Astrophysical Reaction Rates from In-Beam γ-Ray Spectroscopy: Experimental Studies of Proton Capture Reactions on Stable Rb Isotopes

Cross sections are one of the key parameters for understanding nucleosynthesis in stars. Since the relevant energy ranges in stellar environments are very low and often involve exotic nuclei far from stability, it is often not possible to determine the reaction rates directly through experiments. Therefore, they are usually estimated by extrapolation or with the help of theoretical models. The statistical Hauser-Feshbach model, which is based on nuclear input parameters such as the nuclear level density, the γ-ray strength function, and the optical model potentials, is largely used to describe such processes. This work investigates the extent to which proton capture processes in the stable rubidium isotopes 85Rb and 87Rb can be described by the Hauser–Feshbach model. The γ-process, which occurs during supernova explosions, is currently considered to be the main mechanism for the nucleosynthesis of neutron-deficient p nuclei. Since the reaction network of this process mainly comprises photodisintegration reactions at unstable nuclei, the stellar reaction rates are largely calculated on the basis of theoretical models. However, the accuracy of these predictions can be significantly improved if the underlying nuclear models are verified by experimentally determined absolute cross sections. As part of this work, two proton-induced reactions on 85Rb and 87Rb were therefore investigated using the in-beam method at the HORUS spectrometer at the University of Cologne. Since no published data on the cross sections of these reactions existed to date, the present measurements significantly expand the database on proton-induced reactions. In particular, the measurement on 87Rb contributes to the near completion of the systematic investigation of the N = 50 isotone chain. Challenges arose in particular due to target stability, background suppression, and the low count rates in the relevant energy range. For both rubidium isotopes, the measured cross sections show significant deviations from the default predictions of the Hauser–Feshbach model. The results suggest that the proton optical model potential in particular is responsible for these discrepancies. This is supported by a comparative analysis of (p,n) reaction data, which also cannot be reproduced with the default settings. Changes to the other model parameters — such as the nuclear level density or the γ-ray strength function — did not lead to any significant improvement in the theoretical description. The results obtained in this work thus provide important experimental clues for refining optical model potentials in the mass region around A ≈ 85. They form a basis for future work in which scattering and reaction measurements will further reduce model uncertainties and contribute to more precise astrophysical reaction rates.