Heavy elements like gold and platinum are formed under extreme conditions, such as during the collapse, explosion, or collision of stars. These cosmic events initiate the rapid neutron capture process, commonly known as the r-process. In this process, atomic nuclei absorb neutrons successively, leading to the formation of heavier and more unstable nuclei, which eventually decay into lighter, stable forms.
The journey across the nuclide chart typically involves the beta decay of the parent nucleus, followed by the release of two neutrons. However, the atomic nuclei involved in these reactions are rare and unstable, making them challenging to study directly. Consequently, scientists heavily depend on theoretical models, which require validation through laboratory data.
Investigating Rare Nuclei at CERN's ISOLDE Facility
To delve deeper into this phenomenon, researchers from the University of Tennessee (UT) collaborated with scientists from various institutions, including graduate students Peter Dyszel and Jacob Gouge, alongside Professor Robert Grzywacz and others. Their research utilized large quantities of the rare isotope indium-134.
"These nuclei are challenging to produce and necessitate advanced technology to synthesize in adequate amounts," Grzywacz noted.
The experiments were conducted at CERN's ISOLDE Decay Station, where abundant indium-134 nuclei were created using advanced laser separation techniques to ensure their purity. The decay of indium-134 resulted in excited forms of tin-134, tin-133, and tin-132.
Utilizing a neutron detector funded by the National Science Foundation, the team made three significant discoveries. The most notable was the first measurement of neutron energies linked to beta-delayed two-neutron emission.
"The two-neutron emission is groundbreaking," Grzywacz stated.
This emission occurs only in exotic, unstable nuclei. While the energy required to separate two neutrons is minimal, this experiment successfully measured it. "Neutrons tend to scatter, making it difficult to distinguish between one or two emissions," Grzywacz explained, highlighting the novelty of their approach.
This research represents the first comprehensive study of two-neutron emissions along the r-process pathway, offering crucial insights for refining models that explain how stellar events generate heavy elements like gold.
A Long-Sought Neutron State in Tin
The second major discovery involved the first observation of a long-predicted single particle neutron state in tin-133. Grzywacz explained that the nucleus starts in an excited state and must release energy to stabilize.
"Tin exists in an excited state and must cool down. It can eject one or two neutrons, but it should primarily eject two," he added.
This observation challenges the traditional view that the tin nucleus loses memory of its formation. "We say the tin doesn't forget," Grzywacz remarked, indicating that the memory of the decay event persists.
Advanced neutron detectors enabled the identification of this elusive nuclear state, suggesting that current theoretical frameworks are incomplete and must evolve to explain the varying decay behaviors.
A Third Discovery Challenges Existing Models
The research also uncovered a non-statistical population of the newly identified state during decay, deviating from expected patterns. Grzywacz noted the clean decay environment allowed for clearer observations, contrasting with conventional crowded conditions.
These findings imply that as scientists explore less stable regions of the nuclear landscape, particularly with exotic nuclei, existing models may need reevaluation. New theoretical frameworks will likely emerge to better describe these complex systems.
The Curiosity Driving New Discoveries
The pursuit of enhanced models of nuclear structure presents significant opportunities for early-career scientists like Dyszel, who joined Grzywacz's team in 2022 and was the lead author of the findings published in Physical Review Letters.
Dyszel's extensive role involved constructing neutron tracking detectors and analyzing data, showcasing the collaborative nature of the project. "The success of this work is due to my colleagues and collaborators," he acknowledged.
Originally from Jacksonville, Florida, Dyszel's journey into nuclear science began during a chemistry course, leading him to pursue a degree in physics. "Physics has always been my path to understanding how the world works," he shared.