Results from an experiment at the Facility for Rare Isotope Beams answer a fundamental question about the structure of atomic nuclei and help explain how different elements formed in space.
For decades, some atomic nuclei have emitted more low-energy gamma rays than theory could explain. A collaboration led by researchers at the Facility for Rare Isotope Beams (FRIB) has now traced this mysterious excess in zinc-70 to magnetic transitions occurring inside the nucleus.
The findings are reported in a study recently published in Nature. By resolving a long-standing problem in nuclear physics, the result could improve calculations used to understand nuclear structure and the formation of elements in space.
The international collaboration brought together scientists from 25 institutions across the United States, Canada, Italy, Germany, Norway and South Korea.
Magnetic transitions explain the excess
Gamma rays belong to the same broad family of electromagnetic radiation as visible light and radio waves. Excited atomic nuclei release gamma rays as they move into lower and more stable energy states. Physicists describe how often nuclei emit gamma rays at different energies through a measurement called the gamma-ray strength function.
Electromagnetic transitions between nuclear states can be either electric or magnetic, depending on how protons and neutrons rearrange inside the nucleus as energy is released. For decades, experiments have revealed an unexpected rise in low-energy gamma rays from certain nuclei. This feature, known as the low-energy enhancement (LEE), appears in the gamma-ray strength function, but its physical origin had remained uncertain.
Scientists from four national laboratories contributed, including the NNSA laboratories Lawrence Livermore and Los Alamos.
The project reflects FRIB’s broader partnerships with national laboratories, which connect fundamental nuclear research with national security applications while providing practical training for future members of the nuclear workforce.
“This low-energy enhancement wasn’t predicted by theory, so it was kind of a shock to the community when it was first observed,” said Eleanor Ronning, lead author of the study and former FRIB graduate student who is now a postdoctoral research fellow at the National Institute for Nuclear Physics in Padova, Italy. “It is difficult to predict where LEE occurs — we don’t know which nuclei will exhibit it.”
The new measurements provide strong evidence that magnetic transitions produce the enhancement.
“This is a key step forward,” said Andrea Richard, co-lead of the study and assistant professor and interim director of the Edwards Accelerator Laboratory at Ohio University. “We now have a consistent explanation that connects experimental observations with theory.”
The finding changes element formation models
Resolving the origin of LEE matters beyond the internal structure of atomic nuclei. The effect also influences the nuclear reactions that create elements in astrophysical environments.
LEE increases the expected frequency of neutron-capture reactions, in which an atomic nucleus absorbs a neutron. These reactions contribute to the production of heavy elements during events such as supernova explosions and neutron star mergers.
When the enhancement affects many nuclei, its combined influence can substantially alter calculated reaction rates. Those changes affect models of stellar nuclear processes, nuclear energy systems and applications connected with the NNSA’s national security work.
Measuring LEE is difficult because the signal is weak and can be obscured by background noise. Separating it requires highly sensitive instruments and carefully developed analytical methods.
“Our collaboration has been searching for ways to identify the nature of this low-energy enhancement in gamma-ray emission for over a decade,” said Artemis Spyrou, professor of physics at FRIB and in Michigan State University’s Department of Physics and Astronomy. “This result only became possible thanks to the development of new experimental capabilities and new analysis techniques that did not exist when we began.”
Sean Liddick, professor of chemistry at FRIB, interim chairperson of MSU’s Department of Chemistry, and Ronning’s graduate advisor, said the study depended on capabilities available only at FRIB.
“We used a novel experimental technique that combines specialized instruments in a way that effectively used the entire facility,” Liddick said. “It is exciting to see that effort lead to such a clear result.”
Two pathways isolate the hidden signal
The researchers focused on low-energy gamma rays from zinc-70, a nucleus suspected of showing LEE and whose arrangement of known energy levels is already well documented. Rather than producing zinc-70 through only one route, they examined the beta decay of two different states of its parent nucleus, copper-70.
One copper-70 beam contained nuclei in the ground state, while the other contained nuclei in an excited state known as an isomer. These two starting points created separate pathways into zinc-70, filling different combinations of its energy levels and offering complementary views of the nucleus.
Producing highly pure beams of both copper-70 states required FRIB’s Low Energy Beam and Ion Trap (LEBIT), a high-precision mass spectrometer capable of separating nuclei with extremely small differences in mass and energy.
“We used LEBIT in this way for the first time,” said Ryan Ringle, associate professor of physics at FRIB and LEBIT group leader. “It was an interesting challenge to work on, which provided additional training opportunities for our group’s graduate students. This new technique for isomer separation opens the door to study many more nuclei and motivates technical developments to expand our capabilities in this area.”
After the copper-70 nuclei decayed, the Summing NaI (SuN) detector recorded the gamma rays released by zinc-70. The researchers then used two analytical approaches, the beta-Oslo method and the Shape method, to calculate the gamma-ray strength function produced through each starting state.
Comparing the two strength functions allowed the researchers to determine conclusively that magnetic transitions inside zinc-70 generate the low-energy enhancement. The result gives nuclear theory a new experimental benchmark and provides guidance for future studies.
“We look forward to applying this separated-isomers technique to more nuclei,” Liddick said. “Knowing which nuclei should exhibit this low-energy enhancement is key to designing experiments to investigate them at facilities like FRIB and to improve models of how elements are created in astrophysical environments.”
Collaboration builds future research capacity
The project also demonstrates how collaborations involving numerous institutions, graduate students and postdoctoral researchers can sustain complex experiments over many years.
Ronning and Richard helped write the experimental proposal for FRIB’s second call for submissions through its Program Advisory Committee. Both were early-career researchers at the time. Ronning was a graduate student at FRIB, while Richard was a postdoctoral scholar at LLNL.
After completing her first postdoctoral position at MSU, where she worked on nuclear astrophysics and national security, Richard sought to connect fundamental research more closely with the NNSA’s mission. That goal led her to a postdoctoral position at LLNL. She now continues working across basic science and national security from Ohio University.
“The combined expertise of our research teams is what really made it all possible,” Richard said. “It was a privilege to work with the various teams across institutions over the years. It was a formative experience as an early-career researcher.”
For Ronning, publication marked the completion of a project she had followed from its earliest planning stages.
“Working on the entire process — from writing the proposal and running the experiment to publishing the paper in Nature — has been a rewarding experience,” Ronning said.
Reference: “Magnetic character of the low-energy enhancement in 70Zn” by E. K. Ronning, A. L. Richard, S. N. Liddick, A. Spyrou, R. Ringle, H. Arora, H. C. Berg, J. M. Berkman, D. L. Bleuel, K. Bosmpotinis, S. E. Campbell, X. Chen, B. P. Crider, R. J. Coleman, P. A. DeYoung, A. A. Doetsch, H. Erington, T. Gaballah, N. D. Gamage, E. C. Good, B. Greaves, A. C. Hartley, J. Huffman, C. M. Ireland, C. Izzo, R. Jain, A. C. Larsen, J. E. L. Larsson, R. S. Lubna, F. M. Maier, M. J. Mogannam, D. Mücher, M. R. Mumpower, G. Owens-Fryar, T. H. Ogunbeku, D. P. Scriven, M. K. Smith, C. S. Sumithrarachchi, A. Sweet, K. Taft, A. Tsantiri, S. Uthayakumaar and M. Wiedeking, 15 July 2026, Nature.
DOI: 10.1038/s41586-026-10758-3
This research is based upon work supported by the U.S. Department of Energy Office, the U.S. National Science Foundation, the National Nuclear Security Administration, the U.S. Nuclear Data Program, the Research Council of Norway, the Norwegian Nuclear Research Center, the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.
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