Multi-quasiparticle sub-nanosecond isomers in ¹⁷⁸W

We report on the first measurement of the half-lives of K^π=11^- and 12^- four-quasiparticle states in the even-even nucleus ¹⁷⁸W. The sub-nanosecond half-lives were measured by applying the centroid shift method to data taken with LaBr₃ (Ce) scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became experimentally accessible by the combination of several experimental techniques - scintillator fast timing, isomer spectroscopy with a pulsed beam, and the event-by-event calorimetry information provided by the NuBall array. The measured half-lives are 476(44)ps and 275(65)ps for the I^π=11^- and 12^- states, respectively. The decay transitions include weakly hindered E1 and E2 branches directly to the ground-state band, bypassing the two-quasiparticle states. This is the first such observation for an E1 transition. The interpretation of the small hindrance hinges on mixing between the ground-state band and the t-band.