Selective Population and Neutron Decay of the First Excited State of Semi-Magic 23O

Abstract

We have observed an excited state in the neutron-rich semi-magic nucleus 23O for the first time. No such states have been found in previous searches using y-ray spectroscopy. The observation of a resonance in n-fragment coincidence measurements confirms the speculation in the literature that the lowest excited state is neutron unbound and establishes positive evidence for a 2.8(1) MeV excitation energy of the first excited state in 23O. The non-observation of a predicted second excited state is explained assuming selectivity of inner-shell knockout reactions.