Features of Nuclear Deformations Produced by the Alignment of Individual Particles or Pairs

The interacting boson model is first analysed at the level of "real" bosons with a view to establishing the conditions for the occurrence of rotational band structure. The conditions on the effective interactions are much less restrictive than those required for the occurrence of SU (3) symmetry. The wave functions for the aligned bosons are analysed from the microscopic point of view in order to compare with the coupling scheme based on the alignment of individual particles in the deformed potential. In general, these two coupling schemes, both of which yield rotational band structure, lead to major differences in the nuclear properties. The region of validity of the boson coupling scheme is studied in terms of the effective interactions between the nucleons. Only for rather small deformations, is it justified to truncate the boson space to J = 0 and 2.