( p , 2 p ) and ( p , pn ) quasifree knockout reactions with a 57 Ni radioactive beam *

Knockout reactions using light nuclear targets, e.g., Be or C, have proven in the past to be very useful in gaining information on the wave function of the valence nucleons. However, the strong absorption concentrates the reaction probability at the surface. Similar arguments hold for transfer and Coulomb break-up reactions. Nucleon knockout reactions using protons, on the other hand, allow one to determine the spectral functions of protons and neutrons in a wide range from the weakly bound valence nucleons to the deeply bound core states. Thus, in neutron-rich nuclei one gains access to the hitherto unknown region of the strongly bound protons and simultaneously to the valence neutrons. Beside the single-particle shellstructure, nucleon-nucleon correlations may be investigated as well as cluster knockout reactions. For stable nuclei and in normal kinematics, (p,pN) reactions have been used in the past as spectroscopic tool. The R3B collaboration intends to develop and apply the technique of quasi-free scattering at energies around 500700 AMeV (which is high enough to ensure that the conditions for quasi-free scattering are met), where both outgoing nucleons have energies in the range where the nucleon-nucleon cross section is at a minimum, thus maximizing the transparency of the nucleus and minimizing final state interactions. Measurements such as (p,2p), (p,pn), (p,pα) etc. will become possible in a kinematically complete geometry, allowing for a background-free measurement and also for a better control of final state interactions. As a first step, pilot experiments have been performed to investigate the possibilities of employing quasi-free scattering in inverse kinematics at relativistic energies. In the first experiment, the 57 Ni(p,2p) 56 Co and Ni(p,pn)Ni reactions have been investigated at around 500 AMeV using the LAND-R3B setup. Additional detectors have been inserted to detect the nucleons from the quasi-free scattering process. Figure 1 shows a schematic drawing of the arrangement around the CH2 target. The forward direction around the beam pipe was covered by a segmented CsI detector for detection of photons but also for the detection of energetic protons or neutrons. The figure indicates schematically a (p,2p) reaction. A 12-fold segmented plastic scintillator was placed between the beam pipe and the CsI detector to identify protons and trigger on quasi-free knockout events. Protons have been identified clearly in coincidence with outgoing 56 Co or 56 Ni fragments. If 56 Co is identified after the reaction target, two protons are detected in coincidence exhibiting the typical angular correlations among them as expected for quasi-free scattering events. Figure 2 shows a correlation plot for events with 56 Ni in the outgoing channel, i.e., events associated to (p,pn) one-neutron removal reactions. The x and y axes correspond to the segment number of the hit in the plastic detector and the CsI detector, respectively. The plot was generated from the measurement with a CH2 target after subtracting the contribution from carbon which was determined by a measurement with a C target. The diagonal line with equal segment numbers corresponds to coincident hits of protons in the plastic detector and in the CsI detectors following directly behind. Two more correlation lines appear where the plastic and CsI segment numbers differ by ±6, corresponding to a relative azimuthal angle of ∼180o between the two segments. These correlations originate from a (p,pn) reaction where the proton leaves a signal in the plastic segment but the neutron only leaves a signal in the CsI segment. The angular correlations in such (p,pn) reactions are the same as observed for the two-proton events in (p,2p) reactions. This result demonstrates the feasibility to study also quasi-free neutron knockout reactions in a kinematical complete measurement. Corresponding momentum distributions of fragments and coincident γ rays are presently being analyzed.