A scientific team led by Facility for Rare Isotope Beams, or FRIB, researchers identified the origin of a mysterious excess of low-energy gamma rays emitted by the nucleus zinc-70. They found that the excess is caused by magnetic transitions within the nucleus. The study, “Magnetic Character of the Low-Energy Enhancement in 70Zn,” published in Nature, sheds light on a long-standing puzzle in nuclear physics and has far-reaching implications for astrophysics.
The collaboration included scientists from 25 institutions in the United States, Canada, Italy, Germany, Norway and South Korea.
Gamma rays are a type of electromagnetic radiation, like visible light and radio waves. Atomic nuclei in excited states emit gamma rays as they transition into lower, more stable energy states. Physicists study how frequently nuclei emit gamma rays of different energies characterized by the gamma-ray strength function.
Electromagnetic transitions between nuclear states are classified as either electric or magnetic in nature. These transitions represent different ways that the protons and neutrons inside the nucleus rearrange themselves and release gamma rays. For decades, researchers have observed an unexpected increase in the number of low-energy gamma rays emitted by some nuclei. This low-energy enhancement, or LEE, is a feature of the gamma-ray strength function, but its underlying cause has remained unclear.
The experiment involved staff scientists from multiple national laboratories including Lawrence Livermore National Laboratory, or LLNL; Los Alamos National Laboratory (both National Nuclear Security Administration, or NNSA, laboratories); Lawrence Berkeley National Laboratory; and Pacific Northwest National Laboratory. It highlights FRIB’s broader collaborative program with the national laboratories to facilitate an exchange of ideas between basic research and national security applications, while simultaneously providing hands-on training to develop the next generation 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 study provides strong evidence that this enhancement is driven by magnetic transitions.
“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.”
Understanding the low-energy enhancement is important not only for nuclear structure, but also for astrophysics.
LEE causes neutron-capture reactions to occur more frequently than scientists would normally expect. These reactions drive the formation of heavy elements in environments such as supernovae and neutron star mergers.
When the effects of LEE are compounded over many nuclei, they can drastically affect calculations of reaction rates. That, in turn, influences how scientists model nuclear processes in stars, nuclear energy systems and NNSA national security applications.
LEE is difficult to predict and measure. The signal is subtle and often masked by background noise, requiring highly precise instruments and methods to isolate.
“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 experiment relied on capabilities unique to 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.”
The research team set out to investigate the low-energy gamma-ray emission in zinc-70, a nucleus suspected of exhibiting this feature and whose level scheme is well established. To study zinc-70, they examined the beta decay of two distinct states of its parent nucleus, copper-70.
By isolating these two copper-70 states — one in its ground state and one in an excited, or isomeric, state — the team created two different entry paths into the same nucleus. Each pathway populated different configurations of energy levels in zinc-70, providing complementary views of its internal structure.
Creating these two distinct pathways required producing exceptionally pure beams of each copper-70 state using 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.”
The gamma rays emitted by zinc-70 were subsequently captured using the Summing NaI, or SuN, detector. The team applied two analysis techniques — the beta-Oslo method and the Shape method — to extract the gamma-ray strength function for each of the initial states.
By comparing the two gamma-ray strength functions, the researchers conclusively determined the low-energy enhancement is produced by magnetic transitions inside the nucleus. The findings provide a new benchmark for nuclear theory and a roadmap for future experiments.
“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.”
The study highlights the benefit of multiple research institutions and groups, including students and postdoctoral scholars, working together. Ronning and Richard, who co-authored the proposal for the experiment as part of FRIB’s second call for proposals by its Program Advisory Committee, were early-career researchers at the time. Ronning was a graduate student at FRIB, and Richard was a postdoctoral scholar at LLNL.
Following her first postdoctoral position at MSU, where she studied nuclear astrophysics and national security, Richard sought to bridge basic science with NNSA mission goals. This led her to secure a postdoctoral position at LLNL. Today, from her position at Ohio University, she continues to engage with both basic research and national security missions.
“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 project came full circle.
“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.