Understanding Excitations in ${}^{59,61}$Co, ${}^{59}$Ni

Abstract

High spin states in ${}^{59}$Co ($Z=27$), ${}^{59}$Ni ($Z=28$) and ${}^{61}$Co have been populated by the fusion evaporation reactions, ${}^{48}$Ti(${}^{14}$C, p2n)${}^{59}$Co, ${}^{48}$Ti(${}^{14}$C, 3n)${}^{59}$Ni, and ${}^{50}$Ti(${}^{14}$C, p2n)${}^{61}$Co. The 9 MV tandem accelerator at the John D Fox Laboratory, Florida State University (FSU) was used to accelerate the ${}^{14}$C beam and the de-exciting $\gamma$ rays were detected by the FSU detector array consisting of six High Purity Germanium (HPGe) clover detectors, and three single crystals. Directional correlation of the $\gamma$ rays de-exciting oriented states (DCO ratios) and polarization asymmetry measurements helped to establish spin and parities of the excited states whenever possible. The level scheme of ${}^{59}$Co has been expanded with the inclusion of positive parity states up to 31/2${}^{+}$ at around 11 MeV. The ${}^{59}$Ni positive parity states known from previous study were verified with modifications to some of the spins and parities. On the other hand, the negative parity states were extended to 31/2 at an excitation energy of 12 MeV. No new transition was observed for ${}^{61}$Co, but one of the major bands has been reassigned as consisting of positive parity states by reason of this study which is a candidate for magnetic rotation band. Cross shell excitations were observed in the three nuclei studied and the prominent role of excitation to g${}_{9/2}$ orbital crossing the $N=40$ shell gap was established in relation to collective excitation in these nuclei by comparison with large-scale shell model calculations.