Onset of deformation at $N = 112$ in Bi nuclei

Abstract

The high spin states in $^{195}$Bi has been studied by $\gamma$-ray spectroscopic method using the $^{181}$Ta($^{20}$Ne, 6n) fusion evaporation reaction at 130 MeV. The $\gamma\gamma$ coincidence data were taken using an array of 8 clover HPGe detectors. The spin and parity assignments of the excited states have been made from the measured directional correlation from oriented states (DCO) ratios and integrated polarization asymmetry (IPDCO) ratios. The results show, for the first time, the evidence of a rotational like band based on a 13/2$^+$ band head in this nucleus, indicating the onset of deformation at neutron number $N = 112$ for the Bismuth isotopes. The results obtained were found to be consistent with the prediction of the total Routhian surface calculations using Woods Saxon potential. The same calculations also predict a change in shape from oblate to triaxial in $^{195}$Bi at high rotational frequency.