Shape Coexistence and Its Cause inGd151

The ($\alpha , 3n\gamma$) reaction has been used to study the level structure of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Gd}$. Energy spacings and other information indicate a small deformation for the observed $i_{\frac{13}{2}}$ decoupled band; whereas, a large prolate deformation is indicated by the normal rotational band associated with the $h_{\frac{11}{2}}$ hole excitation. A simple explanation for these two shapes in ${}_{}{}^{151}{}_{}{}^{}\mathrm{Gd}$ is given, based on the Nilsson diagram, and the behavior is shown to be related to the well-known shape change occurring between 88 and 90 neutrons.