Low-energy Measurement of the ⁹⁶Zr(α,n)⁹⁹Mo 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 favor a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion timescales, and (α,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 α+nucleus optical potential. There are several sets of parameters to calculate the α+nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude. Recently the 96Zr(α,n)99Mo 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 ≤ Ec.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.

Constraining Nucleosynthesis in Neutrino-driven Winds: Observations, Simulations, and Nuclear Physics

“…It was shown in Mohr et al (2020) that this approach is able to reproduce compound formation cross sections at very low sub-Coulomb energies with deviations below a factor of 2 over a wide range of masses. Recent experimental results (Kiss et al 2021;Szegedi et al 2021) on stable isotope elements show a very good agreement with the predicted cross sections and thus confirm the predictive power of this approach.…”

“…That work motivated numerous experimental studies, and new measurements of few (α, n) reaction cross sections for nuclei close to stability have been recently reported (Kiss et al 2021;Szegedi et al 2021). More experiments are currently proposed or are analyzed in nuclear physics facilities around the world.…”

Constraining nucleosynthesis in neutrino-driven winds: observations, simulations and nuclear physics

“…It was shown in Mohr et al (2020) that this approach is able to reproduce compound formation cross sections at very low sub-Coulomb energies with deviations below a factor of two over a wide range of masses. Recent experimental results (Kiss et al 2021;Szegedi et al 2021) on stable isotope elements show a very good agreement with the predicted cross sections and thus confirm the predictive power of this approach.…”

Horizons: Nuclear Astrophysics in the 2020s and Beyond

“…Successful direct reaction-rate measurements have been performed for charged particle reaction rates of importance to heavy element nucleosynthesis in the weak r-, p-, and νp-processes. For a few special cases, precision measurements with stable ion beams are possible, e.g., the activation cross section determinations of the weak r-process reactions 96 Zr(α, n) and 100 Mo(α, n) at ATOMKI [256,257]. For most cases, rare isotope beams are required.…”