Nuclear magnetic flips explain low-energy
A study finds low-energy gamma rays from atomic nuclei come from magnetic transitions where protons and neutrons flip their internal magnets.
A decades-old puzzle in nuclear physics has been solved by an experiment at the Facility for Rare Isotope Beams (FRIB). The work, published in Nature in August 2026, shows that magnetic transitions inside atomic nuclei are responsible for a mysterious overproduction of low-energy gamma rays.
For years, scientists have observed that certain nuclei emit far more low-energy gamma rays than theoretical models predict. This phenomenon, called low-energy enhancement, has been difficult to explain or forecast. "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 lead author Eleanor Ronning, a former FRIB graduate student. She noted the difficulty in predicting which nuclei will exhibit the effect.
The new research provides strong evidence linking the effect to magnetism within the nucleus.
Separating electric and magnetic decay
To investigate, the team studied the decay of a radioactive copper isotope into zinc. Using FRIB's specialized instruments, they isolated and examined two distinct decay pathways.
In one pathway, the decay proceeded via an electric transition. This involved protons shifting their positions within the nucleus. The second pathway involved a magnetic transition. Here, the neutrons and protons inside the nucleus effectively flipped their tiny internal magnets.
The key finding was that only the magnetic transition produced the signature low-energy enhancement in gamma rays. This demonstrated the magnetic nature of the long-standing mystery. "We now have a consistent explanation that connects experimental observations with theory," said co-lead author Andrea Richard, a former LLNL postdoctoral researcher now at Ohio University.
Implications for models and security
Although the study focused on a single nucleus, the researchers believe the insight can refine nuclear models for a wide range of elements and reactions. These improved models have broad applications.
LLNL scientist Darren Bleuel, an author on the study, highlighted implications for national security. "We can improve the knowledge of our stockpile performance and interpretation of past test program results using the improved theory based on these discoveries," he said. The findings could also enhance nuclear forensics, aiding in the detection and sourcing of nuclear events.
Beyond security, the research aids astrophysics. It can help scientists better model nuclear reactions in extreme cosmic environments like stars, supernovae, and neutron star mergers. These are the processes that create heavy elements. The work also contributes to a better understanding of processes relevant to nuclear energy. The experiment was a collaborative effort, proposed jointly by Ronning and Richard, with LLNL scientists providing expertise and continuous monitoring during its weeklong, around-the-clock run.
