Nuclear Structure Group

Evolution of collectivity in even-even nuclei with angular momentum

In 1971 Johnson, Ryde and Sztarkier discovered back-bending phenomenon which seemed to be due to the Coriolis anti-pairing (CAP) and was predicated by Mottelson and Valatin in 1960. It was suggested that fast nuclear rotation would influence the nuclear pairing correlation. The observed in 158Dy sharp change of moment of inertia with respect to the rotational frequency was assigned to the total disappearance of these correlations. Thus the observed in the yrast line large deviation from the rotational behaviour is explained with the crossing of two rotational bands. The first one is associated to the rotation of super-fluid system while the upper band is explained with the rotations of a normal, without pairing correlations, system. However, in 1972 Stephens and Simon suggested that the Coriolis force influences only the high-j low-Ω valence nucleons (RAL model). In result at a critical frequency the nucleons are decoupled from the core and align their angular momenta with the rotation axis. Since the discovery of the back-bending phenomenon in 158Dy this effect was observed in a large number of nuclei.

The above discussed changes of the intrinsic nuclear structure influence the rotational properties of the nuclei which can be observed in the band evolution. The nuclei may completely change their collective behaviour, for example from vibrational to rotational. In terms of RAL model this means that last unpaired nucleons polarize the shape. Thus the experimental results on the back-bending effect give important information not only about nuclear pairing correlations but also on the nuclear shape.