Supercomputer simulations provide new insights into controversial nuclear magnetic excitation of calcium 48

The world’s most powerful supercomputer is helping resolve conflicting research findings that have puzzled scientists for more than a decade, and could also shed new light on the interiors of collapsing stars.

Nuclear physicists at the Department of Energy’s Oak Ridge National Laboratory recently used Frontier, the world’s most powerful supercomputer, to calculate the magnetic properties of the atomic nucleus of calcium-48. Their results, published in the journal Physical Exam Letterswill not only provide a better understanding of how magnetism manifests itself inside other nuclei, but also help resolve a decade-old disagreement between experiments that have drawn different conclusions about the magnetic behavior of calcium-48. In addition, the research could provide new insights into the subatomic interactions that occur inside supernovae.

“The calcium 48 nucleus has an excited state that decays rapidly because it has strong magnetic interactions and one of the strongest transition forces,” said Gaute Hagen, a computational physicist at ORNL. “We are very interested in the rules that govern how nuclei form. Simulating the fundamental forces inside calcium 48 will help us better understand how it is created and may also give us insight into what other nuclei might exist.”

Calcium-48 is an important isotope used in scientific research. Its nucleus is composed of 20 protons and 28 neutrons, a combination that scientists call “double magic.” Magic numbers, such as 20 and 28, are specific numbers of protons or neutrons that provide stability by forming a complete shell within the nucleus.

Calcium-48’s strong bonding and simple structure also make it an interesting test subject for studying the strong and weak nuclear forces that hold particles together or pull them apart.

Like flipping a switch, the scattering of electrons or photons from calcium-48 energizes and excites the nucleus, making it magnetic and causing it to flip. This action, called a magnetic dipole transition, is dominated by the spin flip of a single neutron.

It’s what’s happening at this precise moment that Hagen and his colleagues are trying to understand – a question that has intrigued the scientific community for more than a decade.

A ten-year-old disagreement

In the early 1980s, scientists studied the magnetic dipole transition of calcium-48 by bombarding the isotope with different beams of protons and electrons. The beams energized the nucleus with about 10 megaelectronvolts, or MeV, just enough to trigger a magnetic signature.

They determined that the strength of the magnetic transition was 4 nuclear magnetons squared. Magnetons are units of measurement used in nuclear physics to describe the magnetic behavior of a nucleus.

But in 2011, nearly three decades later, researchers obtained significantly different results after studying the isotope with gamma rays and exciting the nucleus to the same energy level. They measured a magnetic transition force nearly twice as strong as previously recorded.

“As nuclear physicists, we calculate nuclei from scratch based on state-of-the-art theoretical models of nuclear forces,” said co-investigator Thomas Papenbrock, an ORNL physicist and joint faculty member at the University of Tennessee, Knoxville. “The discrepancies between the different experiments prompted us to find out what we would get if we used these theoretical models to study the magnetic transition.”

Free the border

The Frontier supercomputer, operated by the Oak Ridge Leadership Computing Facility, a DOE Office of Science user facility located at ORNL, is the world’s first exascale machine and can perform more than a quintillion, or a billion billion, calculations per second. The system’s incredible computing power allowed Hagen’s team to run simulations with remarkable efficiency and accuracy.

The team used a model called effective chiral field theory to relate nuclear phenomena to the fundamental theory of the strong nuclear force, quantum chromodynamics. They used a powerful numerical method called coupled clusters to calculate the properties of the calcium-48 nucleus. This approach offers a trade-off between high precision and detail and computational cost, making it an ideal task for Frontier.

Simulations showed that the magnetic transition strength of calcium 48 was consistent with the results of gamma-ray experiments.

But they didn’t just seek to shed light on the magnetic dipole transition. They also studied other factors such as so-called continuum effects that describe how the nucleus interacts with its surroundings. They also studied how pairs of nucleons (the particles present in the nucleus of an atom) interact inside the nucleus during the transition and how they contribute to the overall electromagnetic properties.

The simulations showed that continuum effects reduced the magnetic transition strength by about 10%. And, contrary to previous beliefs that nucleon pair interactions suppress or significantly weaken the magnetic transition strength, the simulations showed that in some cases these effects slightly increased the magnetic transition strength.

“We hope this will prompt experimentalists to reexamine their approach and make important adjustments. Or perhaps over time we might learn that the lower values ​​recorded in the 1980s experiments were actually correct,” Hagen said. “That would mean the theory we are using is incomplete, which would also be a shock in many ways. But one way or another, we will learn a lot from it.”

“We hope that these calculations will spark further discussions between theorists and experimentalists,” Papenbrock added. “For now, the ball is in the experimentalists’ court.”

From subatomic to astronomical

Bijaya Acharya, the study’s first author, is a postdoctoral researcher in ORNL’s Theoretical and Computational Physics Group. One of Acharya’s primary responsibilities was developing the algorithms that allowed the team to study many higher-order quantum effects in the simulations. He specializes in the study of neutrinos, tiny particles created by exploding stars that travel through space at close to the speed of light. Neutrinos are generated by nuclear fusion reactions in the solar core and are also produced by nuclear reactors on Earth.

“We observe the abundance of calcium-48 deep in the core of a collapsing supernova, where there is also significant neutrino exposure,” Acharya said. “The physics that describes the magnetic transition force of calcium-48 also describes how neutrinos interact with matter.”

“This suggests that larger transition forces also imply that neutrinos are more likely to interact with matter. So if the value of the magnetic transition force is larger than previously thought, this means that heating and other factors associated with neutrino interactions in supernova explosions would also be larger, and vice versa for smaller values. And this would of course greatly influence our understanding of these very large systems.”

Stars are like alchemists, says Raphael Hix, a nuclear astrophysicist and group leader at ORNL. The stardust emitted by supernovae contains a wide range of newly created nuclei, including calcium-48 in some cases, and these new heavy elements are the source of the creation of new generations of stars and planets.

“You can’t understand how Mother Nature does this in a star unless you understand the rules she follows to put the nuclei together. That’s basically what Hagen’s calculations are about,” Hix said. “And like in alchemy, someone will turn those calculations into interesting reaction rates, which will then be turned into astrophysical calculations to help us better understand the universe.”