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 (the s process), with most being synthesized in asymptotic giant branch (AGB) stars^3,4,5. However, making accurate abundance predictions for ²⁰⁵Pb has so far been impossible because the weak decay rates of ²⁰⁵Pb and ²⁰⁵Tl 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 ²⁰⁵Tl⁸¹⁺, an exotic decay mode that only occurs in highly charged ions. The measured half-life is 4.7 times longer than the previous theoretical estimate⁸ 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 ²⁰⁵Pb yields. Propagating those yields with basic galactic chemical evolution (GCE) and comparing with the ²⁰⁵Pb/²⁰⁴Pb ratio from meteorites^9,10,11, 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 ²⁰⁵Pb–²⁰⁵Tl decay system as a chronometer in the early Solar System.

寿命为数百万年的放射性原子核可以揭示太阳的形成历史和在其出生时间和地点发生的活跃核合成^1,2。在最古老的陨石中发现衰变特征的原子核中，²⁰⁵Pb 是一个有力的例子，因为它完全由慢速中子捕获（s 过程）产生，其中大多数是在渐近巨分支 （AGB） 恒星^3,4,5 中合成的。然而，到目前为止，对 ²⁰⁵Pb 进行准确的丰度预测是不可能的，因为 ²⁰⁵Pb 和 ²⁰⁵Tl 的弱衰变速率在恒星温度下非常不确定^6,7。为了限制这些衰变速率，我们首次测量了完全电离 ²⁰⁵Tl⁸¹⁺ 的束缚态 ^β− 衰变，这是一种仅发生在高电荷离子中的奇异衰变模式。测得的半衰期比之前的理论估计⁸ 长 4.7 倍，我们 10% 的实验不确定性消除了主要的核物理学限制。凭借新的、有实验支持的衰变率，我们使用 AGB 恒星模型来计算 ²⁰⁵Pb 的产率。用基本的银河系化学演化 （GCE） 传播这些产率，并与陨石^9,10,11 的 ²⁰⁵Pb/²⁰⁴Pb 比率进行比较，我们确定了太阳物质在其母分子云内的隔离时间。我们发现正隔离时间与太阳系早期发现的其他 s 过程短寿命放射性原子核一致。 我们的结果再次证实了太阳诞生的地方是一个长寿的巨大分子云，并支持在太阳系早期使用 ^{205 Pb-205}Tl 衰变系统作为天文台表。