The inclusive breakup for the 11 Li þ 208 Pb reaction at energies around the Coulomb barrier has been measured for the first time. A sizable yield of 9 Li following the 11 Li dissociation has been observed, even at energies well below the Coulomb barrier. Using the first-order semiclassical perturbation theory of Coulomb excitation it is shown that the breakup probability data measured at small angles can be used to extract effective breakup energy as well as the slope of BðE1Þ distribution close to the threshold. Fourbody continuum-discretized coupled-channels calculations, including both nuclear and Coulomb couplings between the target and projectile to all orders, reproduce the measured inclusive breakup cross sections and support the presence of a dipole resonance in the 11 Li continuum at low excitation energy.

The low-lying level structure of the unbound neutron-rich nucleus 13 Be has been investigated via breakup on a carbon target of secondary beams of 14 B was found to populate a broad lowlying structure some 0.7 MeV above the neutron-decay threshold in addition to a less prominent feature at around 2.4 MeV. Based on the selectivity of the reaction and a comparison with (0-3) ω shell-model calculations, the low-lying structure is concluded to arise from closely spaced J π =1/2 + and 5/2 + resonances (Er=0.40±0.03 and 0.85−0.11 MeV), whilst the broad higher-lying feature is a second 5/2 + level (Er=2.35±0.14 MeV). Taken in conjunction with earlier studies, it would appear that the lowest 1/2 + and 1/2 − levels lie relatively close together below 1 MeV.

The levels in 26 Na with single particle character have been observed for the first time using the d( 25 Na,pγ) reaction at 5 MeV/nucleon. The measured

The first measurement of the elastic scattering of the halo nucleus 11 Li and its core 9 Li on 208 Pb at energies near the Coulomb barrier is presented. The 11 Li þ 208 Pb elastic scattering shows a strong reduction with respect to the Rutherford cross section, even at energies well below the barrier and down to very small scattering angles. This drastic change of the elastic differential cross section observed in 11 Li þ 208 Pb is the consequence of the halo structure of 11 Li, as it is not observed in the elastic scattering of its core 9 Li at the same energies. Four-body continuum-discretized coupled-channels calculations, based on a three-body model of the 11 Li projectile, are found to explain the measured angular distributions and confirm that the observed reduction is mainly due to the strong Coulomb coupling to the dipole states in the low-lying continuum of 11 Li. These calculations suggest the presence of a low-lying dipole resonance in 11 Li close to the breakup threshold. One century ago, Rutherford [1] inferred the structure of the atom from the reaction data measured by Geiger and Marsden [2]. Since then, nuclear structure properties have often been deduced from nuclear reaction studies. With the advent of the first accelerated radioactive beams, new nuclear structures were discovered, such as the existence of a halo in some very loosely bound nuclei. In fact, 25 years ago, Hansen and Jonson [3] interpreted the large interaction cross section observed in 11 Li with light targets by Tanihata et al.[4] as due to the high probability of the outermost nucleons to be at large distances from the central core, which they referred to as a halo structure. The halo structure is a threshold phenomenon due to the low binding energy of the last nucleons. Halo nuclei have several features in common, such as a rather compact core, an extended neutron distribution, and very few, if any, excited states.The discovery of halo nuclei brought renewed interest in the modeling of nuclear reactions. This peculiar structure should affect the reaction properties at near-Coulomb barrier energies. Due to their low binding energy, the description of reactions involving halo nuclei should incorporate the coupling into the continuum. Current approaches to reaction theory involve different approximations whose validity needs to be checked when applied to halo nuclei.The most neutron-rich bound lithium isotope, 11 Li, is a fascinating case. Predicted to be unbound, it was identified in 1966 [5] and it is the archetype of a Borromean halo nucleus; i.e., the two different binary subsystems, 9 Li-n and n-n, are unbound, whereas the three-body system is bound by S 2n ¼ 369:15 AE 0:65 keV [6]. The ground state density distribution of 11 Li extends well beyond its core; i.e., the rms matter radius for the 9 Li isotope is 2:44 AE 0:06 fm Due to the loosely bound structure, the neutron halo is easily polarizable in the strong electric field of a heavy PRL 109, 262701 (2012) P H Y S I C A L

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The quenching of the N =20 shell gap in neutron-rich nuclei is investigated by studying the singleparticle structure of 27 Ne via neutron transfer using a 26 Ne beam. Two low-lying negative parity intruder states have been observed, the lowest of which is identified as J π = 3/2 − , confirming earlier speculations. A level identified as 7/2 − is observed higher in energy than the 3/2 − , contrary to the ordering at β-stability and at an energy significantly different to the predictions of previous shellmodel calculations. The measured energies and deduced spectroscopic factors are well reproduced in full (0,1)-ω 0s-0p-0d-1s-0f -1p calculations in which there is a significant ad-hoc reduction (∼ 0.7 MeV) in the N =20 shell gap. Neutron-rich nuclei often exhibit structural behaviour significantly different to stable nuclei, with a striking example being the "island of inversion" in the A ≃ 32 region of neutron rich nuclei [1,2]. The nuclei in this "island" are deformed rather than spherical owing to residual interactions and quenching of the N =20 magic number through the migration in energy of the shell-model orbitals [3]. This migration is known to be due, in part, to nucleon-nucleon tensor forces and to three-body N N N forces [4,5]. In adjacent more weakly bound nuclei, the migration may also be affected by the proximity of the continuum [6].The N =20 shell gap seen in nuclei near stability arises from the separation of the 0d 3/2 orbital and the negative parity orbitals (0f 7/2 , 1p 3/2 ,.. [10]. Guided by shell model expectations, the 885 keV level was tentatively identified as the lowest 1/2 + state and the 765 keV level was inferred to have a negative parity of 1/2 − , 3/2 − or 5/2 − . The 1/2 + is weak since it is populated mainly via pair-excited components in the 26 Ne ground state, in single-step transfer. In single-neutron knockout at higher energies, both levels were seen and the angular momentum of the removed nucleon was assigned as ℓ=0 or 1 [11], consistent with the results from the (d,p) reaction study [10]. These two excited states were also observed in the p( 28 Ne, 27 Neγ) reaction at intermediate energy [12] but no further information concerning spins was obtained [37]. No evidence for a 7/2 − state has been reported, despite a clear prediction that it should exist at an energy close to the 3/2 − state [11,15]. The most direct means to probe the evolution of orbital energies is to measure the strength of single-particle states where a neutron is transferred into orbitals that are otherwise empty. The present experiment was designed along these lines to populate the 7/2 − and 3/2 − 0f -1p shell states and any other strong single-particle levels via (d,p) transfer and to determine their properties.A beam of 26 Ne ions (∼100% pure) at 9.8A MeV was provided by the SPIRAL facility at GANIL and

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