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Breakup reactions were used to study the ground-state configuration of the neutron-rich isotope 23 For decades the nuclear shell model has been one of the most important tools to interpret a wealth of experimental data. With the advent of radioactive-beam facilities, the evolution of nuclear shell structure far from the valley of stability could be studied. At present, such studies can be extended to the limits of bound nuclei on the neutron-rich side (the neutron dripline) for nuclear charges up to Z 8, oxygen. An important tool in this context are high-energy knockout reactions of single neutrons from near-dripline nuclei. Brown et al. [1] have shown that the residue momentum distributions and the corresponding cross sections can be analyzed in such a way that both the l values and the single-particle occupation probabilities of the levels can be deduced. This, however, requires that the level from which the breakup occurred is identified uniquely by measuring rays in coincidence with the residue [2,3].An interesting case to study is the series of neutronrich oxygen isotopes near the neutron dripline. As mentioned above, the experimental confirmation of such a theoretical analysis requires an exclusive knockout experiment where the individual levels are identified by measuring in coincidence the deexciting rays. We have performed such an exclusive experiment at the fragment separator FRS at Gesellschaft fü r Schwerionenforschung (GSI) in Darmstadt, Germany. The primary beam was 40 Ar at 1 GeV=nucleon delivered by the SIS synchrotron with an average intensity of 1:5 10 10 particles=spill.

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One-nucleon removal reactions at relativistic energies have been used as a spectroscopic tool for obtaining information about the ground state of neutron-rich oxygen isotopes. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after breakup were measured along with the one-nucleon removal cross-sections. The relative contributions of the remaining fragments in their ground and excited states have been determined for a few cases from measurements of γ rays in coincidence with the longitudinal momentum distribution. The controversial case of 23O has been directly addressed. Comparison of our exclusive momentum distributions for the one-neutron removal channel to theoretical momentum distributions calculated with an Eikonal model for the knockout process rendered a spin and parity of Iπ = 1/2+ for the 23O ground state. This result resolves the existent experimental discrepancy and supports sub-shell closure at N = 14 with a fairly pure 2s1/2 neutron in 23O.