The 'Ni nucleus has been identified among the products of deep-inelastic reactions of Ni projectiles bombarding '3OTe and~o 'Pb targets. Three new states, including the high-lying 2+ (2033 keV) and the 0.86 ms 5 isomer, indicate a substantial subshell closure at neutron number N = 40. The level structure and the observed very slow E3 transition speed are discussed within the shell model. PACS numbers: 27.50.+e, 21.60.Cs, 23.20.Lv, 25.70.Lm In spherical nuclei the 1g9/2 orbital is distinctly separated in energy from all other single-particle levels. This gives rise to the well established magicity of the neutron and proton numbers N, Z = 50 and points towards a somewhat less pronounced closure at N, Z = 40. For protons the Z = 40 subshell closure is clearly demonstrated by the well known level structure of the 9OZr nucleus [1], for which the lowest excitation is the 1.76 MeV 0+ state, the first 2+ state appears at 2.19 MeV, and the lowest lying particle-hole (p,~2g9t2) excitation produces the longlived 5 isomeric state. The study of similar features in

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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...

The even 52-56 Ti isotopes have been studied with intermediate-energy Coulomb excitation and absolute B(E2; 0 + → 2 + 1) transition rates have been obtained. These data confirm the presence of a subshell closure at neutron number N = 32 in neutron-rich nuclei above the doubly magic nucleus 48 Ca and provide no direct evidence for the predicted N = 34 closure. Large-scale shell model calculations with the most recent effective interactions are unable to reproduce the magnitude of the measured strengths in the semimagic Ti nuclei and their strong variation with neutron number.