The oscillating behaviour of backbending

The anomaly of the moment of inertia usually called backbending varies in its strength regularly as a function of the neutron number for the rare earth nuclei. This oscillating behaviour of backbending due to the rotational alignment of two isub(13/2) neutrons is explained in a particle number and angular momentum conserving model. The neutrons in the isub(13/2)-shell are described by shell model wave functions and are weakly coupled to the rotation of the core. It is shown that the alignment is arranged by the nucleus by scattering two isub(13/2) neutrons mainly from the Ω = 1/2 and Ω = 3/2 isub(13/2) orbits into the core. The aligned band has therefore in average two neutrons less in the isub(13/2)-shell. Interaction between the valence nucleon and the core is due to the pairing force. It is shown that this interaction has j- 3/2 zeros. No interaction between the ground band and the aligned band means strong backbending while a strong interaction means no backbending. In the second part of this talk we explain the behaviour of the second anomaly of the moment of inertia by the alignment of two hsub(11/2) protons corresponding to the same oscillating behaviour as discussed before. In this way we can explain that for the N = 90 isotones 15666 Dy shows no backbending, 15868 Er shows upbending and 17070Yb shows a strong second anomaly of moment of inertia. (orig.)