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The title is : Quasi-free proton knockout reaction of 23,25F


The change of the neutron dripline from oxygen to fluorine indicates the 1d5/2 proton affects the neutron shell structure. We aim to know how the neutron sd-shell structure is changed by the 1d5/2 proton in neutron-rich 23F and 25F nucleus using proton spectroscopy. The spectroscopy is free from the effects of the proton shell structure, because the 1d5/2 proton in 23F or 25F is a single-particle state due to the 𝑍=8 magicity and even neutron number. If the neutron shell structure is not changed by the proton in 23,25F, after the sudden removal of that proton, the spectroscopic factor of that proton should be unity and not fragmented. Therefore, the effect on the neutron-shell from the proton will be shown on the spectroscopy.

The quasi-free 23,25F(p,2p) direct knockout reactions in inverse kinematics were performed in RIBF, RIKEN Nishina Center. Secondary beams of 23F and 25F were produced at ~280A MeV. The missing four-momentum of the residual oxygen (22O or 24O) was reconstructed using coincidence measurement of the incident nucleus and the two scattered protons. The excitation energy of the residue was then deduced.

From the experimental results, the occupation number of the 1d5/2 proton of 25F was 0.1 ± 0.3 and the proton is indeed in single-particle state. Meanwhile, the spectroscopic strength of the 1d5/2 proton of 23F or 25F were fragmented. These pointed that the change of the sd-shell neutron structure due to the 1d5/2 proton is the reason of the fragmentation. The change of neutron shell suggests the disappearance of 𝑁=16 magicity. The nuclear structures of the 25F and 23F demonstrated the Type-1 shell evolution. The comparison with the present shell model interactions (SFO, USDB, and SDPF-MU) indicated that the tensor force should be stronger. Also, the spectroscopic strength of the p-orbit was ~0.8 in 23,25F, this shows that the short-range correlation in neutron-rich nuclei is as same as stable nuclei.