Description of transitional nuclei

The description of transitional nuclei, which have a deformation between spherical and strongly deformed nuclei, is reviewed. This article concentrates on models with a phenomenological core and one or two nucleons moving in its potential. For the core a rigid axial or triaxial rotor, the Bohr-Mottelson model or the interacting boson approximation is used. The motion of the additional nucleons are described as quasi-particles moving in the rotating deformed potential of the core. The properties of the odd nucleons are determined by two forces: (i) the coupling to the quadrupole moments of the core (strong coupling) and (ii) the influence of the Coriolis and centrifugal forces which like to decouple the odd nucleon from the motion of the core (decoupling). Models are presented which can describe at the same time decoupling and strong coupling and the competition between both. The model is applied to odd-mass nuclei, to the two-quasi-particle negative-parity states in even-mass nuclei, to zero- and two-quasi-particle positive-parity states in even-mass nuclei and to doubly odd-mass nuclei. (author)