A multi-institutional collaboration has identified the physical origin of the low-energy enhancement observed in atomic nuclei, a phenomenon that has puzzled physicists for decades. By isolating zinc-70 nuclei, researchers at the Facility for Rare Isotope Beams (FRIB) confirmed that magnetic transitions are responsible for the unexpected surplus of low-energy gamma rays.
Atomic nuclei release gamma rays as they transition from excited states to more stable configurations. Physicists characterize this process through the gamma-ray strength function, which measures the frequency of emissions across different energy levels. For years, experimental data showed an unexplained rise in low-energy gamma rays in specific nuclei, a feature known as the low-energy enhancement (LEE).
The study, published in Nature, utilized high-precision instrumentation to distinguish between electric and magnetic transitions. Researchers analyzed the decay of copper-70 into zinc-70, employing two distinct starting states to isolate the signal. This method allowed the team to map the energy levels of zinc-70 with high clarity. The data provided conclusive evidence that magnetic transitions drive the observed enhancement.
The project relied on the Low Energy Beam and Ion Trap (LEBIT) at FRIB to produce pure beams of copper-70 isomers. Ryan Ringle, associate professor of physics at FRIB and LEBIT group leader, noted that this application of the mass spectrometer marked a new experimental milestone. The team subsequently used the Summing NaI (SuN) detector to record gamma-ray emissions, applying the beta-Oslo and Shape analytical methods to interpret the results.
The beta-Oslo method is particularly critical for this research, as it allows for the extraction of the nuclear level density and the gamma-ray strength function from beta-decay data. By analyzing the statistical properties of the gamma-ray cascades, the team could isolate the specific contribution of magnetic dipole transitions within the low-energy regime. This rigorous mathematical approach effectively filters out background noise that previously obscured the signal in earlier, less sensitive experiments.
Eleanor Ronning, the study’s lead author and a postdoctoral fellow at the National Institute for Nuclear Physics in Padova, Italy, emphasized the difficulty of predicting which nuclei exhibit LEE. She noted that the phenomenon was not initially predicted by theoretical models, creating a complex challenge for the nuclear physics community. The new findings now provide a consistent framework that aligns experimental observations with theoretical expectations.
Andrea Richard, co-lead of the study and assistant professor at Ohio University, stated that the results represent a notable step forward for the field. The collaboration spanned 25 institutions, including Lawrence Livermore and Los Alamos national laboratories, reflecting the integration of fundamental research with national security applications. This partnership model provided essential training for early-career researchers who managed the project from proposal to publication.
The resolution of the LEE mystery carries implications for astrophysical models of element formation. Because the enhancement increases the probability of neutron-capture reactions, it influences the synthesis of heavy elements during cataclysmic events like neutron star mergers and supernova explosions. Accurate calculations of these reaction rates are vital for understanding the chemical evolution of the universe.
The findings also impact nuclear energy systems and related national security research. By refining the gamma-ray strength function, scientists can improve the accuracy of models describing stellar nuclear processes. This research demonstrates how advanced experimental capabilities enable the investigation of phenomena that were previously obscured by background noise.
The study highlights the necessity of specialized instrumentation for isolating weak signals within complex nuclear environments. By successfully separating the isomers of copper-70, the team established a new protocol for future investigations. This methodology allows physicists to probe the internal structure of nuclei that were previously inaccessible to standard detection techniques.
Future work will focus on applying the isomer-separation technique to a broader range of nuclei to test the universality of these magnetic transitions. Sean Liddick, professor of chemistry at FRIB, indicated that identifying which nuclei exhibit LEE is essential for designing future experiments. The team intends to use these benchmarks to further improve predictive models of nuclear structure and cosmic element production.
