Collectivity at the prolate-oblate transition: The Lifetime of W

The neutron-rich rare isotope $^{190}$W is discussed as a candidate for a prolate-oblate transitional nucleus with maximum γ-softness. The collectivity of this isotope is assessed for the first time by the measurement of the reduced E2 transition probability of its first ${2}^{+}$ state to the ground state. The experiment employed the FAst TIming Array (FATIMA), comprised of 36 LaBr$_{3}$(Ce) scintillators, which was part of the DESPEC setup at GSI, Darmstadt. The ${4}_{1}^{+}$ and ${2}_{1}^{+}$ states of $^{190}$W were populated subsequently to the decay of its 127(12) μs isomeric ${J}^{\pi }={10}^{-}$ state. The mean lifetime of the ${2}_{1}^{+}$ state was determined to be $\tau =274\left(28\right)$ ps, which corresponds to a $B\left(E2;{2}_{1}^{+}\to {0}_{1}^{+}\right)$ value of 95(10) W.u. The results motivated a revision of previous calculations within an energy-density functional-based interacting boson model-2 approach, yielding E2 transition properties and spectroscopic quadrupole moments for tungsten isotopes. From comparison to theory, the new data suggest that $^{190}$W is at the transition from prolate to oblate structure along the W isotopic chain, which had previously been discussed as a nuclear shape-phase transition.