Description of nuclear octupole and quadrupole deformation close to the   axial symmetry: Critical-point behavior of 224Ra and 224Th
  The model, introduced in a previous paper, for the description of the octupole and quadrupole degrees of freedom in conditions close to the axial symmetry, is applied to situations of shape phase transitions where the quadrupole amplitude can reach zero. The transitional nuclei 224,226Ra and 224Th are discussed in the frame of this model. Their level schemes can be reasonably accounted for assuming a square-well potential in two dimensions. Electromagnetic transition amplitudes are also evaluated and compared with existing experimental data. 
P.G. Bizzeti and A.M. Bizzeti-Sona