Evolution of the N=50 gap from Z=30 to Z=38 and extrapolation toward 78Ni

The evolution of the N = 50 gap is analyzed as a function of the occupation of the proton pi f(5/2) and pi p(3/2) orbits. It is based on experimental atomic masses, using three different methods of one-or two-neutron separation energies of ground or isomeric states. We show that the effect of correlations, which is maximized at Z = 32 could be misleading with respect to the determination of the size of the shell gap, especially when using the method with two-neutron separation energies. From the methods that are the least perturbed by correlations, we estimate the N = 50 spherical shell gap in Ni-78(28)50. Whether Ni-78 would be a rigid spherical or deformed nucleus is discussed in comparison with other nuclei in which similar nucleon-nucleon forces are at play.