Evidence for weak deformation in the 88-neutron nucleusTb153

The decay of 6.3-${\mathrm{h}}^{153}$Dy to levels in ${}_{}{}^{153}{}_{}{}^{}\mathrm{Tb}$ has been studied with high-resolution detectors. Sources were prepared by the ${}_{}{}^{154}{}_{}{}^{}\mathrm{Gd}({}_{}{}^3{}_{}{}^{}\mathrm{He}, 4n)$ reaction and chemically purified by ion exchange. A total of 329 $\gamma$ rays were assigned to ${}_{}{}^{153}{}_{}{}^{}\mathrm{Dy}$ decay; another 71 $\gamma$ rays may be associated with ${}_{}{}^{153}{}_{}{}^{}\mathrm{Dy}$ decay. The proposed decay scheme of 54 levels includes 319 of the 400 $\gamma$ rays. The $Q$ value for $\beta$ decay was measured to be 2250 ± 50 keV. Eleven low-lying positive-parity levels were interpreted in the Coriolis-mixed, particle-plus-rotor model as Nilsson states and rotational band members. Nilsson orbitals so interpreted include $\frac{5}{2^{+}}\mathrm{}\left[402\right]\mathrm{}$, ground state; $\frac{3}{2^{+}}\mathrm{}\left[411\right]\mathrm{}$, 147.4 keV; $\frac{5}{2^{+}}\mathrm{}\left[413\right]\mathrm{}$, 218.6 keV; and $\frac{7}{2^{+}}\mathrm{}\left[404\right]\mathrm{}$, 254.2 keV. The order and spacing of the negative-parity levels are consistent with rotation-alignment model predictions. For both positive- and negative-parity levels a deformation $\beta$ of about 0.15 is required to fit the data.