Scientists detect first-ever beta-delayed neutron emission from rare fluorine isotope

A research team at the Facility for Rare Isotope Beams (FRIB) is the first ever to observe a beta-delayed neutron emission from fluorine-25, a rare, unstable nuclide. Using the FRIB Decay Station Initiator (FDSi), the team found contradictions in prior experimental findings. The results led to a new line of inquiry into how particles in exotic, unstable isotopes remain bound under extreme conditions. Led by Robert Grzywacz, professor of physics at the University of Tennessee, Knoxville (UTK), the team included Jack Peltier, undergraduate student at UTK, Zhengyu Xu, postdoctoral researcher at UTK, Sean Liddick, professor of chemistry at FRIB and interim chairperson of MSU’s Department of Chemistry, and Rebeka Lubna, scientist at FRIB. The team published its results (“The evidence of N = 16 shell closure and β-delayed neutron emission from ²⁵F”) in Physics Letters B.

“The different results on decay lifetime we obtained for fluorine-25 were similar to previously measured decay of oxygen-24. And while we are not entirely certain why we found this difference between previously published results, we have conducted numerous checks on our results and are confident in our findings,” Grzywacz said.

Magic nuclei help scientists map the island of inversion

As an atom’s electrons follow specific orbits around a nucleus, these electrons form “shells” at different energy levels. The number of electrons a single shell can hold varies depending on the element in question and the energy level of the environment. When a shell is at or near its capacity for electrons, it is more atomically stable.

Something similar happens with protons and neutrons within an atomic nucleus. Similar to the number of orbiting electrons outside the nucleus, the shell inside the nucleus describes the number of protons or neutrons that can fit in the nucleus at a given energy level. Over time, researchers also noticed that certain numbers of either protons or neutrons increase the stability of the nucleus in the same way that filled electron shells increase stability. Scientists rely on these “magic numbers” of protons and neutrons to evaluate and predict the stability of a nuclide. Nuclei can also be doubly magic, meaning that both protons and neutrons have filled their respective shells, resulting in further increased stability.

While magic numbers are a helpful guide for identifying stable nuclides, nuclear physicists need a combination of theory and experiment to describe the most unstable nuclei that may only exist for milliseconds before decaying. In the 1970s, for example, researchers studying exotic lithium and sodium isotopes discovered an “island of inversion” where certain nuclides showed remarkable stability compared to their chaotic, short-lived neighbors. Since that time, researchers have used these outlying nuclides to better understand their chaotic, short-lived neighbors in the atomic chart.

However, researchers in recent years have started seeing signs that well-established magic numbers may not apply to every nuclide or under all circumstances. Recently, Grzywacz and his collaborators discovered just such an anomaly while studying oxygen-24, a nucleus expected to be unstable and short-lived near the edge of the island of inversion. To the team’s surprise, oxygen-24 behaved as if it were doubly magic when studied using instruments at FRIB’s predecessor, the National Superconducting Cyclotron Laboratory.

In a following study at FRIB, the team incidentally saw evidence that fluorine-25 may also demonstrate more stability than expected. “When we run experiments at FRIB, we are usually generating many isotopes at the same time and delivering them to the experimental system,” Liddick said. “In proposing an experiment, we focus on a particular nucleus that we think is going to have a high impact on the system, but we know there will always be additional results to look through in the data. In this case, not only could we look at oxygen-24, but also observe the decay of other isotopes into neighboring nuclei.”

Using FDSi, the team studied the beta decay of fluorine-25, the isotope with 16 neutrons, observing its daughter, neon-25, eject a neutron for the first time experimentally. The team’s experiment not only contradicted prior reaction experiments conducted in 2020, but it also increased the probability that 16 could serve as a robust magic number for neutrons in the isotopes near oxygen-24. “This work shows that there may be more to the story here than we originally thought,” Xu said. “In publishing this work, we wanted to let the community know that there are other things going on in this border region of the island of inversion that we might not fully understand yet. We need to continue to couple experimental and theoretical work to investigate these phenomena more thoroughly.”

Cross-institution collaboration pushes the boundaries of nuclear physics

Liddick pointed out that while use of FDSi at FRIB was pivotal in this research, a broad collaboration between FRIB, UTK, Argonne National Laboratory, and Oak Ridge National Laboratory conceived and implemented the instrument. “My role in the system is acting as the FRIB local contact,” Liddick said. “I interact with the facility and make sure that everyone is on the same page when we are running experiments, but these large collaborations take a lot of people and expertise to work. All these institutions have their own detection systems, and they want to bring them here to FRIB for experiments and engage their students in detector development and the research itself.”

For Peltier, running an experiment in collaboration with a multi-institutional team of physicists not only resulted in a journal publication early in his academic career, but it also added insights into his future career path. “This experience has both sharpened my focus on a field of study and expanded the world within that field. I now know I want to focus on nuclear physics, and I’ve been exposed to these different focus areas within the field that I was unaware of previously,” he said.

Eric Gedenk is a freelance science writer.

Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), with financial support from and furthering the mission of the DOE-SC Office of Nuclear Physics. Hosting the most powerful heavy-ion accelerator, FRIB enables scientists to make discoveries about the properties of rare isotopes in order to better understand the physics of nuclei, nuclear astrophysics, fundamental interactions, and applications for society, including in medicine, homeland security, and industry. User facility operation is supported by the DOE-SC Office of Nuclear Physics as one of 28 DOE-SC user facilities.

This research used resources of the National Superconducting Cyclotron Laboratory (NSCL), which was a National Science Foundation (NSF)-supported user facility.

The U.S. Department of Energy Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of today’s most pressing challenges. For more information, visit energy.gov/science.