Current estimates suggest the Sun formed from a progenitor molecular cloud in about ten to twenty million years. These calculations are based on the behavior of long-lived radionuclides produced by the astrophysical s-process in AGB stars - stars nearing the end of their life cycle. The traces of these decayed radionuclides appear as slight excesses in meteorites, detected as decay products. The 205Pb nucleus, produced exclusively by the s-process without contamination from other processes, is pivotal for these studies.
On Earth, 205Pb decays to 205Tl by proton conversion with the aid of an atomic electron. The minuscule energy difference between these nuclei is overcome by 205Pb's higher electron binding energy, reversing the decay relationship if electrons are stripped away. This happens in AGB stars, where temperatures of hundreds of millions of Kelvin fully ionize atoms, enabling the decay of 205Tl to 205Pb. However, under typical lab conditions, 205Tl remains stable, making such measurements previously impossible.
The rare decay, where the electron produced is captured in an atomic orbit, is termed bound-state beta decay. Moreover, the nuclear decay predominantly leads to an excited state in 205Pb, just 2.3 kiloelectronvolts above the ground state. The "seesaw" model of 205Tl and 205Pb's interaction depends on temperature, electron density, and the nuclear transition strength, which remained elusive until now.
An international collaboration of scientists from 37 institutions across 12 countries performed the experiment under conditions achievable only at GSI/FAIR's ESR and FRS. "The measurement of 205Tl81+ had been proposed in the 1980s, but it has taken decades of accelerator development and the hard work of many colleagues to bring to fruition," noted Professor Yury Litvinov of GSI/FAIR. He emphasized the innovative techniques developed, including producing bare 205Tl, separating it via the FRS, and managing its storage and observation at the ESR.
Dr. Riccardo Mancino from the Technical University of Darmstadt and GSI/FAIR commented on the importance of this data, explaining, "Knowing the transition strength, we can now accurately calculate the rates at which the seesaw pair 205Tl-205Pb operates at the conditions found in AGB stars."
Teams from Konkoly Observatory, INAF Osservatorio d'Abruzzo, and the University of Hull incorporated the new decay rates into their AGB models. "The new decay rate allows us to predict with confidence how much 205Pb is produced in AGB stars and finds its way into the gas cloud which formed our Sun," said Dr. Maria Lugaro of Konkoly Observatory. Comparing this with meteorite data yields a Sun formation interval of ten to twenty million years, aligning with other slow neutron capture process findings.
"Our result highlights how groundbreaking experimental facilities, collaboration across many research groups, and a lot of hard work can help us understand the processes in the cores of stars. With our new experimental result, we can uncover how long it took our Sun to form 4.6 billion years ago," said Guy Leckenby, doctoral student from TRIUMF and lead author of the study.
While the measured decay half-life is vital for understanding 205Pb accumulation, further nuclear reaction studies, such as the neutron capture rate on 205Pb, are planned at the ESR. These efforts underscore the remarkable capabilities of GSI/FAIR's heavy-ion storage rings in bringing cosmic phenomena into the laboratory.
The team dedicated their work to the late Fritz Bosch, Roberto Gallino, Hans Geissel, Paul Kienle, Fritz Nolden, and Gerald J. Wasserburg, who championed this research for many years.
Research Report:High-temperature 205Tl decay clarifies 205Pb dating in early Solar System
Related Links
GSI Helmholtz Centre for Heavy Ion Research
Solar Science News at SpaceDaily
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