First Measurement of ⁸⁷Rb(<i>α</i>, <i>xn</i>) Cross Sections at Weak <i>r</i>-process Energies in Supernova <i>ν</i>-driven Ejecta to Investigate Elemental Abundances in Low-metallicity Stars

Abstract Observed abundances of Z ∼ 40 elements in metal-poor stars vary from star to star, indicating that the rapid and slow neutron capture processes may not contribute alone to the synthesis of elements beyond iron. The weak r -process was proposed to produce Z ∼ 40 elements in a subset of old stars. Thought to occur in the ν -driven ejecta of a core-collapse supernova, ( α , xn ) reactions would drive the nuclear flow toward heavier masses at T = 2−5 GK. However, current comparisons between modeled and observed yields do not bring satisfactory insights into the stellar environment, mainly due to the uncertainties of the nuclear physics inputs where the dispersion in a given reaction rate often exceeds 1 order of magnitude. Involved rates are calculated with the statistical model where the choice of an α -optical-model potential ( α OMP) leads to such a poor precision. The first experiment on 87 Rb( α , xn ) reactions at weak r -process energies is reported here. Total inclusive cross sections were assessed at E c.m. = 8.1−13 MeV (3.7−7.6 GK) with the active target MUlti-Sampling Ionization Chamber. With an N = 50 seed nucleus, the measured values agree with statistical model estimates using the α OMP Atomki-V2 . A reevaluated reaction rate was incorporated into new nucleosynthesis calculations, focusing on ν -driven ejecta conditions known to be sensitive to this specific rate. These conditions were found to fail to reproduce the lighter heavy element abundances in metal-poor stars.