New astronomical observations point to a nucleosynthesis picture that goes beyond what was accepted until recently. The intermediate “$i$” process was proposed as a plausible scenario to explain some of the unusual abundance patterns observed in metal-poor stars. The most important nuclear physics properties entering $i$-process calculations are the neutron-capture cross sections and they are almost exclusively not known experimentally. Here we provide the first experimental constraints on the ${}^{139}\mathrm{Ba}(n,\gamma ){}^{140}\mathrm{Ba}$ reaction rate, which is the dominant source of uncertainty for the production of lanthanum, a key indicator of $i$-process conditions. This is an important step towards identifying the exact astrophysical site of stars carrying the $i$-process signature.

• Received 31 January 2023
• Revised 6 June 2023
• Accepted 4 April 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.202701