Speaker
Description
Neutron capture cross sections are one of the key input parameters for an accurate description of the slow (s) process of stellar nucleosynthesis, which is responsible for the production of about half of the elemental solar abundances between Fe and Bi in AGB stars [1]. In this contribution we will present the results of the measurement of the capture cross section of the thallium isotopes $^{204}$Tl and $^{205}$Tl, performed at the n_TOF facility (CERN) between 2015 and 2018, and the implications that the new cross section data have for the production of the important s-process only lead isotopes $^{204}$Pb and $^{205}$Pb.
It is well-known that the s-process of elements heavier than Sr occurs in the Asymptotic Giant Branch (AGB) stage of low mass (1.5 to 3 solar masses) stars [1]. In the short, but intense, neutron irradiation taking place in the recurrent Thermal Pulse episodes, both thallium isotopes become unstable with very short half-lives, and thus they act as branching points of the s-process flow, i.e. nuclei in which capture reactions compete with the decay process. Consequently, the capture cross section of both isotopes strongly affects the s-process abundance of their daughter isotopes, which in the case of $^{204}$Tl is $^{204}$Pb. The s-only stable isotopes such as $^{204}$Pb play a pivotal role in studying the s-process, because they can be used to benchmark state-of-the-art AGB models by comparing nucleosynthesis calculations with observed abundances of these nuclei. A new calculation of the stellar $^{204}$Tl$(\mathrm{n},\gamma)$ cross section, based on the results of the first ever measurement of $^{204}$Tl$(\mathrm{n},\gamma)$ conducted at n_TOF, and the consequences of this result for the $^{204}$Pb production in AGB stars were all reported in a recent publication in Physical Review Letters [2]. With the new results, the uncertainty arising from the $^{204}$Tl$(\mathrm{n},\gamma)$ cross-section on the s-process abundance of $^{204}$Pb could be reduced from $\sim$30\% down to +8\%/ -6\%, and the s-process calculations are in agreement with the latest solar $^{204}$Tl solar abundance of Pb reported by K. Lodders in 2021 [3]. Therefore, presently there is no need to invoke additional nucleosynthesis mechanisms or fractionation effects in order to explain the $^{204}$Pb abundance observed in the solar system.
In the case of $^{205}$Tl, the activation of its bound state beta decay to $^{205}$Pb at stellar temperatures makes the $^{205}$Pb abundance very sensitive to the cross section of the $^{205}$Tl$(\mathrm{n},\gamma)$ reaction. Apart from being s-only, $^{205}$Pb is radioactive with a long half-life of 17.2 Myr, and therefore it has potential to be used as a cosmochronometer of the s process. Recently, the results of the first ever measurement of the stellar decay of $^{205}$Tl, conducted at GSI, were published by Leckenby et al. in Nature [4]. The new results allowed the authors to make the first accurate estimation of the s-process isolation time based on s-process calculations and the current $^{205}$Pb meteoritic abundance. In this context, the latest results of the new measurement of the stellar $^{205}$Tl$(\mathrm{n},\gamma)$ cross section that will be presented in this contribution will permit to further reduce the still relevant nuclear data uncertainty affecting the $^{205}$Pb s-process stellar production.
[1] F. Käppeler, R. Gallino, S. Bisterzo, and W. Aoki, Rev. Mod. Phys. 83, 157 (2011).
[2] A. Casanovas-Hoste et al., Phys. Rev. Lett. 133, 052702 (2024).
[3] K. Lodders, Space Sci. Rev. 217, 44 (2021).
[4] G. Leckenby et al., Nature 635, 321 (2024).
