Evolution of the N=50 shell gap energy towards ${}^{78}$Ni

Abstract

Atomic masses of the neutron-rich isotopes ${}^{76-80}$Zn, ${}^{78-83}$Ga, ${}^{80-85}Ge,$^{81-87}$Asand$^{84-89}$SehavebeenmeasuredwithhighprecisionusingthePenningtrapmassspectrometerJYFLTRAPattheIGISOLfacility.Themassesof$^{82,83}$Ga,$^{83-85}$Ge,$^{84-87}$Asand$^{89}$Se 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.

4 pages, 4 figures. To be published in Physical Review Letters