U.S. Team Traces Source of Anomalous Gamma Rays from Zinc-70, Nuclear Magnetic Transitions May Improve Models of Heavy Element Formation

An international research team led by scientists from the Facility for Rare Isotope Beams (FRIB) at Michigan State University has recently identified the source of the large number of low-energy gamma rays emitted by zinc-70 nuclei. The study suggests that magnetic transitions occurring within the nucleus are the key reason for this anomalous signal enhancement. The findings were published in the journal Nature under the title "Magnetic character of the low-energy enhancement in 70Zn."

Gamma rays are electromagnetic radiation released when excited atomic nuclei transition to lower, more stable energy levels. For a long time, scientists have observed an anomalous increase in the number of low-energy gamma rays in some atomic nuclei, known as the "low-energy enhancement" (LEE), but its physical mechanism had remained unclear. This study provides new experimental evidence for this nuclear physics problem.

The research team focused on zinc-70. Zinc-70 is believed to exhibit the low-energy enhancement effect, and its level structure is relatively well understood. Rather than directly studying zinc-70, the researchers analyzed the β decay of two different energy states of its parent nucleus, copper-70: one being the ground state and the other an excited state or isomer. These two decay paths populate different combinations of energy levels in zinc-70, allowing the researchers to observe its nuclear structure from complementary perspectives.

To obtain a high-purity beam of copper-70 ions, the team used the Low-Energy Beam and Ion Trap (LEBIT) facility at FRIB for preparation, and employed a sum NaI(Tl) detector to record the gamma rays emitted by zinc-70. Subsequently, the researchers applied two analysis methods—the β-Oslo method and the shape method—to determine the gamma-ray strength functions corresponding to different initial states. By comparing the two sets of results, the team confirmed that the low-energy enhancement primarily originates from magnetic transitions within the nucleus.

The researchers stated that these results provide a new experimental benchmark for nuclear theoretical models and offer a viable pathway for further investigating the low-energy enhancement phenomenon in other atomic nuclei. Since the low-energy enhancement affects neutron capture reaction rates, the findings will help improve models of heavy element production in extreme astrophysical environments such as supernova explosions and neutron star mergers.

The project involved researchers from 25 institutions across the United States, Canada, Italy, Germany, Norway, South Korea, and other countries. The research team believes that with the development of related experimental techniques and isomer separation methods, it will be possible to test this mechanism in more atomic nuclei in the future, thereby further refining nuclear structure and nuclear astrophysics models.