Transition from collective to noncollective rotation at high spin in N approx. = 87 nuclei

The systematics of the (E2) #betta# ray transition energies and the available lifetime data are used to characterize the excitation modes of the light rare-earth nuclei (N greater than or equal to 82) at different spins. The results, which include our recently obtained data on 149Gd, 154Ho, 155Er, 157Yb and 158Yb nuclei, indicate that, at low spins, the nuclear excitation mode (shapes) change from single-particle excitations (weakly oblate) in N less than or equal to 85 nuclei to quasi-vibrational (soft triaxial) in N = 86, weakly rotational (prolate) in N = 87, and rotational (prolate) in the N greater than or equal to 88 systems. At higher angular momenta, all these nuclei show a general tendency to traverse the (epsilon,#betta#) plane towards the oblate axis, and to eventually adopt the aligned coupling mode of excitation