Quasi-free knockout reactions with high-energy radioactive beams

Thomas Aumann

Technical University Darmstadt,  Germany

The measurement of proton-induced reactions in complete kinematics will be an important part of the physics program at the future R3B  (Reactions with Relativistic Radioactive Beams) experiment at FAIR. In particular the quasi-free knockout processes of the type (p,2p), (p,pn), (p,pα) will be used to investigate the single-particle and cluster structure of neutron-proton asymmetric nuclei and the role of nucleon-nucleon correlations in nuclei.

At the present LAND/R3B setup we have performed experiments using a prototype installation for the detection of high-energy protons and neutrons in (p,2p) and (p,pn) reactions, respectively.  Recoil protons have been detected with an array of Si-Strip detectors and the 4π NaI spectrometer “Crystal Ball”. Measurements have been performed with a ¹²C beam as a benchmark as well as with the proton-dripline nucleus ¹⁷Ne.  For comparison, breakup of ¹⁷Ne induced by a carbon target has been studied as well in complete kinematics. For ¹²C, proton knockout from different single-particle states has been identified including knockout from the 0s state by reconstructing the excitation-energy spectrum of ¹¹B utilizing γ spectroscopy and the invariant-mass method.  In case of proton knockout from ¹⁷Ne, the excitation energy of the unbound ¹⁶F clearly shows contributions from halo and core knockout.  From the analysis of the momentum distributions and cross sections the occupancies for the s and p configurations in the ¹⁷Ne ground state have been determined.  Examples for knockout reactions populating unbound states beyond the neutron dripline will be discussed as well.