Beyond the neutron drip line: The unbound oxygen isotopes O 25 and O 26
The very neutron rich oxygen isotopes O 25 and O 26 are investigated experimentally and theoretically. The unbound states are populated in an experiment performed at the R3B LAND setup at GSI via proton knockout reactions from F 26 and F 27 at relativistic energies around 442 and 414 MeV/nucleon, respectively. From the kinematically complete measurement of the decay into O 24 plus one or two neutrons, the O 25 ground state energy and width are determined, and upper limits for the O 26 ground state energy and lifetime are extracted. In addition, the results provide indications for an excited state in O 26 at around 4 MeV. The experimental findings are compared to theoretical shell model calculations based on chiral two and three nucleon (3N) forces, including for the first time residual 3N forces, which are shown to be amplified as valence neutrons are added.