Low-energy Measurement of the 96Zr(α,n)99Mo Reaction Cross Section and Its Impact on Weak r-process Nucleosynthesis

Lighter heavy elements beyond iron and up to around silver can form in neutrino-driven ejecta in core-collapse supernovae and neutron star mergers. Slightly neutron-rich conditions favor a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion timescales, and (alpha,n) reactions become critical to move matter toward heavier nuclei. The rates of these reactions are calculated with the statistical model and their main uncertainty, at energies relevant for the weak r-process, is the alpha+nucleus optical potential. There are several sets of parameters to calculate the alpha+nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude. Recently the Zr-96(alpha,n)Mo-99 reaction has been identified as a key reaction that impacts the production of elements from Ru to Cd. Here, we present the first cross section measurement of this reaction at energies (6.22 MeV <= E-c.m. <= 12.47 MeV) relevant for the weak r-process. The new data provide a stringent test of various model predictions which is necessary to improve the precision of the weak r-process network calculations. The strongly reduced reaction rate uncertainty leads to very well-constrained nucleosynthesis yields for Z = 44-48 isotopes under different neutrino-driven wind conditions.

Automated summary

The text discusses the formation of lighter heavy elements beyond iron and up to around silver in core-collapse supernovae and neutron star mergers, as well as the importance of α+n reactions in the weak r-process. It presents a statistical model for calculating reaction rates and identifies the main uncertainty at relevant energies. The Zr-96(α,n)Mo-99 reaction is highlighted as a key reaction impacting the production of elements from Ru to Cd, with the first cross section measurement presented for testing various model predictions.