Effect of proton-proton Coulomb repulsion on soft dipole excitations of light proton-rich nuclei

We perform three-body model calculations for soft dipole excitations of the proton-rich Borromean nucleus ${}^{17}$Ne. To this end, we assume that ${}^{17}$Ne takes the ${}^{15}\mathrm{O}+p+p$ structure, in which the two valence protons are excited from the $0^{+}$ ground state configuration to $1^{-}$ continuum states. We employ a density-dependent contact force for the nuclear part of the pairing interaction, and discretize the continuum states with the box boundary condition. We show by explicitly including the Coulomb interaction between the valence protons that the Coulomb repulsion does not significantly alter the $E1$ strength distribution. We point out that the effect of the Coulomb repulsion in fact can be well simulated by renormalizing the nuclear pairing interaction.

Figure 1

Comparison of the $E1$ strength distributions for ${}^{17}$Ne obtained with several treatments for the Coulomb interaction between the valence protons. The solid line is obtained by fully including the Coulomb interaction, while the dashed line is obtained by switching off the Coulomb repulsion. The thin solid line is the result of the renormalized nuclear interaction, which is readjusted to reproduce the ground state energy without the Coulomb interaction. The dotted line denotes the result without any pairing interaction. These distributions are smeared with the Cauchy-Lorentz function, Eq. (12), with $\Gamma =1.0$ MeV.