Study of the deformation-driving $\nu d_{5/2}$ orbital in $^{67}_{28}$Ni$_{39}$ using one-neutron transfer reactions

The $νg_{9/2} , d_{5/2}, s_{1/2}$ orbitals are assumed to be responsible for the swift onset of collectivity observed in the region below $^{68}$Ni. Especially the single-particle energies and strengths of these orbitals are of importance. We studied such properties in the nearby $^{67}$Ni nucleus, by performing a ($d, p$)-experiment in inverse kinematics employing a post-accelerated radioactive ion beam (RIB) at the REX-ISOLDE facility. The experiment was performed at an energy of 2.95 MeV / u using a combination of the T-REX particle detectors, the Miniball $\gamma$-detection array and a newly-developed delayed-correlation technique as to investigate $\mu$s-isomers. Angular distributions of the ground state and multiple excited states in $^{67}$Ni were obtained and compared with DWBA cross-section calculations, leading to the identification of positive-parity states with substantial $νg_{9/2}$ (1007 keV) and $νd_{5/2}$ (2207 keV and 3277 keV) single- particle strengths up to an excitation energy of 5.8 MeV. 50% of the $νd_{5/2}$ single-particle strength relative to the $νg_{9/2}$-orbital is concentrated in and shared between the first two observed 5/2$^{+}$ levels. A comparison with extended Shell Model calculations and equivalent ($^{3}$He, $d$) studies in the region around $^{90}_{40}$Zr$_{50}$ highlights similarities for the strength of the negative-parity pf and positive-parity $g_{9/2}$ state, but differences are observed for the $d_{5/2}$ single-particle strength.