Evolution of the N=50 Shell Gap Energy towards ⁷⁸Ni

Atomic masses of the neutron-rich isotopes ^76–80Zn, ^78–83Ga, ^80–85Ge, ^81–87As, and ^84–89Se have been measured with high precision using the Penning trap mass spectrometer JYFLTRAP at the IGISOL facility. The masses of ^82,83Ga, ^83–85Ge, ^84–87As, and ⁸⁹Se were measured for the first time. These new data represent a major improvement in the knowledge of the masses in this neutron-rich region. Two-neutron separation energies provide evidence for the reduction of the N=50 shell gap energy towards germanium (Z=32) and a subsequent increase at gallium (Z=31). The data are compared with a number of theoretical models. An indication of the persistent rigidity of the shell gap towards nickel (Z=28) is obtained.