The so-called alpha cluster structure appears in the excited states of light nuclear system, and the structures have been extensively investigated in the N=Z systems. Furthermore, the alpha cluster structures are also discussed in neutron-excess (N>Z) systems extensively.
In the present report, we focus on the mirror systems, which are obtained by replacing neutrons to protons in neutron-excess system, and demonstrate that the Coulomb shift of the mirror systems is a new probe to catch the sign of the clustering phenomena. Here we discuss the Coulomb shift of the mirror systems, such as (18O = alpha + 14C) - (18Ne = alpha + 14O).
The interaction potential of alpha and the residual nucleus is derived from the double folding (DF) model with the effective nucleon-nucleon interaction of the density-dependent Michigan 3-range Yukawa. The validity of the DF potential is checked by applying it to the alpha + 14C elastic scattering. The scattering calculation with the DF potential nicely reproduces the observed differential cross sections.
The energy level of these mirror systems are calculated from the orthogonality condition model (OCM), in which the functional space allowed by the Pauli's exclusion principle is exactly constructed. Here we have constructed the Pauli's allowed space by employing the mathematical technique of the SU(3) group representation in nuclei. The resonant 0+ levels and their decay width are identified by imposing the absorbing boundary condition. Above the alpha decay threshold, two resonant 0+ states are obtained, which seem to be consistent to the recent observations of the alpha + 14C elastic scattering and the multi-nucleon transfer reaction.
We have evaluated the energy shift of the 0+ states in the mirror systems of 18O - 18Ne, which arises from the difference of the Coulomb interaction. The OCM calculation predicts that the Coulomb shift for the resonant 0+ states is prominently reduced in comparison to the shift for the low-lying bound states. This reduction of the Coulomb shift for the resonant 0+ state is induced by the development of the alpha clustering. Therefore this result strongly suggests that the Coulomb shift is new probe to identify the cluster degrees of freedom.
In this report, we will discuss the relation of the reduction of the Coulomb shift and the development of the cluster degrees of freedom in detail.