Evidence for weak deformation in the 88-neutron nucleusTb153

Abstract

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 $\ensuremath{\gamma}$ rays were assigned to $^{153}\mathrm{Dy}$ decay; another 71 $\ensuremath{\gamma}$ rays may be associated with $^{153}\mathrm{Dy}$ decay. The proposed decay scheme of 54 levels includes 319 of the 400 $\ensuremath{\gamma}$ rays. The $Q$ value for $\ensuremath{\beta}$ decay was measured to be 2250 \ifmmode\pm\else\textpm\fi{} 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 $\frac{5}{{2}^{+}}[402]$, ground state; $\frac{3}{{2}^{+}}[411]$, 147.4 keV; $\frac{5}{{2}^{+}}[413]$, 218.6 keV; and $\frac{7}{{2}^{+}}[404]$, 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 $\ensuremath{\beta}$ of about 0.15 is required to fit the data.RADIOACTIVITY $^{153}\mathrm{Dy}$ [from $^{154}\mathrm{Gd}(^{3}\mathrm{He}, 4n)$]; measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, ${I}_{\mathrm{ce}}$, ${Q}_{\ensuremath{\beta}}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$, and ${\ensuremath{\beta}}^{+}\ensuremath{-}\ensuremath{\gamma}$ coin; deduced ${\ensuremath{\alpha}}_{K}$, $log\mathrm{ft}$. $^{153}\mathrm{Tb}$ deduced levels, $J$, $\ensuremath{\pi}$. Enriched target, Ge(Li) and Si(Li) detectors.