Shell structure effects on the high-spin rotational states in ytterbium nuclei

The ground state rotational band members up to spin 18+ have been identified in 170Yb(N=100) by studying the gamma rays from the 160Gd(14C, 4n) 170Yb reaction. The nuclear moment of inertia of the band shows an abrupt increase at high-spin values similar to that observed in nuclei with N=90-96, but in contrast with the smooth increase in the N=98 nucleus 168Yb. This dependence on neutron number is difficult to explain in terms of the decoupling of neutrons in the i13/2 orbital alone, but is consistent with the Coriolis antipairing description of a phase transition from a superfluid to a normal state if account is taken of the reduction in pairing correlations caused by the shell structure peculiar to 168Yb.