High-temperature 205Tl decay clarifies 205Pb dating in early Solar System

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, 205 Pb is a powerful example, as it is produced exclusively by slow neutron captures (the s process), with most being synthesized in asymptotic giant branch (AGB) stars 3 – 5 . However, making accurate abundance predictions for 205 Pb has so far been impossible because the weak decay rates of 205 Pb and 205 Tl 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 ionized 205 Tl 81+ , an exotic decay mode that only occurs in highly charged ions. The measured half-life is 4.7 times longer than the previous theoretical estimate 8 and our 10% experimental uncertainty has eliminated the main nuclear-physics limitation. With new, experimentally backed decay rates, we used AGB stellar models to calculate 205 Pb yields. Propagating those yields with basic galactic chemical evolution (GCE) and comparing with the 205 Pb/ 204 Pb 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 other s -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 the 205 Pb– 205 Tl decay system as a chronometer in the early Solar System. Measurement of the bound-state β − decay of 205 Tl 81+ gives a new, longer half-life, allowing for the calculation of accurate stellar 205 Pb yields and the isolation time of the early Solar System.