Electric Monopole Transition from the Superdeformed Band in Ca 40

The electric monopole (E0) transition strength ρ2 for the transition connecting the third 0+ level, a "superdeformed"band head, to the "spherical"0+ ground state in doubly magic Ca40 is determined via e+e- pair-conversion spectroscopy. The measured value ρ2(E0;03+→01+)=2.3(5)×10-3 is the smallest ρ2(E0;0+→0+) found in A<50 nuclei. In contrast, the E0 transition strength to the ground state observed from the second 0+ state, a band head of "normal"deformation, is an order of magnitude larger ρ2(E0;02+→01+)=25.9(16)×10-3, which shows significant mixing between these two states. Large-scale shell-model (LSSM) calculations are performed to understand the microscopic structure of the excited states and the configuration mixing between them; experimental ρ2 values in Ca40 and neighboring isotopes are well reproduced by the LSSM calculations. The unusually small ρ2(E0;03+→01+) value is due to destructive interference in the mixing of shape-coexisting structures, which are based on several different multiparticle-multihole excitations. This observation goes beyond the usual treatment of E0 strengths, where two-state shape mixing cannot result in destructive interference.