“`html
Have you ever been curious about how long it took for our Sun to form in its stellar nursery? A global group of scientists has made strides toward an answer. They successfully measured the bound-state beta decay of fully-ionized thallium (205Tl81+) ions at the Experimental Storage Ring (ESR) of GSI/FAIR. This measurement significantly impacts the creation of radioactive lead (205Pb) in asymptotic giant branch (AGB) stars, aiding in the estimation of the Sun’s formation timeline. The findings have been featured in the journal Nature.
Current estimates suggest the Sun formed from its parent molecular cloud over a span of several tens of millions of years. This estimate arises from long-lived radionuclides created shortly before the Sun’s formation via a process known as the astrophysical s-process. This s-process occurred in nearby AGB stars, which are intermediate mass stars nearing the end of their life cycles. The radionuclides, having decayed long ago since the Sun’s birth 4.6 billion years ago, left detectable signatures as slight excess amounts of decay products in meteorites. The ideal candidate for study is a radionuclide produced solely by the s-process, free from contamination by other nucleosynthesis processes. The only fitting “s-only” nuclide is 205Pb.
On Earth, 205Pb decays into 205Tl by converting a proton and an electron into a neutron and an electron neutrino. The difference in energy between 205Pb and its daughter 205Tl is minimal; however, the higher binding energies of the electrons in 205Pb (with atomic number Z=82, compared to the 81 electrons in 205Tl) influences the decay direction. In simpler terms, if all electrons are removed, the roles of the parent and daughter nuclides switch, leading 205Tl to undergo beta minus decay to become 205Pb. This process occurs in AGB stars where temperatures reach hundreds of millions of Kelvin, enough to completely ionize the atoms. The amount of 205Pb generated in AGB stars critically depends on the decay rate of 205Tl to 205Pb. However, this decay can’t be measured under standard lab settings since 205Tl is stable there.
The decay of 205Tl can only take place if the resulting electron is captured into one of the bound atomic orbits in 205Pb. This represents an exceptionally rare decay process known as bound-state beta decay. Furthermore, the nuclear decay results in an excited state in 205Pb, positioned only 2.3 kiloelectronvolts above its ground state, and is strongly favored over decay to the ground state. The connection between 205Tl and 205Pb can be likened to a stellar seesaw model, where both decay pathways are possible, and the favored direction depends on conditions such as temperature and electron density, alongside the nuclear transition strength, which was previously unknown.
Thanks to a brilliant experiment carried out by an international team of 37 institutions from twelve countries, this unknown has now been unveiled. Bound-state beta decay is measurable only when the decaying nucleus is stripped of all its electrons and maintained under these peculiar conditions for several hours. This is uniquely achievable at GSI/FAIR’s heavy-ion Experimental Storage Ring (ESR) combined with the fragment separator (FRS). “The measurement of 205Tl81+ was proposed back in the 1980s, but it took decades of accelerator advancements and the dedication of many colleagues to bring it to fruition,” states Professor Yury Litvinov from GSI/FAIR, who is the spokesperson for the experiment. “A comprehensive set of novel techniques was essential to create the necessary conditions for a successful experiment, including the generation of bare 205Tl through a nuclear reaction, followed by its separation in the FRS and subsequent accumulation, cooling, storage, and monitoring in the ESR.”
“Now that we understand the transition strength, we can accurately calculate how the 205Tl-205Pb system operates under conditions typical of AGB stars,” remarks Dr. Riccardo Mancino, who conducted the calculations as a post-doctoral researcher at the Technical University of Darmstadt and GSI/FAIR.
Researchers from the Konkoly Observatory in Budapest (Hungary), the INAF Osservatorio d’Abruzzo (Italy), and the University of Hull (UK) have derived the 205Pb production yield in AGB stars by incorporating the new 205Tl/205Pb stellar decay rates in their advanced AGB astrophysical models. “The updated decay rate empowers us to confidently predict how much 205Pb is produced in AGB stars and eventually ends up in the gas cloud forming our Sun,” explains Dr. Maria Lugaro from Konkoly Observatory. “By comparing this with the 205Pb quantities inferred from meteorites, we conclude that the time frame for the Sun’s formation from its progenitor molecular cloud ranges from ten to twenty million years, consistent with other radioactive isotopes formed through the slow neutron capture process.”
“Our findings emphasize the significance of cutting-edge experimental facilities, collaborative efforts across numerous research groups, and dedicated hard work in enhancing our understanding of processes occurring in the cores of stars. With our new experimental results, we can reveal the timeline for the formation of our Sun 4.6 billion years ago,” states Guy Leckenby, a PhD student from TRIUMF and the lead author of the publication.
The measured half-life of the bound-state beta decay is vital for analyzing the accumulation of 205Pb in the interstellar medium. Nonetheless, other nuclear reactions are equally important, such as the neutron capture rate on 205Pb, with experimental plans utilizing the surrogate reaction method in the ESR. These findings underscore the unique opportunities provided by the heavy-ion storage rings at GSI/FAIR, allowing the vast universe to be explored in the lab.
The efforts are dedicated to the memory of late colleagues Fritz Bosch, Roberto Gallino, Hans Geissel, Paul Kienle, Fritz Nolden, and Gerald J. Wasserburg, who supported this research for many years.
“`
