Gold cannot form until certain unstable atomic nuclei break apart. Exactly how those nuclear transformations unfold has long been difficult to determine. Now, nuclear physicists at the University of Tennessee (UT) report three discoveries in a single study that clarify important parts of this process. Their findings could help researchers build improved models of the stellar events that create heavy elements and better predict the behavior of exotic atomic nuclei.
Heavy elements such as gold and platinum are forged under extraordinary conditions, including when stars collapse, explode, or collide. These events trigger the rapid neutron capture process (or r-process for short). During this process, an atomic nucleus absorbs neutrons in rapid succession. As the nucleus grows heavier and more unstable, it eventually breaks down into lighter and more stable forms.
Along this pathway across the nuclide chart, a common sequence involves beta decay of the parent nucleus followed by the release of two neutrons. The atomic nuclei involved in these reactions are extremely rare and unstable, making them difficult or even impossible to study directly in experiments. Because of this, scientists rely heavily on theoretical models, which must be tested and refined using laboratory data.
Studying Rare Nuclei With CERN's ISOLDE Facility
To investigate the process more closely, UT researchers collaborated with scientists from several institutions. The team included UT Graduate Students Peter Dyszel and Jacob Gouge, Professor Robert Grzywacz, Associate Professor Miguel Madurga, and Research Associate Monika Piersa-Silkowska. Their work also built on data analysis methods developed by Research Assistant Professor Zhengyu Xu.
The researchers began with large quantities of the rare isotope indium-134.
"These nuclei are hard to make and require a lot of new technology to synthesize in sufficient quantities," Grzywacz explained.
The team carried out the experiments at the ISOLDE Decay Station at CERN, which produced abundant indium-134 nuclei and used advanced laser separation techniques to ensure their purity. When indium-134 undergoes decay, it generates excited forms of tin-134, tin-133, and tin-132.
Using a neutron detector funded through the National Science Foundation Major Research Instrumentation program and constructed at UT, the scientists uncovered three major findings. The most significant result was the first measurement of neutron energies associated with beta-delayed two-neutron emission.
"The two-neutron emission is the biggest deal," Grzywacz said.
Beta-delayed two-neutron emission occurs only in exotic nuclei, which are unstable and exist only briefly. The energy needed to separate two neutrons from the nucleus is extremely small, but in this experiment it was large enough to measure.
"The reason this is hard is because neutrons like to bounce around. It's hard to tell if it's one or two," Grzywacz explained. In earlier attempts, "no one measured energies," so this approach "opens a completely new field."
This research marks the first detailed study of two-neutron emission from a nucleus that lies along the r-process pathway. The results provide valuable insight for improving models that describe how stellar events create heavy elements such as gold.
A Long-Sought Neutron State in Tin
The team's second major discovery was the first observation of a long predicted single particle neutron state in tin-133. According to Grzywacz, the nucleus begins in an excited state and must release energy to stabilize.
"Tin is in an excited state. (It) has to cool off. It can spit out a neutron, or, with enough energy, it can spit out two neutrons. It should always spit two neutrons, but it doesn't."
Traditionally, scientists believed the tin nucleus simply released neutrons to cool down, effectively losing any trace of the earlier beta decay event. In that scenario the nucleus behaves like an "amnesiac nucleus," with no memory of how it was formed.
"We say the tin doesn't forget," Grzywacz said. "This 'shadow' of indium doesn't completely disappear. The memory is not erased."
Advanced neutron detectors allowed researchers to detect this elusive nuclear state. The observation suggests that current theoretical explanations are incomplete and that scientists need a more sophisticated framework to explain why some decays release one neutron while others release two.
"People were searching for it for 20 years and we found it," Grzywacz said. "Those two neutrons allowed us to see this state."
He noted that the newly observed state represents an intermediate stage in the two-neutron emission sequence. It also represents the final elementary excitation of the tin-133 nucleus, helping complete the nuclear structure picture and improving the accuracy of theoretical calculations.
A Third Discovery Challenges Existing Models
The study also revealed a third important result. Researchers observed a non statistical population of this newly identified state. In simple terms, the way the state is populated during decay does not follow the patterns that scientists typically expect.
Grzywacz explained that the decay environment in this experiment is relatively clean. The nuclear states are separated rather than crowded together.
"You're not making split-pea soup," he said. "Still, in most cases it behaves like split-pea soup. Somehow this statistical mechanism happens. Why is it statistical, even though it shouldn't be and why in our cast it isn't"?
The findings suggest that as scientists explore regions of the nuclear landscape farther from stability, particularly among exotic nuclei such as Tennessine, existing models may no longer apply. New theoretical approaches will likely be required to describe these extreme systems.
The Curiosity Driving New Discoveries
The search for improved models of nuclear structure and element formation offers major opportunities for early career scientists such as Dyszel. He joined Grzywacz's research group in 2022 and served as the first author of the Physical Review Letters paper describing the discoveries.
His responsibilities during the experiment were extensive. Dyszel built frames for neutron tracking detectors and assembled them within the experimental apparatus. He installed electronic systems, constructed beta detectors, performed test measurements, helped develop data acquisition software, adjusted timing systems, and analyzed the resulting data. Despite his broad role, the project remained a collaborative effort involving many researchers.
"The success of this work is due in part to my colleagues and collaborators, whose guidance and constructive input were crucial," he said.
Originally from Jacksonville, Florida, Dyszel joined UT after earning a bachelor's degree in physics from the University of North Florida. His interest in nuclear science began earlier during a general chemistry course, when he first learned about beta decay. The idea that nuclear transformations could create entirely new elements with different properties captured his attention, initially leading him to consider a degree in chemistry.
"It was not until I started my bachelor's degree that I had stepped foot into a physics class, which instantaneously drove me towards a degree in physics," he explained. "I've always been interested in understanding how the world works, and physics has been, and continues to be, the path I want to follow in pursuit of that curiosity."