It has been a long-standing mystery in nuclear physics: why do the nuclei of some atoms emit more low-energy gamma rays than they should?
The answer can be found in a new study from an international scientific team led by the Facility for Rare Isotope Beams (FRIB) and including authors from Lawrence Livermore National Laboratory (LLNL). Published in Nature, the work sheds light on the internal structure of atomic nuclei and has far-reaching implications for national security and astrophysics.
Gamma rays are a type of electromagnetic radiation like visible light and radio waves. Atomic nuclei in excited states emit gamma rays as they radioactively decay into lower, more stable energy states.
For decades, researchers have observed an unexpected increase in the number of low-energy gamma rays emitted by some nuclei. But the underlying cause of this so-called "low-energy enhancement" has remained unclear. Not all nuclei show the effect, and scientists can't predict when it will pop up.
"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 study provides strong evidence that this enhancement is driven by magnetic transitions in the nuclei.
"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."
To unlock this long-sought solution, the team measured the gamma-ray emission from a radioactive copper isotope as it decayed into zinc. The unique capabilities and specialized instruments at FRIB allowed the team to isolate two different states of decay.
One state followed an electric transition: as the copper decayed, the protons inside its nucleus rearranged their positions. The second state followed a magnetic transition. In that case, the neutrons and protons inside the nucleus essentially flipped their internal magnets.
Only the decay with the magnetic transition showed the low-energy enhancement of gamma rays, proving that the phenomenon is magnetic in nature.
The experiment was proposed jointly by Ronning and Richard. In addition to Richard's work as a postdoctoral researcher, other scientists at LLNL provided expertise and worked around the clock to help monitor the 24/7, weeklong experiments.
While this study only examines a single nucleus, it will lead to improvements across the nuclear landscape.
"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 results will also inform how scientists model nuclear processes in stars, supernovae and neutron star mergers - including the reactions that drive the formation of heavy elements - as well as processes relevant to nuclear energy.
