The yrast state investigations of odd-A Co isotopes around N = 28

Abstract: A systematic comparison of the odd-$A$ cobalt isotopes $^{53-65}$Co’s negative parity halo states energy level structures indicates a level inversion between $9/2^-$ and $11/2^-$ in $^{55}$Co, and suggests that $^{53,57}$Co might exhibit strong collective effects. Shell model calculations based on the GXPF1A effective interaction reproduce well these nuclei’s halo state energy levels, along with the corresponding experimental values for magnetic moments and electric quadrupole moments. The shell model results show that the dominant proton configuration component in the ground state $7/2^-$ wave functions of $^{53-65}$Co is $ pi left(1f_{7/2} right)^7$. The excited states $9/2^-$ and $11/2^-$ in $^{55}$Co involve a competition between $1f_{7/2}$ proton excitation and $1f_{7/2}$ neutron excitation, leading to a possible level inversion between the $9/2^-$ and $11/2^-$ states. Moreover, using the Constrained Hartree-Fock (CHF) method to study the quadrupole deformation characteristics of $^{53,55,57}$Co, and combining the average occupancy numbers and configurations obtained from shell model calculations, the reasons why the excited state energies of $^{55}$Co are higher compared to other Co isotopes were analyzed.