World’s fastest supercomputer decodes decade-long calcium-48 magnetic mystery

Nuclear physicists at Oak Ridge National Laboratory (ORNL) have achieved a significant breakthrough by utilizing the world’s most powerful supercomputer, Frontier.

They have inched closer to resolving a long-standing disagreement in the field of nuclear physics. Their research is focused on deciphering the magnetic properties of the calcium-48 atomic nucleus.

“The calcium-48 nucleus has an excited state that decays quickly because it has strong magnetic interactions and one of the highest transition strengths,” said Gaute Hagen, a computational physicist at ORNL.

The research’s findings aim to enhance our understanding of magnetism within nuclei and settle a decade-long debate between experiments that produced conflicting results about calcium-48’s magnetic behavior.

This breakthrough could reveal the subatomic interactions inside supernovae, massive star explosions that spread heavy elements across the universe.

“We’re very interested in the rules that govern how nuclei are made. Simulating the fundamental forces inside calcium-48 will help us better understand how it’s created and perhaps also give us some insight into what other nuclei could exist,” added Hagen.

‘Doubly magic’ calcium-48

Calcium-48, a special type of atom used in research, has a very stable structure, making it useful for studying the forces that hold atoms together or break them apart.

“Its nucleus is composed of 20 protons and 28 neutrons — a combination that scientists call doubly magic,” said researchers in a press release.

However, the magnetic behavior of calcium-48 has been a subject of scientific inquiry since the early 1980s.

Initial experiments, conducted using beams of protons and electrons, suggested a magnetic transition strength of 4 nuclear magnetons squared. This value shows how strong the magnetic field gets inside the nucleus during a specific change.

However, in 2011, experiments using gamma rays reported a magnetic transition strength nearly twice as strong. This significant discrepancy created confusion among scientists about the true magnetic behavior of calcium-48.

The team employed chiral effective field theory, which connects observed nuclear phenomena to the fundamental theory of the strong nuclear force, quantum chromodynamics.

“The discrepancies between the different experiments motivated us to find out what result we would get if we used those theoretical models to study the magnetic transition,” said co-investigator Thomas Papenbrock, an ORNL physicist.

Addressing the discrepancy

To address this conundrum, The ORNL team utilized the Frontier supercomputer, the world’s first exascale machine capable of performing over a quintillion calculations per second.

“The system’s incredible computing power enabled Hagen’s team to conduct simulations with remarkable efficiency and precision,” highlighted the press release .

They also used the coupled-cluster method to calculate the properties of the calcium-48 nucleus accurately.

The results of these simulations were definitive. They showed that the magnetic transition strength of calcium-48 matched the results from gamma-ray experiments, settling the long-standing debate in nuclear physics.

Deeper insights into nuclear physics

This research went beyond just clarifying the magnetic behavior of calcium-48. It also revealed insights into continuum effects, which explain how the nucleus interacts with its environment.

Moreover, the simulations exhibited the intricate dance of nucleon pairs (protons and neutrons) within the nucleus during the magnetic transition.

The simulations showed that continuum effects reduced the magnetic transition strength by about 10%.

“And, contrary to previously held beliefs that nucleon pair interactions significantly suppress or weaken the magnetic transition strength, the simulations showed that in some cases, these effects slightly increased the magnetic transition strength.”

Impact on astrophysics

Additionally, the study has implications for astrophysics. Calcium-48 is found abundantly within the cores of collapsing supernovae, where neutrinos, elusive subatomic particles, play a crucial role.

“The physics that describes the magnetic transition strength in calcium-48 also describes how neutrinos interact with matter,” said Bijaya Acharya, the study’s first author.

“So, if the value of the magnetic transition strength is larger than previously thought, that means that reheating and other factors associated with neutrino interactions in supernova explosions would also be larger, and vice versa for smaller values,” Acharya explained.

By unraveling the fundamental rules governing the assembly of nuclei, scientists gain insights into the processes that shape the universe, from the creation of stars and planets to the abundance of elements.

“Someone will turn these calculations into interesting reaction rates, and then those reaction rates will be turned into astrophysics calculations to help us better understand the universe,” concluded ORNL nuclear astrophysicist Raphael Hix.