In a groundbreaking study, nuclear physicists at the University of Tennessee have unveiled three major discoveries that shed light on the intricate process of gold formation. This research, which clarifies the nuclear transformations necessary for creating heavy elements, could significantly enhance models of stellar events and predict the behavior of exotic atomic nuclei.
Heavy elements like gold and platinum are forged under extraordinary conditions, such as during star collapses, explosions, or collisions. These cosmic events trigger the rapid neutron capture process, or r-process, where an atomic nucleus rapidly absorbs neutrons. As the nucleus becomes heavier and more unstable, it eventually breaks down into lighter, more stable forms.
Understanding the R-Process: A Complex Pathway
The r-process involves a sequence where the parent nucleus undergoes beta decay, followed by the release of two neutrons. The atomic nuclei participating in these reactions are extremely rare and unstable, making direct study challenging. Consequently, scientists rely heavily on theoretical models, which must be validated and refined with laboratory data.
Studying Rare Nuclei With CERN’s ISOLDE Facility
To delve deeper into this process, UT researchers collaborated with scientists from several institutions, including UT Graduate Students Peter Dyszel and Jacob Gouge, Professor Robert Grzywacz, Associate Professor Miguel Madurga, and Research Associate Monika Piersa-Silkowska. Their work also leveraged data analysis methods developed by Research Assistant Professor Zhengyu Xu.
The team focused on the rare isotope indium-134, conducting experiments at CERN’s ISOLDE Decay Station. This facility produced abundant indium-134 nuclei and used advanced laser separation techniques to ensure purity. When indium-134 decays, it generates excited forms of tin-134, tin-133, and tin-132.
“These nuclei are hard to make and require a lot of new technology to synthesize in sufficient quantities,” Grzywacz explained.
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 was the first measurement of neutron energies associated with beta-delayed two-neutron emission.
“The two-neutron emission is the biggest deal,” Grzywacz said.
Key Discoveries: A New Field of Study
Beta-delayed two-neutron emission occurs only in exotic nuclei, which are unstable and exist briefly. Although the energy needed to separate two neutrons from the nucleus is typically small, 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. “No one measured energies before, so this approach opens a completely new field.”
This research marks the first detailed study of two-neutron emission from a nucleus along the r-process pathway, providing valuable insights for improving models that describe how stellar events create heavy elements like 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. However, advanced neutron detectors allowed researchers to detect this elusive nuclear state.
“We say the tin doesn’t forget,” Grzywacz said. “This ‘shadow’ of indium doesn’t completely disappear. The memory is not erased.”
The observation suggests that current theoretical explanations are incomplete, necessitating 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.”
The newly observed state represents an intermediate stage in the two-neutron emission sequence, helping complete the nuclear structure picture and improving theoretical calculations.
A Third Discovery Challenges Existing Models
The study also revealed a third important result: a non-statistical population of this newly identified state. In simple terms, the way the state is populated during decay does not follow typical patterns.
“You’re not making split-pea soup,” Grzywacz 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 case 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 like 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, including building frames for neutron tracking detectors, installing electronic systems, constructing beta detectors, performing test measurements, developing data acquisition software, and analyzing data.
“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 during a general chemistry course, when he first learned about beta decay.
“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.”
