Scheme illustrating the operation of the two branches at ⁸⁵Kr (ground state) and ⁸⁶Rb that determine the Rb production by the s-process. Black filled nuclei are stable, while the empty ones are unstable. Arrows show possible nuclear reactions, particularly neutron captures or β-decays. In case of low neutron density (N_n < 10⁹ cm⁻³), both branches are closed and the s-process follows the chain ⁸⁴Kr →⁸⁵Kr →⁸⁵Rb →⁸⁶Rb →⁸⁶Sr. Owing to the large neutron-capture cross section of ⁸⁵Kr compared to those of the neutron-magic nuclei (N = 50) ⁸⁸Sr, ⁸⁹Y, and ⁹⁰Zr, a low amount of Rb is produced. On the contrary, at higher neutron density, the two branches are open and the s-process sequence deviates toward ⁸⁶Kr and ⁸⁷Rb, both of which are neutron magic nuclei, and therefore both present very low neutron-capture cross sections. In the latter case, large overabundances of Kr and Rb are found.