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

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 Hf168 and W174 rare earth isotopes. Measurements were performed using fast LaBr3 scintillation detectors. Combined with the recently remeasured B(E2;21+→01+) values for Hf and W isotopes the new data remove discrepancies observed in the differentials of B(E2) values for these isotopes.