Electric monopole transition from the superdeformed band in 40Ca
Abstract
The electric monopole (E0) transition strength 2 for the transition connecting the third 0+ level, a "superdeformed" band head, to the "spherical" 0+ ground state in doubly magic 40Ca has been determined via e+e- pair-conversion spectroscopy. The measured value, 2(E0; 0+3 0+1)~=~2.3(5)×10-3, is the smallest 2(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, 2(E0; 0+2 0+1)~=~25.9(16)×~10-3, which shows significant mixing between these two states. Large-Scale Shell Model (LSSM) calculations were performed to understand the microscopic structure of the excited states, and the configuration mixing between them; experimental 2 values in 40Ca and neighboring isotopes were well reproduced by the LSSM calculations. The unusually small 2(E0; 0+3 0+1) 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.
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