Quasiparticle alignment effects in rotational bands of 159 Yb

High-spin states in the transitional nucleus ¹⁵⁹Yb were measured by means of heavy-ion induced reactions. The three reactions used were ^{116Cd(48Ti,5n)159Yb at 220 MeV, 98Mo(64Ni,3n)159Yb at 265 MeV, and 147Sm(16O,4n)159Yb at 90 MeV. The first two measurements were performed at the Holifield Heavy Ion Research Facility at Oak Ridge National Laboratory, using the Spin Spectrometer multi-detector array. The third reaction was performed at the McMaster University Tandem Accelerator Laboratory.}

The previously known ii3/2 rotational band (Trautmann et al. 1975) has been extended up to 61/2 h, and possibly 69/2 h. Three new side bands have been established with spins and parities determined. All of the bands (structures) have been assigned intrinsic i_13/2 and h_9/2 neutron configurations.

A systematic analysis of the neighboring transitional nuclei has been performed. The properties of these nuclei were compared at the 4-6 quasiparticle level, in order to learn about the shape-driving effects of quasiparticles at higher rotational frequencies. By contrast, earlier comparisons were done only at the 1-4 quasiparticle, level and at moderate rotational frequencies.

Band crossings due to the rotational alignment of the secondary vi_13/2 pair and the primary πh_11/2 pair are seen in ¹⁵⁹Yb. The primary neutron i_{13/2 crossing is not observed until after it has been completed. The crossings due to the secondary neutron pair and the proton pair are seen to be degenerate in the vi13/2 band. In addition, a deficiency is observed in the alignment gain due to those two crossings. In order to understand these two features, Cranked Shell Model calculations and equilibrium shape calculations have been performed.}

The observed structure in the bands of ¹⁵⁹Yb after the proton crossing is interpreted within the context of the theory of band terminating states, based upon calculations of the type done by Bengtsson and Ragnarsson, 1985.

Evidence for the occurence of a third neutron alignment process is presented for the neighboring N = 90 nuclei.