Strange gamma rays could reveal how stars forge heavy elements

Researchers led by scientists at the Facility for Rare Isotope Beams, or FRIB, have identified the source of a puzzling abundance of low-energy gamma rays released by the zinc-70 nucleus. Their results show that magnetic transitions occurring inside the nucleus produce the unexpected signal.

The findings, published in Nature in the study "Magnetic Character of the Low-Energy Enhancement in 70Zn," resolve a long-standing question in nuclear physics and could improve scientific models of how heavy elements are created in space.

The international collaboration brought together researchers from 25 institutions across the United States, Canada, Italy, Germany, Norway and South Korea.

A Surprising Gamma-Ray Signal

Gamma rays belong to the same broad family of electromagnetic radiation as visible light and radio waves. When an excited atomic nucleus moves to a lower and more stable energy state, it can release energy in the form of gamma rays.

Scientists measure how often nuclei emit gamma rays at different energies using a quantity known as the gamma-ray strength function.

Transitions between nuclear energy states are described as either electric or magnetic. Each type reflects a different way that protons and neutrons reorganize inside a nucleus before releasing gamma rays.

For decades, scientists have detected an unexplained rise in the number of low-energy gamma rays emitted by certain nuclei. Known as the low-energy enhancement, or LEE, this feature appears in the gamma-ray strength function, but researchers had not been able to determine what produced it.

The experiment included staff scientists from several national laboratories, including Lawrence Livermore National Laboratory (LLNL) and Los Alamos National Laboratory (both National Nuclear Security Administration, or NNSA, laboratories); Lawrence Berkeley National Laboratory; and Pacific Northwest National Laboratory.

The work also reflects FRIB's larger partnership with national laboratories, which encourages the exchange of knowledge between fundamental research and national security applications. At the same time, the program gives students and early-career scientists practical training that supports the future 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 within the nucleus are responsible for 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."

Why the Discovery Matters for Astrophysics

Explaining LEE is important for understanding the internal structure of nuclei, but its significance extends far beyond nuclear physics.

The enhancement can increase the frequency of neutron-capture reactions beyond what scientists would normally predict. These reactions play a central role in producing heavy elements during extreme cosmic events, including supernovae and neutron star mergers.

Across many different nuclei, the combined effects of LEE can substantially change calculated reaction rates. Those changes can affect models of nuclear activity inside stars, nuclear energy systems, and NNSA national security applications.

Measuring a Faint and Elusive Effect

LEE has been exceptionally difficult to study because scientists cannot easily predict where it will appear. Its signal is also weak and can be obscured by background noise, making highly precise equipment and advanced analysis methods essential.

"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 experimental resources 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 Paths Into Zinc-70

The researchers focused on zinc-70, a nucleus believed to show the low-energy enhancement and one whose arrangement of energy levels is already well understood. Instead of studying zinc-70 directly, they examined the beta decay of two separate states of its parent nucleus, copper-70.

The team isolated copper-70 in two forms: its ground state and an excited, or isomeric, state. These two states provided separate pathways into zinc-70.

Each pathway filled a different combination of energy levels within zinc-70, allowing researchers to observe its nuclear structure from two complementary perspectives.

Producing the two pathways required exceptionally pure beams of both copper-70 states. The researchers created them with FRIB's Low Energy Beam and Ion Trap, or LEBIT, a high-precision mass spectrometer.

"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."

Magnetic Transitions Reveal the Answer

The gamma rays released by zinc-70 were recorded with the Summing NaI, or SuN, detector. Researchers then used two analytical approaches, the beta-Oslo method and the Shape method, to determine the gamma-ray strength function associated with each initial state.

Comparing the two strength functions allowed the team to determine conclusively that magnetic transitions inside the nucleus produce the low-energy enhancement.

The result gives nuclear theorists a new experimental benchmark and provides a strategy for investigating the phenomenon in additional nuclei.

"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."

Training Future Nuclear Scientists

The project also demonstrates the value of collaboration among institutions, established researchers, students and postdoctoral scholars.

Ronning and Richard helped write the experimental proposal during FRIB's second call for proposals by its Program Advisory Committee. Both were early in their careers 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 in nuclear astrophysics and national security, Richard pursued opportunities that connected fundamental science with NNSA mission objectives. That path led to another postdoctoral position at LLNL.

Now based at Ohio University, she continues to contribute to both basic research and national security efforts.

"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, the publication marked the completion of a project she had helped guide from its earliest 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.

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.

Reporting from ScienceDaily contributed to this story.