An international collaboration of scientists succeeded in the measurement of the bound-state beta decay of fully-ionised thallium (thallium-205 81+) ions at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. The experiment, conducted at the Experimental Storage Ring (ESR) of GSI/FAIR and analysed in partnership with TRIUMF, Vancouver, revealed that the half-life of bare thallium-205 (81+) is 291 days. This measurement has profound effects on the production of radioactive lead (lead-205) in asymptotic giant branch (AGB) stars, which were simulated by collaborators at Konkoly Observatory, Budapest, and can be used to help determine how long the Sun took to form in the early Solar System. The results have been published in the journal Nature.
(image: First author and TRIUMF graduate research assistant Guy Leckenby working at the Experimental Storage Ring at GSI/FAIR, Darmstadt)
Bound-state beta decay is an exotic decay mode that only occurs in highly charged ions and can turn a stable atom like thallium-205 (+0) into a radioactive ion when all electrons are removed (as in thallium-205 (81+)). This unique decay mode has so far only been observed at the ESR, which is currently the only device capable of storing millions of fully ionised ions for several hours.
“The measurement of thallium-205 (81+) 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,” says Professor Yury Litvinov of GSI/FAIR, spokesperson of the experiment. “The thallium-205 beam had to be created in GSI/FAIR’s Fragment Separator (FRS) in a nuclear reaction with many injections into the ESR required to reach a sufficient number of stored ions. The FRS team developed a groundbreaking new setting to achieve the required beam intensity for a successful experiment.”
The experiment was conducted in 2020 during the opening weeks of the COVID-19 lockdowns. “COVID definitely threw a spanner in the works, but the dedication of the local team saved the day,” says Guy Leckenby, doctoral student from TRIUMF and first author of the publication. “We perfected the analysis over three years, which was a worthwhile effort as the measured half-life of 291(+33)(-27) days is 5 times as long as what was previously being used. This highlights the importance of making an experimental measurement.”
(image: artist Danielle Adams worked with paper author Guy Leckenby to create a depiction of an AGB star feeding the early Solar System)
“By knowing the half-life, we can now accurately calculate the rates transforming thallium-205 into lead-205 and back in different plasma environments inside stars,” says Dr. Riccardo Mancino, who computed the rates as a post-doctoral researcher in theoretical physics at TU Darmstadt. “With the new experimental result, we were able to provide significantly improved rates for the AGB models.”
The asymptotic giant branch (AGB) refers to stars of ½ to eight times the mass of our Sun at the end of their life cycle and is the site where some elements heavier than iron are produced in a process called slow neutron capture. Researchers from the Konkoly Observatory in Budapest (Hungary), the INAF Osservatorio d'Abruzzo (Italy), and the University of Hull (UK), implemented the new 205Tl/205Pb stellar decay rates in their state-of-the-art AGB astrophysical models. “Whilst the different models see slightly different results, the confidence in the new decay rates means we can be sure that AGB stars produced the lead-205 that once made its way into the gas cloud which formed our Sun,” explains Dr. Maria Lugaro, researcher at Konkoly Observatory. “Given the uncertainties in the amount of extinct lead-205 we currently see in meteorites, it seems that our new lead-205 result is giving a time interval for the collapse of the pre-solar gas cloud that is consistent with other radioactive species produced by the slow neutron capture process. In short, we are starting to assemble evidence for exactly how long it took for our Sun to form over 4.5 billion years ago.”
The measured bound-state beta decay half-life is essential to analyze the accumulation of lead-205 in the interstellar medium. However, more research accounting for the full history of the galaxy is needed to fully comprehend it. In addition to planned simulations of galactic chemical evolution, a further measurement of the neutron capture rate on lead-205 by utilizing the surrogate reaction method in the ESR has been proposed. Numerous additional high-impact experiments are proposed for the new heavy-ion storage rings planned at the future accelerator facility FAIR, which is currently under construction at GSI.
The work is dedicated to deceased colleagues Fritz Bosch, Roberto Gallino, Hans Geissel, Paul Kienle, Fritz Nolden, and Gerald J. Wasserburg, who were supporting this research for many years.
Reference: G. Leckenby, R.S. Sidhu, R.J. Chen, R. Mancino, B. Szanyi et al., Nature 635:8038 (2024)