Single-particle structure observed in radioactive-beam experiments

It has been demonstrated that single-nucleon knockout reactions in inverse kinematics at energies above 50 MeV/nucleon are a powerful spectroscopic tool for identifying single-particle structure. Partial and differential cross sections to individual final levels are obtained by measuring the momentum of the projectile residue with a high-resolution spectrograph in coincidence with gamma rays. The shape of the longitudinal momentum spectra identifies the orbital angular momentum l, while the absolute cross sections determine single-nucleon spectroscopic factors. Examples of results with this technique are given. The method has an extremely high sensitivity, and experiments have been carried out with incident beams of less than one atom/second. An analysis of precise data from high-energy experiments suggests that the method furnishes absolute occupancies and will allow the study of short-range correlations caused by the hard core in the nucleon-nucleon interaction. For the deeply bound p-shell neutron states of O-16 and C-12 the occupancies are reduced by factors Rs of 0.56(3) and 0.51(3), respectively, relative to the many-body shell model with effective interactions. Data for weakly bound neutron and proton states in radioactive nuclei suggests Rs values much closer to unity. We expect that the nuclear knockout method, in addition to being a tool for structure studies, will contribute to the understanding of the fundamentals of the shell model.