Magnetic clues inside atomic nuclei help explain how elements form in stars

Scientists have uncovered new insights into how elements form in stars through the discovery of magnetic transitions within atomic nuclei. A team led by the Facility for Rare Isotope Beams (FRIB) has identified the source of an unusual surplus of low-energy gamma rays emitted by the nucleus of zinc-70. Their findings, detailed in a paper titled “Magnetic Character of the Low-Energy Enhancement in 70 Zn” published in Nature, suggest that these gamma rays originate from magnetic transitions within the nucleus. This revelation addresses a longstanding mystery in nuclear physics and offers critical information for understanding stellar nucleosynthesis. The research involved a broad international collaboration, bringing together experts from 25 institutions across the United States, Canada, Italy, Germany, Norway, and South Korea. Scientists from major national laboratories, including Lawrence Livermore National Laboratory, Los Alamos National Laboratory, Lawrence Berkeley National Laboratory, and Pacific Northwest National Laboratory, contributed to the study. The work underscores the growing synergy between fundamental research and practical applications, particularly in areas related to national security and energy production. Gamma rays are high-energy photons released when atomic nuclei shift from higher to lower energy states. These emissions are studied using the concept of the gamma-ray strength function, which describes the likelihood of gamma ray emission at specific energies. For years, physicists have noticed an anomaly known as the low-energy enhancement (LEE), an unexpected rise in low-energy gamma rays emitted by certain nuclei. Despite extensive research, the exact mechanism behind this phenomenon remained unclear. The latest study demonstrates that the LEE is primarily driven by magnetic transitions rather than electric ones. Magnetic transitions involve changes in the orientation of protons and neutrons within the nucleus, releasing gamma rays as a result. This finding challenges previous assumptions and suggests that magnetic interactions play a more significant role in nuclear behavior than previously thought. The results provide a clearer link between experimental data and theoretical models, offering a robust framework for future studies. The implications of this discovery extend beyond nuclear physics into astrophysics. LEEs influence neutron-capture processes, which are essential for creating heavier elements in extreme cosmic environments such as supernovae and neutron star collisions. By increasing the frequency of these reactions, LEEs can significantly alter the rates at which elements form. Accurate modeling of these processes is crucial for understanding the life cycles of stars and the distribution of chemical elements throughout the universe. However, measuring LEEs presents considerable technical challenges. The signals are faint and easily obscured by background interference, necessitating advanced instrumentation and refined analytical techniques. Over the past decade, researchers have worked tirelessly to refine their ability to detect and interpret these subtle phenomena. The breakthrough described in the recent study was made possible by the development of new experimental tools and innovative data analysis methods, marking a pivotal moment in the field. The study’s lead author, Eleanor Ronning, emphasized the significance of the discovery. As a former FRIB graduate student and current postdoctoral researcher at the National Institute for Nuclear Physics in Padova, Italy, she noted that the LEE had initially surprised the scientific community. “It was kind of a shock to the community when it was first observed,” she explained. “It is difficult to predict where LEE occurs, we don’t know which nuclei will exhibit it.” Co-lead Andrea Richard, assistant professor and interim director of the Edwards Accelerator Laboratory at Ohio University, highlighted the importance of connecting experimental findings with theoretical predictions. “We now have a consistent explanation that connects experimental observations with theory,” she stated. This alignment strengthens the foundation for further exploration into the complex interplay between nuclear structure and astrophysical processes.