No abstract

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 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.

The nucleus 52 Fe with ͑N = Z =26͒ has been investigated using intermediate-energy Coulomb excitation in inverse kinematics. A reduced transition probability of B͑E2;0 1 + → 2 1 + ͒ = 817͑102͒ e 2 fm 4 to the first excited 2 + state at 849.0(5) keV was deduced. The increase in excitation strength B͑E2 ↑ ͒ with respect to the even-mass neighbor 54 Fe (B͑E2 ↑ ͒ = 620͑50͒ e 2 fm 4 ) agrees with shell-model expectations as the magic number N =28 is approached. This measurement completes the systematics of reduced transition strengths to the first excited 2 + state for the even-even N = Z nuclei up to mass A = 56.

We report the first detailed study of the relative importance of the stripping and diffraction mechanisms involved in nucleon knockout reactions, by the use of a coincidence measurement of the residue and fast proton following one-proton knockout reactions. The measurements used the S800 spectrograph in combination with the HiRA detector array at the NSCL. Results for the reactions 9Be(9C,8B+X)Y and 9Be(8B,7Be+X)Y are presented and compared with theoretical predictions for the two reaction mechanisms calculated using the eikonal model. The data show a clear distinction between the stripping and diffraction mechanisms and the measured relative proportions are very well reproduced by the reaction theory. This agreement adds support to the results of knockout reaction analyses and their applications to the spectroscopy of rare isotopes.

The reaction 9 Be 28 Mg; 26 Ne X has been studied at 82 MeV=nucleon together with two similar cases, 30 Mg and 34 Si. Strong evidence that the reactions are direct is offered by the parallel-momentum distributions of the reaction residues and by the inclusive cross sections. The pattern of the partial cross sections for 28 Mg suggests the presence of correlations. A preliminary theoretical discussion based on eikonal reaction theory and the many-body shell model is presented. The reaction holds great promise for the study of neutron-rich nuclei.

The γ -ray spectroscopy of 25 Si and 29 S has been performed using single neutron knockout reactions with intermediate energy beams of the exotic isotopes 26 Si and 30 S. Two γ rays have been observed in 25 Si and three in 29 S. These are the first γ rays observed in these two isotopes. These two nuclei appear to be well deformed, and possible future intermediate-energy Coulomb excitation measurements would confirm their rotational nature.