New Insights into the Origin of Rare Proton-Rich Isotopes

A recent experiment at the Facility for Rare Isotope Beams (FRIB) has advanced the understanding of proton-rich isotopes, known as p-nuclei, which have puzzled astrophysicists for over six decades. Led by Artemis Tsantiri, now a postdoctoral fellow at the University of Regina, the research team conducted the first measurement of proton capture on arsenic-73, resulting in the formation of selenium-74. This study, published in *Physical Review Letters*, involved over 45 researchers from 20 institutions across the United States, Canada, and Europe.

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The formation of p-nuclei, which range from selenium-74 to mercury-196, cannot be explained by the neutron-capture processes that create many elements heavier than iron. The leading hypothesis for their origin is the gamma process, occurring during specific supernova explosions. In these events, extreme temperatures generate intense gamma radiation that strips neutrons from heavy nuclei, resulting in a higher proton-to-neutron ratio. Over time, nuclear transformations convert some protons into neutrons, leading to the production of p-nuclei.

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In the experiment, researchers directed a beam of arsenic-73 into a chamber filled with hydrogen gas, where the proton capture reaction was observed for the first time in a laboratory setting. The process involved arsenic-73 absorbing a proton to form selenium-74 in an excited state, which then emitted a gamma ray as it transitioned to a stable configuration. This measurement is crucial for understanding the abundance of selenium-74 in the solar system, as it helps clarify both its production and destruction mechanisms during the gamma process.

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The incorporation of experimental data into astrophysical models reduced the uncertainty in the predicted abundance of selenium-74 by half. However, the model still fails to fully account for the observed abundance, suggesting that existing theories regarding the gamma process may require further refinement.

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Artemis Spyrou, a professor of physics at FRIB and Tsantiri's research advisor, remarked, “These results bring us a step closer to understanding the origins of some of the rarest isotopes in the universe.” Tsantiri emphasized the significance of the experiment, stating, “Even though the origin of the p-nuclei has been a topic of study for over 60 years, measurements of important reactions on short-lived isotopes are almost non-existent.”

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While the findings represent a significant step forward, some experts caution that the reduction in uncertainty does not resolve all questions regarding the formation of p-nuclei. Critics argue that the reliance on theoretical models may still overshadow empirical data, necessitating further experimental validation.

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The research was supported by the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, and the Natural Sciences and Engineering Research Council of Canada. Future studies are anticipated to build on these findings, potentially leading to a more comprehensive understanding of the processes that govern the formation of rare isotopes in the universe.

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“Even though the origin of the p-nuclei has been a topic of study for over 60 years, measurements of important reactions on short-lived isotopes are almost non-existent,” — Artemis Tsantiri, Postdoctoral Fellow, University of Regina

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“These results bring us a step closer to understanding the origins of some of the rarest isotopes in the universe,” — Artemis Spyrou, Professor of Physics, FRIB

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“Tsantiri’s work is a nice example of the multidisciplinary collaborations needed for advancing the field, and of the kind of professional development opportunities for early career researchers at FRIB.” — Artemis Spyrou, Professor of Physics, FRIB

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