Onset of deformation atN=112in Bi nuclei

Abstract

The high spin states in ${}^{195}$Bi were studied by $\ensuremath{\gamma}$-ray spectroscopic method using the ${}^{181}$Ta(${}^{20}$Ne, 6$n$) fusion-evaporation reaction at 130 MeV. The $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence data were taken using an array of eight clover high-purity germanium detectors. The spin and parity assignments of the excited states were made from the measured directional correlation from oriented states ratios and integrated polarization asymmetry ratios. The results show, for the first time, the evidence of a rotational-like band based on a 13/2${}^{+}$ bandhead in this nucleus, indicating the onset of deformation at the 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 the Woods-Saxon potential. The same calculations also predicted a change in shape from oblate to triaxial in ${}^{195}$Bi at high rotational frequency.