the Network

The high-performance computing work also ties closely with research in applied mathematics and computer science, giving the collaboration its distinctive character and expanding both the team and the tools available for their studies.

“This is a very special joint project,” Nazarewicz explains. “What makes NUCLEI so unique is the involvement of physicists, computer scientists, and applied mathematicians. This is unparalleled worldwide. It’s not only that we have this nice combination, but we are also strongly aligned with big experimental initiatives.”

Those big experiments are pursuing science with global implications. One facility is investigating how nuclear particles can be used to model, diagnose, and cure diseases. Another is attempting to create fusion (and energy) by compressing and heating a pea-sized capsule of deuterium and tritium with 192 laser beams. It’s the equivalent of making a star right here on Earth.

The NUCLEI website houses the team’s publications, reports, conference presentations, and open-source computer codes. This allows the entire nuclear physics community to reap the benefits of the project’s funding.

UT physics students are frequent contributors to this effort, and last year were co-authors on a high-profile paper in Nature that redefined the limits of the nuclear landscape with the expectation that 6,900 (plus or minus 500) nuclei exist, more than double the number currently known.

“The scope of NUCLEI is very broad,” Nazarewicz says, “and of course we involve all our students and post-docs in this work.” This translates into more opportunities for other scientists who have discovered the benefits of having a deep bench.

“People finally realized they can accomplish so much more,” he says. “We know each other so well now and we work so well as a team.” Simply put, the whole that NUCLEI represents is much greater than the sum of its parts.

Visit NUCLEI online for more information at computingnuclei.org