Gold cannot form until certain unstable atomic nuclei decompose. It has long been difficult to understand exactly how these nuclear changes unfold. Now, nuclear physicists at the University of Tennessee (UT) have reported three discoveries in a single study that reveal key parts of this process. Their findings could help researchers build improved models of stellar phenomena that produce heavy elements and more accurately predict the behavior of exotic nuclei.
Heavy elements such as gold and platinum are forged under unusual conditions such as star collapse, explosions, and collisions. These events trigger a rapid neutron capture process (r-process for short). During this process, the atomic nucleus absorbs neutrons one after another. As the nucleus becomes heavy and unstable, it eventually breaks down into lighter, more stable forms.
A typical sequence along this path across the nuclide chart involves beta decay of the parent nucleus followed by the emission of two neutrons. The nuclei involved in these reactions are so rare and unstable that they are difficult or impossible to study directly in experiments. For this reason, scientists rely heavily on theoretical models, which must be tested and refined using laboratory data.
Researching rare nuclei at CERN’s ISOLDE facility
To investigate this process more closely, UT researchers collaborated with scientists from several institutions. The team included University of Tokyo graduate students Peter Diesel and Jacob Googe, Professor Robert Googe, Associate Professor Miguel Madurga, and Researcher Monika Pielsa-Silkowska. Their research is also based on data analysis techniques developed by research assistant professor Zhengyu Xu.
The researchers started with large amounts of the rare isotope indium-134.
“These nuclei are difficult to make, and many new technologies are needed to synthesize them in sufficient quantities,” Grzywacz explained.
The research team conducted the experiment at CERN’s ISOLDE decay station, where abundant indium-134 nuclei were produced and advanced laser separation techniques were used to ensure their purity. When indium-134 decays, excited states of tin-134, tin-133, and tin-132 are formed.
Using a neutron detector funded through the National Science Foundation’s Major Research Instrumentation Program and built at UT, scientists made three major discoveries. The most important result was the first measurement of the neutron energy associated with beta-delayed 2 neutron emission.
“Emitting two neutrons is the biggest deal,” Grzywacz said.
Emission of beta-delayed 2 neutrons occurs only in exotic nuclei that are unstable and exist only for short periods of time. The energy required to separate two neutrons from an atomic nucleus is very small, but it was enough energy to be measured in this experiment.
“The reason this is difficult is because the neutrons tend to bounce around, so it’s hard to tell if it’s one or two,” Grzywacz explained. This approach “opens up a whole new field” because “no one had measured energy” in previous attempts.
This study is the first detailed study of the emission of two neutrons from a nucleus along the r-process path. The results provide valuable insights for improving models that explain how stellar phenomena produce heavy elements such as gold.
The neutron state of tin that has been sought for many years
The team’s second major discovery was the first observation of the long-predicted single-particle neutron state of tin-133. Grzywacz said atomic nuclei start out in an excited state and must release energy to stabilize.
“Tin is in an excited state. It needs to be cooled. Tin can spit out one neutron, and with enough energy it can spit out two neutrons. It should always spit out two neutrons, but it doesn’t.”
Previously, scientists thought that the tin nucleus simply emitted neutrons to cool down, effectively eliminating any trace of the early beta decay event. In that scenario, the nucleus behaves like an “amnestic nucleus” and has no memory of how it was formed.
“They say you never forget a can,” Grzyvac said. “This ‘shadow’ of indium will not disappear completely. The memory will not disappear.”
Advanced neutron detectors have enabled researchers to detect this elusive nuclear state. This observation suggests that current theoretical explanations are incomplete and that a more sophisticated framework is needed to explain why some decays release one neutron and others release two neutrons.
“People have been looking for it for 20 years, and we found it,” Grzywacz said. “Thanks to these two neutrons, we were able to see this state.”
He pointed out that the newly observed state represents an intermediate step in a two-neutron emission sequence. It also represents the final elementary excitation of the tin-133 nucleus, which helps complete the picture of the nuclear structure and improves the accuracy of theoretical calculations.
A third discovery challenges existing models
The study also revealed a third important finding. Researchers observed a non-statistical population of this newly identified state. Simply put, the state’s population during the collapse does not follow the pattern that scientists typically expect.
Grzywacz described the experiment’s collapse environment as relatively clean. Nuclear-weapon states are not clustered together, but separated.
“You’re not making split pea soup,” he said. “And yet, most of the time, it behaves like split pea soup. Somehow this statistical mechanism happens. Why does it become statistical when it shouldn’t be statistical, and why isn’t it statistical with our cast?”
This finding suggests that existing models may no longer apply as scientists explore areas of the nuclear landscape that are far from stable, especially exotic nuclei such as tennessine nuclei. New theoretical approaches may be required to explain these extreme systems.
Curiosity creates new discoveries
The search for improved models of nuclear structure and element formation provides great opportunities for early career scientists like Diesel. He will join Grzywacz’s research group in 2022 and will be physical review letter A paper describing the findings.
His responsibilities during the experiment were extensive. Dyszel built a frame for the neutron tracking detector and assembled it inside the experimental setup. He installed the electronic systems, built the beta detector, performed test measurements, assisted in the development of the data acquisition software, calibrated the timing system, and analyzed the resulting data. Despite his extensive role, the project remained a collaborative effort involving many researchers.
“The success of this research is also due to my colleagues and collaborators, whose guidance and constructive feedback was invaluable,” he said.
A native of Jacksonville, Fla., Diesel comes to UT after earning a bachelor’s degree in physics from the University of North Florida. His interest in nuclear science began when he first learned about beta decay in a general chemistry course. The idea that nuclear transmutation could create entirely new elements with different properties caught his attention and initially led him to consider a degree in chemistry.
“When I started my bachelor’s degree, I walked into a physics class for the first time, and it instantly inspired me to pursue a physics degree,” he explained. “I’ve always been interested in understanding how the world works, and physics is, and continues to be, the path I want to take to pursue that curiosity.”
