Systematics of quasirotational states inN=88nuclei

The decays of 4.2-min ${}_{}{}^{152}{}_{}{}^{}\mathrm{Tb}$, 3.3-min ${}_{}{}^{154}{}_{}{}^{}\mathrm{Ho}$, 11.8-min ${}_{}{}^{154}{}_{}{}^{}\mathrm{Ho}$, and 82-sec ${}_{}{}^{156}{}_{}{}^{}\mathrm{Tm}$ have been studied by $\gamma$-ray and conversion electron spectroscopy using the He-jet technique. High-spin states in ${}_{}{}^{152}{}_{}{}^{}\mathrm{Gd}$ were also studied in the ${}_{}{}^{152}{}_{}{}^{}\mathrm{Sm}$($\alpha$,$4n\gamma$) reaction at 50 MeV and the ${}_{}{}^{150}{}_{}{}^{}\mathrm{Sm}$($\alpha$,$2n\gamma$) reaction at 28 MeV. The systematic behavior of quasirotational bands in $N=88\mathrm{}$ nuclei has been studied, and observed features in the location of $\beta$- and $\gamma$-band states are shown to be explainable from a consideration of available orbitals in a Nilsson diagram. Microscopic calculations employing fourth and sixth order versions of the boson expansion model of Tamura, Kishimoto, and Weeks reproduce the known low-lying positive parity states in ${}_{}{}^{150}{}_{}{}^{}\mathrm{Sm}$, ${}_{}{}^{152}{}_{}{}^{}\mathrm{Gd}$, ${}_{}{}^{154}{}_{}{}^{}\mathrm{Dy}$, and ${}_{}{}^{156}{}_{}{}^{}\mathrm{Er}$. Experimentally observed negative-parity states are explained remarkably well by a quadrupole-octupole coupling model, but a backbending plot suggests a possible change in intrinsic structure above spin 11.