Evolution of E2 strength in the rare-earth isotopes Hf-174,Hf-176,Hf-178,Hf-180

Mean lifetimes of yrast states of the isotopes Hf-174,Hf-176,Hf-178,Hf-180 have been measured using fast-electronic scintillation timing. Excited states of Hf-174,Hf-176,Hf-178 were populated via beta decay, while 180 Hf was populated via Coulomb excitation. The lifetimes of the 2(1)(+) and 4(1)(+) states of all isotopes and the lifetimes of the 6(1)(+) states of Hf-174,Hf-178 were measured, using the slope and the centroid shift methods. The mean lifetime, tau(4(1)(+)) = 85(13) ps, of Hf-178 has been determined for the first time. In addition, the mean lifetimes of the 2(1)(-) and the 3(1)(-) states of Hf-176 have been determined. Systematic uncertainties on the evolution of data as a function of neutron number were reduced by using the same setup for all the isotopes of interest. The data are in agreement with other recent lifetime measurements where available and shows a shift of the maximum of collectivity for the Hf isotopic chain from neutron midshell at N = 104 to N = 100.