Both one-proton and one-neutron knockout reactions were performed with fast beams of two asymmetric, neutron-deficient rare isotopes produced by projectile fragmentation. The reactions are used to probe the nucleon spectroscopic strengths at both the weakly and strongly bound nucleon Fermi surfaces. The one-proton knockout reactions 9 Be( 28 S, 27 P)X and 9 Be( 24 Si, 23 Al)X probe the weakly bound valence proton states and the one-neutron knockout reactions and 9 Be( 28 S, 27 S)X and 9 Be( 24 Si, 23 Si)X the strongly bound neutron states in the two systems. The spectroscopic strengths are extracted from the measured cross sections by comparisons with an eikonal reaction theory. The reduction of the experimentally deduced spectroscopic strengths, relative to the predictions of shell-model calculations, is of order 0.8-0.9 in the removal of weakly bound protons and 0.3-0.4 in the knockout of the strongly bound neutrons. These results support previous studies at the extremes of nuclear binding and provide further evidence that in asymmetric nuclear systems the nucleons of the deficient species, at the more-bound Fermi surface are more strongly correlated than those of the more weakly bound excess species.

PACS numbers: to be definedMany properties of the atomic nucleus, such as vibrations, rotations and incompressibility can be interpreted as due to a two-component quantum liquid of protons and neutrons. Electron scattering measurements on stable nuclei demonstrate that their central densities are saturated, as for liquid drops. In exotic nuclei near the limits of mass and charge, with large imbalances in their proton and neutron numbers, the possibility of a depleted central density, or a "bubble" structure, was discussed in a recurrent manner since the seventies. Here we report first experimental evidence that points to a depletion of the central density of protons in the short-lived nucleus 34 Si. The proton-to-neutron density asymmetry in 34 Si offers the possibility to place constraints on the density and isospin dependence of the spin-orbit force -on which nuclear models have disagreed for decades-and on its stabilizing effect towards limits of nuclear existence.Microscopic systems composed of atoms or clusters can exhibit intrinsic structures that are bubble-like, with small or depleted central densities. For example, the fullerene molecules, composed of C atoms, are structures with extreme central depletion [1]. In nuclear physics, depletions also arise in nuclei with well-developed cluster structures when clusters are arranged in a triangle or ring-like structure -such as in the triple-α Hoyle state [2,3]. Unlike such a non-homogeneous, clustered system, central density depletions or bubble-like structures would be much more surprising in homogeneous systems, such as typical atomic nuclei with properties characteristic of a quantum liquid [4].This hindrance of bubble formation in atomic nuclei is inherent in the nature of the strong force between nucleons, which is strongly repulsive at short distances (below 0.7 fm), attractive at medium range (≈1.0 fm) and vanishes at distances beyond 2 fm. In a classical picture, the medium-ranged attraction of nuclear forces implies that nucleons interact strongly and attractively only with immediate neighbors, leading to a saturation of the nuclear central density, ρ 0 . Quantum mechanically, the delocalization of nucleons [5] leads to a further homogeneity of the density. Extensive precision electron scattering studies from stable nuclei [6] confirm that their central densities are essentially constant, with ρ 0 ≈ 0.16 fm −3 , independent of the number of nucleons A. As a consequence, like a liquid drop, the nuclear radii and volumes increase as A 1/3 and as A, respectively. Thus, a priori, bubble-like nuclei with depleted central densities are unexpected.Historically, the possibility of forming bubble nuclei was investigated theoretically in intermediate-mass [7][8][9][10], superheavy [11] and hyperheavy systems [12]. In general, central depletions will arise from a reduced occupation of single particle orbits with low angular momentum . These wave functions extend throughout the nuclear interior whereas those with high-are more excluded by centrifugal forces. For...

The breakdown of the N = 20 magic number in the so-called island of inversion around 32 Mg is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal-and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of 30 Mg and 32 Mg, with the latter consisting nearly purely of 2p-2h N = 20 cross-shell configurations. Single-neutron knockout experiments on 30,32 Mg projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions.

The structure of 19,20,22 C has been investigated using high-energy (around 240 MeV/nucleon) one-and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from 19 C and 20 C and two-neutron removal from 22 C. Two-neutron removal from 20 C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass A − 1 reaction residues A−1 C and, following one-neutron emission after one-neutron removal, the mass A − 2 two-neutron removal residues A−2 C. The smaller contributions of direct two-neutron removal, that populate the A−2 C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground-state configurations and one-neutron overlaps with low-lying states in 18−21 C. These suggest significant νs

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...