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Abstract Radioactive nuclei with lifetimes on the order of millions of years can reveal the formation history of the Sun and active nucleosynthesis occurring at the time and place of its birth^1,2. Among such nuclei whose decay signatures are found in the oldest meteorites,²⁰⁵Pb is a powerful example, as it is produced exclusively by slow neutron captures (thesprocess), with most being synthesized in asymptotic giant branch (AGB) stars^3–5. However, making accurate abundance predictions for²⁰⁵Pb has so far been impossible because the weak decay rates of²⁰⁵Pb and²⁰⁵Tl are very uncertain at stellar temperatures^6,7. To constrain these decay rates, we measured for the first time the bound-state β⁻decay of fully ionized²⁰⁵Tl⁸¹⁺, an exotic decay mode that only occurs in highly charged ions. The measured half-life is 4.7 times longer than the previous theoretical estimate⁸and our 10% experimental uncertainty has eliminated the main nuclear-physics limitation. With new, experimentally backed decay rates, we used AGB stellar models to calculate²⁰⁵Pb yields. Propagating those yields with basic galactic chemical evolution (GCE) and comparing with the²⁰⁵Pb/²⁰⁴Pb ratio from meteorites^9–11, we determined the isolation time of solar material inside its parent molecular cloud. We find positive isolation times that are consistent with the others-process short-lived radioactive nuclei found in the early Solar System. Our results reaffirm the site of the Sun’s birth as a long-lived, giant molecular cloud and support the use of the²⁰⁵Pb–²⁰⁵Tl decay system as a chronometer in the early Solar System. 

The r-process has been shown to be robust in reproducing the abundance distributions of heavy elements, such as europium, seen in ultra-metal poor stars. In contrast, observations of elements 26 < Z < 47 display overabundances relative to r-process model predictions. A proposed additional source of early nucleosynthesis is the weak r-process in neutrino-driven winds of core-collapse supernovae. It has been shown that in this site ( α ,n) reactions are both crucial to nucleosynthesis and the main source of uncertainty in model-based abundance predictions. Aiming to improve the certainty of nucleosynthesis predictions, the cross section of the important reaction 86 Kr( α ,n) 89 Sr has been measured at an energy relevant to the weak r-process. This experiment was conducted in inverse kinematics at TRIUMF with the EMMA recoil mass spectrometer and the TIGRESS gamma-ray spectrometer. A novel type of solid helium target was used.