OHIO physicist’s research published in Nature advances understanding of nuclear behavior

An Ohio University physicist’s research is helping answer a longstanding question in nuclear physics, providing new insight into the behavior of atomic nuclei and the processes that shape the universe.

Andrea Richard, assistant professor in the Department of Physics and Astronomy and researcher at the Edwards Accelerator Lab at Ohio University, co-authored a study published in the journal Nature that provides evidence about the nature of an unusual feature in gamma-ray emissions from atomic nuclei. The research identified the feature as magnetic in character, an important finding for researchers studying nuclear structure and reactions.

The team studied copper-70, a radioactive isotope that decays into zinc-70, examining a phenomenon known as the low-energy enhancement in the gamma-ray strength function. This function describes the probability of gamma-ray emission at different energies as atomic nuclei transition between energy states.

Although scientists had observed the enhancement experimentally, its underlying cause remained debated. Some theories suggested it was electric in nature, while others proposed a magnetic origin. The team’s findings provide experimental evidence that the enhancement is associated with magnetic transitions.

The result gives researchers new information to incorporate into nuclear reaction models used to investigate how elements are synthesized in stars and to study processes relevant to nuclear energy. 

Richard’s research connects fundamental questions about atomic nuclei with broader questions about the universe and spans nuclear structure, statistical nuclear physics and nuclear astrophysics, with potential applications in nuclear energy and national security.

A collaborative approach to research
The study grew from a collaboration between Richard and Eleanor Ronning, who was a graduate student at Michigan State University when the project began and is now working as a post-doc at the National Institute for Nuclear Physics (INFN) in Italy. While working as a postdoctoral researcher at Lawrence Livermore National Laboratory, Richard partnered with Ronning to develop a proposal for an experiment at the Facility for Rare Isotope Beams (FRIB).

The facility, which began operations in May 2022, is the highest power rare-isotope accelerator facility in the United States. Researchers compete for limited experimental time by submitting proposals that outline their scientific questions, explain the significance of the work and detail how the experiment will be conducted.

Applicants work with beam physicists to estimate how much experimental time they will need and develop a comprehensive scientific case for their proposed research. Proposals undergo a competitive review process, with only a select number receiving time to conduct their experiments.

Richard and Ronning developed their proposal together, working with Michigan State faculty members Dr. Sean Liddick, Dr. Artemis Spyrou, and Dr. Ryan Ringle to refine the experimental details and establish how the project would support Ronning’s doctoral dissertation. Their proposal was ultimately accepted, allowing them to carry out the experiment and analyze the resulting data.

Richard said the opportunity to develop and execute the project together was particularly meaningful because both researchers were early in their careers.

“We wrote the proposal together and then carried out the experiment together with our collaborators,” Richard said. “We met online every week, and it was exciting to be able to take an idea from the proposal stage, get it accepted through such a competitive process, execute the experiment and ultimately publish the results in Nature.”

For Ronning, the opportunity to help lead the project from its initial concept through publication was an important part of her scientific training.

“It was satisfying to start with an idea and work with my Ph.D. supervisor and Andrea and turn it into something that became my own little project,” Ronning said.

She said the experience gave her the opportunity to take on substantial responsibility early in her career, from developing the proposal to planning the experiment and analyzing the results.

“I never thought we’d be published in a journal like Nature,” Ronning said. “It was truly a dream come true.”

Ronning also credited Richard with helping her develop the skills and confidence to lead research projects. She said Richard was consistently willing to answer questions and help her navigate unfamiliar coding tools and research techniques, even while working as a postdoctoral researcher herself.

Their collaboration developed into a close working relationship in which they could share ideas and learn from one another.

Richard said involving students in every stage of a project helps them develop a stronger sense of ownership over their research. Rather than joining an existing study and simply receiving data to analyze, students can help shape the scientific question, write the proposal, compete for experimental resources, conduct measurements and interpret the findings.

“We were involved in every aspect of it, from proposing the science to getting the beam time, conducting the experiment and analyzing the data,” Richard said. “That sense of ownership is really valuable, especially for early-career researchers. You’re not just handed data to analyze; you’re helping develop the scientific question and carrying it through the entire process.”

An unexpected discovery
Isotopes are different versions of the same chemical element, distinguished by the number of neutrons in their atomic nuclei. While stable isotopes make up much of the matter found in the world around us, radioactive isotopes decay and can provide researchers with valuable information about how nuclei behave and ultimately how elements are created.

“For this experiment, we produced copper-70, a radioactive isotope, at FRIB and studied how it decays,” Richard said. “What makes copper-70 particularly interesting is that its nucleus can exist in several different energy states, including its ground state and two isomeric states. Those states have different nuclear properties, and we wanted to understand how each one decays and what that could tell us about the nucleus.”

As copper-70 decays, the emission of gamma rays provides information about the energy and properties of the resulting zinc-70 nucleus. Researchers use these emissions to study the ways nuclei transition between energy states and to better understand the forces and structures that govern nuclear behavior.

The team initially set out to investigate a different feature of the decay, but its analysis revealed an unexpected pattern known as the low-energy enhancement in the gamma-ray strength function. The enhancement appears as an increase in gamma-ray strength at low energies and has been the subject of debate among nuclear physicists.

“When we started the project, the low-energy enhancement wasn’t our primary motivation,” Richard said. “We were interested in testing a different feature, and the enhancement emerged from the data. It looks like an upbend in the gamma-ray strength function, almost like a check mark, and researchers have been trying to understand what causes it since it was first observed experimentally.”

The nature of the enhancement has important implications for understanding how atomic nuclei emit gamma rays. Competing theoretical explanations have suggested that the phenomenon could be magnetic or electric in character. The new study provides evidence that the enhancement observed in zinc-70 is magnetic, helping researchers better understand the mechanisms behind the phenomenon.

The finding could have implications beyond the study of a single isotope. Gamma-ray strength functions help scientists calculate nuclear reaction rates used in astrophysics, including models of how elements are synthesized in stars. A better understanding of the underlying physics could also inform research into nuclear reactions and energy.

“Understanding its nature helps us better describe how nuclei behave and improves the information we use in calculations across a range of fields, including astrophysics,” Richard said.

Building on the findings of another Bobcat
The research also builds on decades of work in the field. Alexander Voinov, an OHIO physicist who collaborates with Richard, has studied gamma-ray strength functions and nuclear level densities for approximately 35 years.

In 2004, Voinov and collaborators from the University of Oslo published experimental results showing unusual behavior in gamma-ray strength functions that contradicted theoretical models available at the time. Although the findings initially attracted skepticism, interest in the research area has grown as more scientists have investigated the phenomenon.

“Now many scientific groups, including young generations of scientists, are involved in this area of research worldwide, including our lab,” Voinov said.

Voinov continues to study gamma-ray strength functions and nuclear level densities, conducting experiments at the Edwards Accelerator Laboratory and collaborating with research groups around the world, as well as working directly with Richard. His work focuses on improving nuclear reaction models with applications in astrophysics, national security and nuclear energy.

“I’m glad we share common scientific interests and that I have the opportunity to collaborate with her in this area of research now that she has joined our lab as an assistant professor,” Voinov said.

At OHIO, Richard leads a research group of six graduate students and four undergraduate students investigating statistical nuclear physics. Their work includes studying different isotopes and mass regions, nuclear level densities and measurements relevant to nuclear astrophysics, nuclear energy, and nuclear security.

Richard and her collaborators plan to build on the recent findings by studying additional nuclei and investigating related phenomena, including the scissors mode, to better understand how nuclear structure influences gamma-ray strength functions. The team hopes to determine whether the magnetic behavior observed in the original experiment extends to other nuclei and how the phenomenon changes across different nuclear systems.

Richard, a native of Zanesville, Ohio, earned her master’s degree in experimental nuclear science in 2014 and her Ph.D. in 2018 from OHIO. She was a postdoctoral researcher at Michigan State University’s National Superconducting Cyclotron Laboratory (now FRIB) and Lawrence Livermore National Laboratory before returning to OHIO in January 2024 to join the Department of Physics and Astronomy.