The cross sections for single-neutron removal from the very neutron-rich nucleus 31Ne on Pb and C targets have been measured at 230 MeV/nucleon using the RIBF facility at RIKEN. The deduced large Coulomb breakup cross section of 540(70) mb is indicative of a soft E1 excitation. Comparison with direct-breakup model calculations suggests that the valence neutron of 31Ne occupies a low-l orbital (most probably 2p(3/2)) with a small separation energy (S(n) approximately < 0.8 MeV), instead of being predominantly in the 1f(7/2) orbital as expected from the conventional shell ordering. These findings suggest that 31Ne is the heaviest halo system known.

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Single nucleon pickup reactions were performed with a 18:1 MeV=nucleon 14 O beam on a deuterium target. Within the coupled reaction channel framework, the measured cross sections were compared to theoretical predictions and analyzed using both phenomenological and microscopic overlap functions. The missing strength due to correlations does not show significant dependence on the nucleon separation energy asymmetry over a wide range of 37 MeV, in contrast with nucleon removal data analyzed within the sudden-eikonal formalism. DOI: 10.1103/PhysRevLett.110.122503 PACS numbers: 24.50.+g The existence of single-particle-like modes in nuclei, near the Fermi surface, is particularly important because these are at the basis of the nuclear shell model and thus govern the low energy nuclear dynamics. Yet, they result from nontrivial many-body correlations, which affect energy ordering and filling of active orbits. Spectroscopic factors (SFs) are a unique tool to address the question of correlations as they are strictly linked to the notion of shell occupancies and can be probed using direct reaction cross section measurements [1,2]. Information for stable nuclei was formerly provided by the electromagnetic probe (e, e 0 p) [3][4][5]. Even for closed shell nuclei like 16 O or 208 Pb, a cross section reduction by 30%-40% relative to an independent-particle-based model was observed. Different origins are now well established, like short range correlations [1] and couplings to collective modes at high excitation energy [6] or to the continuum [7]. Single nucleon pickup reactions were also used for stable nuclei yielding results consistent with (e, e 0 p) measurements [8,9].For nuclei away from the valley of stability, new approaches have been developed in inverse kinematics at various incident energies, knockout and transfer reactions. From knockout reactions at intermediate energy, a reduction factor R s was deduced as the ratio between the experimental cross section and a theoretical value obtained in a sudden-eikonal approach [10]. A strong dependence was claimed for R s versus the asymmetry (difference in separation energy) ÁS ¼ ðS p À S n Þ with ¼ þ1 (À1) for proton (neutron) removal reactions, with a reduction as high as 70% for large positive ÁS values. This reduction is still not understood and was first accounted for by possible missing correlations in shell-model calculations [10]. Different conclusions were drawn from (i) the possibility of dissipative processes beyond the sudden approximation [11,12], and (ii) transfer reactions at lower incident energies showing no ÁS dependence of R s [13]. From a theoretical point of view, ab initio calculations suggest only a mild dependence of SFs on ÁS [7,14], with equal SFs found for the nucleon removals from 56 Ni [6] despite significant ÁS values (AE 9:5 MeV). Coupled-cluster calculations [7] pointed out a further decrease of proton SFs for isotopes at the neutron dripline, due to coupling to the continuum. This has the substantial effect of enhancing the dependence on...