Sprecher
Beschreibung
Lead-205 initially looks like a very promising candidate to be used as a chronometer for the early Solar System due to its unique position among astrophysically short-lived radionuclides as an s-only isotope probing the termination of the s process [1]. Unfortunately, the 2.3 keV 1/2− first excited state in $^{205}$Pb reduces the half-life in stellar environments by around 6 orders of magnitude, which could severely inhibit $^{205}$Pb production. However, Yokoi et. al. [2] pointed out that the bound-state beta decay of $^{205}$Tl could counter-balance this decay by producing $^{205}$Pb. To clarify the complex production of 205Pb, we measured the bound-state beta decay of $^{205}$Tl$^{81+}$ at the Experimental Storage Ring in GSI, Darmstadt. From the measured half-life, we calculated new weak decay rates for a wide range of astrophysical conditions. AGB stellar nucleosynthesis models based on these new rates saw approximately a factor 2 increase in $^{205}$Pb production (when legacy rates were controlled). With new production ratios, we predicted an updated steady-state interstellar medium (ISM) $^{205}$Pb/$^{204}$Pb ratio. By comparing the ISM ratio to the ratio measured in the earliest meteorites, we derived, for the first time, a positive time interval for the isolation period of the solar material from enrichment. Our new results are also preliminarily consistent with other s-process chronometers. Looking forward, we now aim to investigate the $^{205}$Pb$(n,\gamma)$ cross-section, currently unmeasured at astrophysical energies, using surrogate reactions at the ESR with the NECTAR collaboration.
[1] M. Lugaro, et al. Progress in Particle and Nuclear Physics, 102:1–47, 2018.
[2] K. Yokoi, et al. Astronomy and Astrophysics, 145:339–346, 1985.
