Influence of the N=50 neutron core on dipole excitations in Rb-87

Dipole excitations in the semimagic N=50 nucleus 87Rb were investigated at the Stuttgart Dynamitron facility using bremsstrahlung with an end-point energy of 4.0 MeV. The widths Γ or the reduced excitation probabilities B(Π1)↑ of 18 states were determined for the first time. The magnetic dipole excitations are well reproduced in the framework of the shell model, however, these calculations cannot describe the observed electric dipole excitations. The 1/2+ state at 3060 keV is proposed to be the weak coupling of an
f5/2 proton hole to the 3− octupole vibrational state in the N=50 core 88Sr. The relatively strong E1 transition from that state to the ground state is explained as mainly the neutron h11/2→g9/2 transition. The breakup of the N=50 core and neutron excitations into the h11/2 shell are essential to describe electric dipole excitations, but neutron-core excitations do not play an important role for the structure of magnetic dipole excitations.