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

Abstract

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 favour a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion time scales, and ($\alpha$,n) reactions become critical to move matter towards 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 ${}^{96}$Zr($\alpha$,n)${}^{99}$Mo 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 $\le$ E${}_{c.m.}$ $\le$ 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.

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• Measurement of the 91Zr(p,gamma)92mNb cross section motivated by type Ia supernova nucleosynthesis
• Optical potential for incident and emitted low-energy $\alpha$ particles. III. Non-statistical processes induced by neutrons on Zr, Nb, and Mo nuclei
• Measurements of the ${}^{96}$Zr($\alpha$,n)${}^{99}$Mo cross section for astrophysics and applications
• Measurement of $\beta$-particles to determine cross sections relevant to the weak r-process
• Bayesian Analysis of the ${}^{86}$Sr$(\alpha ,\alpha )$ Reaction to Constrain the ${}^{86}$Sr$(\alpha ,n)$ Cross Section at Astrophysical Energies
• The total neutron production from the alpha induced reaction on natural Zirconium
• Astrophysical Reaction Rates for Charged-Particle Induced Reactions on Proton-Rich Nuclides