The bound-state $\beta$ decay is an exotic decay mode that only occurs in highly charged ions and can turn a stable atom such as 205Tl into a radioactive ion when all electrons are removed (as in 205Tl81+). This unique decay mode has so far only been observed at the Experimental Storage Ring (ESR) of GSI/FAIR at the Helmholtz Centre for Heavy Ion Research in Darmstadt which the only device currently capable of storing millions of fully ionised heavy ions for several hours.
A half-life almost five times longer
"The measurement of 205Tl81+ was proposed in the 1980s, and we were already involved in a pilot experiment almost 20 years ago, but we were still far from being able to fill the storage ring with enough thallium ions," says ORIGNS scientist Roman Gernhäuser from TUM. "The 205Tl beam had to be generated in a nuclear reaction at the fragment separator (FRS) at GSI/FAIR and transported into the ESR using many injections to achieve a sufficient number of stored ions. The FRS team has developed a groundbreaking new approach to achieve the required beam intensity for a successful experiment," says Professor Yury Litvinov from GSI/FAIR, spokesperson for the experiment.
The experiment was carried out in 2020 during the first weeks of the COVID-19 restrictions. "We were completely surprised and had to learn within days how to support the local team, who did an incredible job, in permanent video conferences," says Roman Gernhäuser. "Our silicon detector array called CsISiPHOS, which we developed at TUM together with TRIUMF and GSI, was also able to provide valuable data. "We then perfected the analysis over three years, which was worth it. The measured half-life of 291(+33)(-27) days is almost five times longer than expected. This shows the importance of an experimental determination," says Guy Leckenby, doctoral student at TRIUMF and first author of the publication.
Laboratory measurements improve star models
“By knowing the half-life, we can now accurately calculate the rates transforming 205Tl into 205Pb and back in different plasma environments inside stars.” says Dr. Riccardo Mancino, who computed the rates at TU Darmstadt and at GSI/FAIR. “With modern computers and the new experimental results, we were able to provide significantly improved rates for the AGB models.”
The asymptotic giant branch (AGB) refers to stars with 0.5 to eight times the mass of our sun at the end of their life cycle. There, elements heavier than iron are created by slow neutron capture in the so-called s-process. Researchers at the Konkoly Observatory in Budapest, the INAF Osservatorio d'Abruzzo and the University of Hull have used the new stellar 205Tl/205Pb stellar decay rates in their astrophysical AGB models to find a time interval for the collapse of the presolar gas cloud that is consistent with other radioactive species. In short, this is a new measurement point for how long the formation of our sun took over 4.5 billion years ago.
The measured half-life of the bound $\beta$ decay is essential for the analysis of the accumulation of lead in the interstellar medium. However, to fully understand it, further research is required taking into account the entire history of the galaxy. In addition to the planned advanced simulations of the chemical evolution of the galaxy, a further measurement of the neutron capture rate of 205Pb using the surrogate reaction method at ESR is proposed. Numerous other high-performance experiments are planned for the new heavy ion storage rings of the future accelerator facility FAIR, which is currently being built at GSI.
The work is dedicated to deceased colleagues Fritz Bosch, Hans Geissel, Paul Kienle, and Fritz Nolden, who were supporting this research for many years
Publication:
G. Leckenby, R.S. Sidhu, R.J. Chen, R. Mancino, B. Szanyi et al. "High-temperature 205Tl decay clarifies 205Pb dating in early Solar System", Nature 2024
Nature Podcast (Interview with Guy Leckenby)
Press Release GSI
Contact:
Dr. Roman Gernhäuser
Technical University Munich / Excellence Cluster ORIGINS
email: roman.gernhaeuser(at)ph.tum.de
