First Nb 94 ( n , γ ) Measurement: Constraining the Nucleosynthetic Origin of Mo 94 in Presolar Grains

Isotopic measurements of presolar silicon carbide grains from dying stars have revealed a puzzling overabundance of ^{94}Mo that stellar nucleosynthesis models have failed to reproduce for two decades. This discrepancy challenged our understanding of the slow neutron-capture process (s-process) that forges approximately half of the elements heavier than iron. The key uncertainty lies at ^{94}Nb, a radioactive branching point where competition between neutron capture and beta decay governs the ^{94}Mo production, yet the neutron-capture cross section had never been measured. Here, we report the first experimental determination of the ^{94}Nb(n,γ)^{95}Nb cross section important for Mo isotopic abundances. The measurement was enabled by a coordinated effort involving high-purity target preparation at Institute of Solid State and Materials Research Dresden, radioactive sample production at the Institut Laue-Langevin Grenoble, radiochemical characterization at Paul Scherrer Institut Villigen, and the time-of-flight CERN n_TOF facility using for the first time segmented total-energy detectors. Incorporation of the resulting Maxwellian-averaged cross section into fully coupled nucleosynthesis models of low-mass asymptotic giant-branch stars brings them into agreement with the presolar grain data. These results remove a major nuclear-physics input uncertainty at the ^{94}Nb branching point and provide a firmer foundation for understanding the origin of ^{94}Mo in the Solar System.