High-precision mass measurements of nickel, copper, and gallium isotopes and the purported shell closure at N=40

High-precision mass measurements of more than 30 neutron-rich nuclides around the Z=28 closed proton shell were performed with the triple-trap mass spectrometer ISOLTRAP at ISOLDE/CERN to address the question of a possible neutron shell closure at N=40. The results for Ni-57,Ni-60,Ni-64-69 (Z=28), Cu-65-74,Cu-76 (Z=29), and Ga-63-65,Ga-68-78 (Z=31) have a relative uncertainty of the order of 10(-8). In particular, the mass of Cu-76 was measured for the first time. We analyze the resulting mass surface for signs of magicity, comparing the behavior of N=40 with that of known magic numbers and with midshell behavior. While the classic indications from the mass surface show no evidence for a shell closure at N=40, there is evidence for a weak-and very localized-effect for Z=28, consistent with findings from nuclear spectroscopy studies.