Nuclear physicists at the University of Tennessee have made three key findings about the rapid neutron-capture process that forms heavy elements like gold in stellar events. Their research, conducted at CERN's ISOLDE facility, clarifies how unstable atomic nuclei decay. The results, published in Physical Review Letters, could refine models of element formation in the universe.
Heavy elements such as gold and platinum originate in extreme cosmic events, like colliding stars, through the rapid neutron-capture process, known as the r-process. In this chain reaction, atomic nuclei absorb neutrons quickly, becoming unstable and decaying into stable forms. Scientists have long struggled to detail these nuclear transformations, particularly for rare, short-lived isotopes.
A team from the University of Tennessee, including graduate students Peter Dyszel and Jacob Gouge, professor Robert Grzywacz, associate professor Miguel Madurga, and research associate Monika Piersa-Silkowska, addressed this gap. They built on data analysis by research assistant professor Zhengyu Xu. The experiments used large quantities of the rare isotope indium-134, produced and purified at CERN's ISOLDE Decay Station.
"These nuclei are hard to make and require a lot of new technology to synthesize in sufficient quantities," Grzywacz explained. Indium-134 decays into excited states of tin-134, tin-133, and tin-132. Using a specialized neutron detector built at UT and funded by the National Science Foundation, the researchers achieved three breakthroughs.
First, they measured neutron energies in beta-delayed two-neutron emission for the first time in an r-process nucleus. "The two-neutron emission is the biggest deal," Grzywacz said, noting the challenge of distinguishing one or two neutrons due to their behavior. This opens new avenues in studying exotic nuclei.
Second, they observed a predicted single-particle neutron state in tin-133, sought for 20 years. "People were searching for it for 20 years and we found it," Grzywacz stated. This state acts as an intermediate in two-neutron emission, showing the nucleus retains a 'memory' of its formation, challenging the idea of an 'amnesiac nucleus.'
Third, the team found a non-statistical population of this state, deviating from expected patterns in cleaner decay environments. Grzywacz likened typical decays to 'split-pea soup' but noted this case did not follow suit.
Dyszel, the study's lead author from Jacksonville, Florida, handled much of the experimental setup and data analysis. His work highlights opportunities for early-career scientists in nuclear physics. The findings suggest current models may need updates for exotic nuclei, improving predictions of heavy element creation.