Experimental study of the knockout reaction mechanism using O-14 at 60 MeV/nucleon

Background: For the deeply bound one-nucleon removal at intermediate energies using a Be-9 or C-12 target, a strong reduction of cross section was observed relative to the prediction of eikonal theoretical model. The large disagreement has not been explained and the systematic trend is inconsistent with results from transfer reactions. The recently observed asymmetric parallel momentum distribution of the knockout residue indicates the significant dissipative core-target interaction in the knockout reaction with a composite target, implying new reaction mechanisms beyond the eikonal reaction descriptions. Purpose: To investigate the reaction mechanism for deeply bound nucleon removal at intermediate energies. Method: Neutron removal from O-14 using a C-12 target at 60 MeV/nucleon was performed. Nucleon knockout cross sections were measured. The unbound excited states of O-13 were reconstructed by using the invariant mass method with the residues and the associated decay protons measured in coincidence. The measured cross sections are compared with an intra-nuclear cascade (INC) prediction. Results: The measured cross section of (O-14, C-11) is 60(9) mb, which is 3.5 times larger than that of (O-14, O-13) channel. This 2pn-removal cross section is consistent with INC prediction, which is 66 mb with the main contribution being non-direct reaction processes. On the other hand, the upper limit of the cross section for one-neutron removal from O-14 followed by proton evaporation is 4.6(20) mb, integrated up to 6 MeV above the proton separation energy of O-13. The calculated total cross section for such reaction processes by the INC model is 2.5 mb, which is within the measured upper limit. Conclusions: The data provide the first constraint on the role of core excitation and evaporation processes in deeply bound nucleon removal from asymmetric nuclei. The experiment results suggest that non-direct reaction processes, which are not considered in the eikonal model, play an important role in deeply bound nucleon removal from asymmetric nuclei at intermediate energies.RCNP accelerator group; JSPS long-term fellowship at the RIKEN Nishina Center [L-13520]; National Science Foundation [PHY 1102511]; U.S. NSF [1415656]; U.S. DOE [DE-FG02-08ER41533]; National Natural Science Foundation of China [U1332129, 11535004, 11275011, 11275001]; National Basic Research Program of China [2013CB834402]SCI(E)ARTICLEjleehc@hku.hk; yeyl@pku.edu.cn49