We examine a quantum phase transition in gamma-soft nuclei, where the O(6) limit is simultaneously a dynamical symmetry of the U(6) group of the interacting boson model and a critical point of a prolate-oblate phase transition. This is the only example of phase transitional behavior that can be described analytically for a finite s,d boson system.

The nucleus 94 Mo was investigated using a powerful combination of γ-singles photon scattering experiments and γγ-coincidence studies following the βdecay of 94m Tc. The data survey short-lived J π = 1 + , 2 + states and include branching ratios, E2/M 1 mixing ratios, lifetimes and transition strengths. The proton-neutron mixed-symmetry (MS) 1 + scissors mode and the 2 + MS state are identified from M 1 strengths. A γ transition between MS states was observed and its rate was measured. Nine M 1 and E2 strengths involving MS states agree with the O(6) limit of the Interacting Boson Model-2 using the proton boson E2 charge as the only free parameter.

We show that the second-order phase transition between spherical and deformed shapes of atomic nuclei is an isolated point following from the Landau theory of phase transitions. This point can occur only at the junction of two or more first-order phase transitions which explains why it is associated with one special type of structure and requires the recently proposed first-order phase transition between prolate and oblate nuclear shapes. Finally, we suggest the first empirical example of a nucleus located at the isolated triple-point.

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Characterisation of the few doubly magic nuclei, known and predicted, provides a benchmark for our knowledge of the fundamental forces that drive the evolution of shell closures with proton-to-neutron asymme