Intruder state collectivity at a double subshell closure from the beta decay of 0- 96Yg to the levels of Zr96

Levels of Zr96 populated following the - decay of the 0- ground state of Y96 have been investigated by -ray and conversion electron singles, -ray multispectral scaling, - and -e- coincidence, and - angular correlation measurements. These data have been used to establish the population of eight excited levels of Zr96, to confirm the 0+ assignment of the 2695-keV level, and to determine a half-life of 5.4(1) s for Y96. The deduced beta transition strengths establish the decay to the ground state, with logft of 5.6, as one of the fastest first-forbidden decays known and support the notion of double subshell closure at Z=40 and N=56. The observed strong hindrance of transitions to excited 0+ states is indicative of shape coexistence. The observed level pattern of the band built on the 02+ state exhibits similarities to nearby vibrational-like nuclei having four valence protons and four valence neutrons. A four-particle, four-hole interpretation of the 02+ state is supported by the data, and we determine a deformation parameter, 20.2, for this intruder bandhead.