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Lighter heavy elements beyond iron and up to around silver can form inneutrino-driven ejecta in core-collapse supernovae and neutron star mergers.Slightly neutron-rich conditions favour a weak r-process that follows a pathclose to stability. Therefore, the beta decays are slow compared to theexpansion time scales, and ($\alpha$,n) reactions become critical to movematter towards heavier nuclei. The rates of these reactions are calculated withthe statistical model and their main uncertainty, at energies relevant for theweak r-process, is the $\alpha$+nucleus optical potential. There are severalsets of parameters to calculate the $\alpha$+nucleus optical potential leadingto large deviations for the reaction rates, exceeding even one order ofmagnitude. Recently the $^{96}$Zr($\alpha$,n)$^{99}$Mo reaction has beenidentified as a key reaction that impacts the production of elements from Ru toCd. Here, we present the first cross section measurement of this reaction atenergies (6.22 MeV $\leq$ E$_\mathrm{c.m.}$ $\leq$ 12.47 MeV) relevant for theweak r-process. The new data provide a stringent test of various modelpredictions which is necessary to improve the precision of the weak r-processnetwork calculations. The strongly reduced reaction rate uncertainty leads tovery well-constrained nucleosynthesis yields for $Z = 44 - 48$ isotopes underdifferent neutrino-driven wind conditions.

Low Energy measurement of the $^{96}\mathrm{Zr}(\alpha,n)^{99}\mathrm{Mo}$ reaction cross section and its impact on weak r-process nucleosynthesis