Mass measurements beyond the major r-process waiting point $^{80}$Zn

High-precision mass measurements on neutron-rich zinc isotopes $^{71m,72-81}$Zn have been performed with the Penning trap mass spectrometer ISOLTRAP. For the first time the mass of $^{81}$Zn has been experimentally determined. This makes $^{80}$Zn the first of the few major waiting points along the path of the astrophysical rapid neutron capture process where neutron separation energy and neutron capture $Q$-value are determined experimentally. As a consequence, the astrophysical conditions required for this waiting point and its associated abundance signatures to occur in $r$-process models can now be mapped precisely. The measurements also confirm the robustness of the $N = 50$ shell closure for $Z = 30$ farther from stability.