Levels in I58 Yb have been established up to 38£ in the yrast band and about 3Qfr in two sidebands. Below spin 20, the similarity to the isotone 156 Er is striking, but up to spin 34 158 Yb remains rather collective while 156 Er becomes triaxial and evolves toward an oblate shape. This appears to result from small shifts in particle-alignment (backbend) frequencies; a proton alignment occurs before neutron ones in 158 Yb, whereas the reverse is true in 156 Er.PACS numbers: 21.10. Re, 23.20.Lv, 27.70. + q High-spin states in the transitional nuclei with 64 ^ Z ^ 70 and 82 ^ N ^ 92 have several interesting properties. First, they can assume a variety of shapes, ranging all the way from oblate to superdeformed prolate. However, questions about the mechanism of the shape changes and the extent to which the different shapes coexist are virtually unexplored. Second, there are reasonably good closed shells at Z = 64 and N = 82 (for spin 0) and, for all the nuclei having up to about twelve additional (valence) nucleons, this produces large shell effects at particular (higher) spins. Finally, the pairing correlations must be quenched with increasing spin, though the details depend strongly on the number and type of nucleons (among other things). Clearly, all three of these properties are interrelated, making high-spin physics of this region rich but complicated.Fortunately the cranked-shell-model (CSM) calculations are now sufficiently well developed to account rather well for the high-spin properties of such nuclei, and they can provide considerable help in sorting out these various effects. The variety of observed shapes in this region comes about due to rather flat (soft) potential-energy surfaces as a function of shape. Thus, small driving forces can produce large changes. One of the interesting shape-driving forces has to do with the nucleon orbits themselves. Even though these can be complicated mixtures of simple shellmodel wave functions (which we label A,B,C, . . .), they have particular (readily calculable) shapes and tend to pull the entire nucleus toward that shape. Sudden shifts in orbit population sometimes come about when the pressure to generate angular momentum causes a pair of high-j orbits to "align"-become fully occupied and align their angular momentum with that of the rest of the core. A secondary effect of this alignment is the shape-driving tendency of the newly occupied orbits. In soft-nuclei like 158 Yb these can produce dramatic shape changes that depend critically on which nucleons align.The nucleus 158 Yb has twelve particles (six protons and six neutrons) outside the above-mentioned closed shells, and lies just at the edge of the region of strongly deformed nuclei. The soft-rotational behavior at spin 0 becomes reasonably good rotational behavior by spin 8 or 10, and this extends up to at least spin 20. A recent study 1 found anomalous 0'quasivibrational") behavior above spin 24, together with some oblate states (band terminations 2 ). We have restudied 158 Yb, and found two new bands as well...
