Evolution of collectivity near mid-shell from excited-state lifetime measurements in rare earth nuclei

Abstract

The $B(E2)$ excitation strength of the first excited ${2}^{+}$ state in even-even nuclei should directly correlate with the size of the valence space and maximize at mid-shell. A previously found saturation of $B(E2)$ strengths in well-deformed rotors at mid-shell is tested through high-precision measurements of the lifetimes of the lowest-lying ${2}^{+}$ states of the $^{168}\text{Hf}$ and $^{174}\text{W}$ rare earth isotopes. Measurements were performed using fast ${\mathrm{LaBr}}_{3}$ scintillation detectors. Combined with the recently remeasured $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ values for Hf and W isotopes the new data remove discrepancies observed in the differentials of $B(E2)$ values for these isotopes.