Monopole migration in69,71,73Cuobserved fromβdecay of laser-ionized68–74Ni

The neutron-rich isotopes ${}^{68–74}\mathrm{Ni}$ have been produced at the LISOL facility in the fission of ${}^{238}\mathrm{U}$ induced by a 30-MeV proton beam. They have been extracted selectively through resonant laser ionization and electromagnetic mass separation. Production rates have been deduced and deviate from theoretical predictions. The $\beta$ decay of the isotopes has been investigated by means of $\beta -\gamma$ and $\gamma -\gamma$ spectroscopy. Half-lives have been determined for ${}^{68–74}\mathrm{Ni},$ and the level scheme has been extended for ${}^{69}\mathrm{Cu}$ and newly established for ${}^{71,73}\mathrm{Cu}.$ The constructed schemes show an unexpected and sharp lowering of the $\pi f_{5/2}$ orbital with respect to the $\pi p_{3/2}$ ground state. This feature is ascribed to the monopole term of the residual proton-neutron interaction and is well reproduced by realistic shell-model calculations.