Electric monopole transition from the superdeformed band in ${}^{40}$Ca

Abstract

The electric monopole ($E0$) transition strength ${\rho }^2$ for the transition connecting the third 0${}^{+}$ level, a "superdeformed" band head, to the "spherical" 0${}^{+}$ ground state in doubly magic ${}^{40}$Ca has been determined via $e^{+}{e}^{-}$ pair-conversion spectroscopy. The measured value, ${\rho }^2(E0;0_3^{+}\to 0_1^{+})=2.3(5)\times 10^{-3}$, is the smallest ${\rho }^2(E0;0^{+}\to 0^{+})$ 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, ${\rho }^2(E0;0_2^{+}\to 0_1^{+})=25.9(16)\times 10^{-3}$, 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 ${\rho }^2$ values in ${}^{40}$Ca and neighboring isotopes were well reproduced by the LSSM calculations. The unusually small ${\rho }^2(E0;0_3^{+}\to 0_1^{+})$ 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.

References in corpus (3)

• ${}^{78}$Ni revealed as a doubly magic stronghold against nuclear deformation
• Discovery of the shape coexisting 0+ state in 32Mg by a two neutron transfer reaction
• Table of electronic factors for E0 electron and electron-positron pair conversion transitions

Cited by in corpus (1)

• Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB