The two-proton knockout reaction 9 Be( 54 Ti, 52 Ca+γ) has been studied at 72 MeV/nucleon. Besides the strong feeding of the 52 Ca ground state, the only other sizeable cross section proceeds to a 3 − level at 3.9 MeV. There is no measurable direct yield to the first excited 2 + state at 2.6 MeV. The results illustrate the potential of such direct reactions for exploring cross-shell proton excitations in neutron-rich nuclei and confirms the doubly-magic nature of 52 Ca.For decades, the cornerstone of nuclear structure has been the concept of single-particle motion in a welldefined potential leading to shell structure and magic numbers governed by the strength of the mean-field spin-orbit interaction [1]. Recent observations in exotic, neutron-rich nuclei have demonstrated that the sequence and energy spacing of single-particle orbits is not as immutable as once thought: some of the familiar magic numbers disappear and new shell gaps develop [2]. Crossshell excitations, arising from the promotion of nucleons across shell gaps, probe changes in shell structure. They are, however, not always readily identifiable in nuclear spectra. This letter demonstrates that two-proton knockout reactions can examine, selectively, cross-shell proton excitations in neutron-rich systems.Single-nucleon knockout reactions with fast radioactive beams are established tools to investigate the properties of halo nuclei [3] and to study beyond meanfield correlations, indicated by the quenching of spectroscopic strengths [4]. Eikonal theory [5] provides a suitable framework for the extraction of quantitative nuclear structure information from such reactions. In contrast, the potential of two-nucleon knockout as a spectroscopic tool has been recognized only recently. Bazin et al. [6] have shown that two-proton removal reactions from beams of neutron-rich species at intermediate energies proceed as direct reactions and that partial cross sections to different final states of the residue provide structure information. More recently, such a reaction was used to infer the magicity of the very neutron-rich 42 Si nucleus [7].In the current experiment, sizable cross sections for the 9 Be( 54 Ti, 52 Ca+γ)X reaction were found to feed only the 52 Ca ground state and a 3 − level with an excitation energy near 4 MeV, bypassing completely the first 2 + level at 2.6 MeV. These observations can be reproduced qualitatively by calculations which assign the 3 − level to the promotion of protons across the Z = 20 shell gap. In addition, the data confirm the presence of a neutron sub-shell closure at N = 32, the subject of much recent attention [8,9,10,11,12,13].The 54 Ti secondary ions were produced by fragmentation of a 130 MeV/nucleon 76 Ge beam, delivered by the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory, onto a 9 Be fragmentation target. The ions were selected in the A1900 largeacceptance fragment separator [14], which was operated with two settings during different phases of the experiment; 1% momentum acceptance and...
