A decades-old puzzle in nuclear physics has centered on a strange question: 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), may provide the answer. Published in Nature, the findings offer new insight into the structure of atomic nuclei and could have important implications for astrophysics, nuclear energy, national security, and nuclear forensics.
Gamma rays are a form of electromagnetic radiation, just like visible light and radio waves. They are released when excited atomic nuclei lose energy and move into lower, more stable states during radioactive decay.
A Longstanding Gamma Ray Mystery
For decades, scientists have noticed that some nuclei emit an unexpectedly large number of low-energy gamma rays. This effect, known as “low-energy enhancement,” has been difficult to explain. It does not appear in every nucleus, and researchers have not been able to reliably predict where it will occur.
“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. “It is difficult to predict where [low-energy enhancement] 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, former postdoctoral researcher at LLNL and current assistant professor at Ohio University. “We now have a consistent explanation that connects experimental observations with theory.”
Separating Electric and Magnetic Nuclear Decay
To investigate the mystery, the researchers measured gamma rays produced as a radioactive copper isotope decayed into zinc. FRIB’s specialized instruments and capabilities allowed the team to separate two distinct decay states and examine them individually.
In one state, the decay involved an electric transition. As the copper transformed, protons inside the nucleus shifted their positions.
The second state involved a magnetic transition. In this case, the neutrons and protons in the nucleus essentially flipped their internal magnets.
Only the magnetic transition produced the low-energy enhancement in gamma rays. That result showed that the phenomenon is magnetic in nature.
Ronning and Richard jointly proposed the experiment. Along with Richard’s work as a postdoctoral researcher, other LLNL scientists contributed their expertise and helped monitor the experiment continuously during the weeklong, 24/7 run.
Implications for Nuclear Science and Security
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 author and 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 the reactions responsible for creating heavy elements — while improving understanding of processes relevant to nuclear energy.
