1211.0057 (H. Nakada et al.)

Crossover from skin-mode to proton-neutron-mode in $E1$ excitations of
neutron-rich nuclei    [PDF]

H. Nakada, T. Inakura, H. Sawai

Character of the low-energy $E1$ excitations is investigated by analyzing the transition densities obtained from RPA calculations in the doubly-magic nuclei. We propose a decomposition method of the $E1$ excitations into the skin mode (i.e. oscillation of the neutron skin against the inner core) and the $pn$ mode (i.e. oscillation between protons and neutrons) via the transition densities. Crossover behavior of the $E1$ excitations is found, from the skin mode at low energy to the $pn$ mode at higher energy. The ratio of the skin-mode strength to the total strength is insensitive to nuclide and to effective interactions in the energy region of the crossover. Depending on the excitation energy, the observed PDRs are not necessarily dominated by the skin mode, as exemplified for $^{90}$Zr.

View original: http://arxiv.org/abs/1211.0057