Spherical to deformed shape transitions in the nucleon pair shell model

A study of the shape transition from spherical to axially deformed nuclei in the even C_e isotopes using the nucleon-pair approximation of the shell model is reported. As long as the structure of the dominant collective pairs is defined in a framework appropriate to deformed nuclei, the model is able to produce a shape transition. The resulting transition is too rapid, however, with nuclei that should be transitional being fairly well deformed. The possibility of using several pairs with each angular momentum to improve the description of the transitional region is discussed as is the possibility of using a larger single-particle space to better describe the moments of inertia in the deformed regime.