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The degree of collectivity in the neutron-rich nuclei 32 Mg and 34 Mg has been determined via intermediateenergy Coulomb excitation in inverse kinematics. Measured energies of the first excited 2 + states and reduced electric quadrupole transition probabilities B(E2; 0 + g.s. → 2 + 1 ) are presented for 32 Mg and 34 Mg. The results agree with previous measurements and confirm the placement of both nuclei within the "island of inversion."

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.

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.

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Intermediate-energy Coulomb excitation is a key method to investigate collectivity in exotic nuclei far from ␤ stability. We report on the measurement of the absolute B(E2;0 1 ϩ →2 1 ϩ ) excitation strength in 46 Ar for five different minimum impact parameters. Our findings underline the validity, feasibility, and perspective of this technique for the study of exotic nuclei also in the regime of higher beam energy.

Monte Carlo shell-model calculations with the modern SDPF-M interaction successfully describe neutron-rich nuclei in the vicinity of N = 20 where normal and intruder configurations coexist at low excitation energy. We report on direct experimental evidence of the population of the 3/2 − intruder state in 27 Ne in the knockout of a single neutron from the ground state of 28 Ne. This low-lying negative parity state is consistent with a narrower shell gap for exotic nuclei with Z N and N ≈ 20. This observation also demonstrates the importance of direct reactions for the study of exotic nuclei and the predictive power of these large-scale shell-model calculations.

The 9 Be 32 Ar; 31 ArX reaction, leading to the 5 2 ground state of a nucleus at the proton drip line, has a cross section of 10.4(13) mb at a beam energy of 65:1 MeV=nucleon. This translates into a spectroscopic factor that is only 24(3)% of that predicted by the many-body shell-model theory. We introduce refinements to the eikonal reaction theory used to extract the spectroscopic factor to clarify that this very strong reduction represents an effect of nuclear structure. We suggest that it reflects correlation effects linked to the high neutron separation energy (22.0 MeV) for this state. DOI: 10.1103/PhysRevLett.93.042501 PACS numbers: 24.50.+g, 21.10.Jx, 27.30.+t The nuclear shell model pictures deeply bound nucleons as being in fully occupied states. At and above the surface of the Fermi sea, configuration mixing then leads to occupancies that gradually decrease to zero. This picture is modified in an important way by several correlation effects that are absent from or are described only approximately by effective-interaction theories, such as the shell model. These correlations arise from shortrange, soft-core, and tensor nucleon-nucleon (NN) interactions and from longer-range couplings involving low-lying and giant resonance collective excitations [1]. They result in the physical nucleon occupancies of deeply bound states being reduced and the strength shifted into states up to quite high energies; see Pandharipande et al. [2]. Absolute measurements of nucleon occupancies may therefore quantify these correlation effects. It should be pointed out that nuclear reaction observables probing the spatial behavior of the nucleonic wave functions agree well with the shell model picture. Examples are the characteristic angular distributions of transfer reactions and the longitudinal momentum distributions of the residues in knockout reactions, which identify the orbital angular momentum l of the nucleon states involved.The main body of evidence on nucleon occupancies has come from studies of the e; e 0 p reaction [2,3]. These have shown that in stable nuclei over a broad mass region, the valence proton states have their occupancies quenched by factors of order 0.6 -0.7 relative to the extreme singleparticle values. Recently, evidence has emerged that nucleon knockout reactions with heavy ions, at intermediate energies and in inverse kinematics, offer new possibilities for studying these effects by extending measurements to rare radioactive species and to neutron states. In general, the occupancy is not directly observable, but it is reflected in the spectroscopic factor C 2 S j that measures the overlap of the initial and final states with quantum numbers (l; j) [4]. The reduction factor R s is defined as the ratio of the experimental and theoretical value for the spectroscopic factor, the latter obtained when the valence nucleons are confined to a single major oscillator shell, the sd shell for the cases discussed here. This is consistent with how the reduction factors are defined in the analysis of e; e 0 p ...