Hidden Magnetism Inside Atoms May Explain Mysterious Gamma Rays

A decades-old puzzle in nuclear physics may finally have an answer: why do some atomic nuclei release more low-energy gamma rays than scientists expect? A new study led by the Facility for Rare Isotope Beams (FRIB), with researchers from Lawrence Livermore National Laboratory (LLNL), published in the journal Nature, offers new insight into the structure of atomic nuclei with implications for astrophysics, nuclear energy, national security and nuclear forensics.

Gamma rays are a form of electromagnetic radiation, like visible light and radio waves, released when excited atomic nuclei lose energy and move into lower, more stable states during radioactive decay. For decades, scientists have noticed that some nuclei emit an unexpectedly large number of low-energy gamma rays, an effect known as "low-energy enhancement" that does not appear in every nucleus. "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 a former FRIB graduate student. "It is difficult to predict where it occurs — we don't know which nuclei will exhibit it."

The new results provide strong evidence that magnetic transitions within the nucleus are responsible for the effect. "This is a key step forward," said Andrea Richard, co-lead of the study, a former postdoctoral researcher at LLNL and now an assistant professor at Ohio University. "We now have a consistent explanation that connects experimental observations with theory."

To investigate, the researchers measured gamma rays produced as a radioactive copper isotope decayed into zinc. FRIB's specialized instruments allowed the team to separate two distinct decay states and examine them individually: one involved an electric transition, in which protons inside the nucleus shifted position, and the other a magnetic transition, in which neutrons and protons essentially flipped their internal magnets. Only the magnetic transition produced the low-energy enhancement in gamma rays, showing the phenomenon is magnetic in nature.

Although the study focused on just one nucleus, the researchers say the findings could improve nuclear models across a much wider range of elements and reactions. "We can improve the knowledge of our stockpile performance and interpretation of past test program results using the improved theory based on these discoveries," said LLNL scientist Darren Bleuel. "In addition, we can improve nuclear forensics — our ability to determine if a nuclear event has occurred and identify the most likely source." The findings could also help scientists better model nuclear reactions in stars, supernovae and neutron star mergers, including reactions responsible for creating heavy elements.