We report on direct time-of-flight based mass measurements of 16 light neutron-rich nuclei. These include the first determination of the masses of the Borromean drip-line nuclei (19)B, (22)C, and (29)F as well as that of (34)Na. In addition, the most precise determinations to date for (23)N and (31)Ne are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in (22)C, with a dominant ν2s(1/2)(2) configuration, and a single-neutron halo in (31)Ne with the valence neutron occupying predominantly the 2p(3/2) orbital. Despite a very low two-neutron separation energy the development of a halo in (19)B is hindered by the 1d(5/2)(2) character of the valence neutrons.

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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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Intermediate energy single-neutron removal from 31 Mg has been employed to investigate the transition into the N=20 island of inversion. Levels up to 5 MeV excitation energy in 30 Mg were populated and spin-parity assignments were inferred from the corresponding longitudinal momentum distributions and γ-ray decay scheme. Comparison with eikonal-model calculations also permitted spectroscopic factors to be deduced. Surprisingly, the 0 + 2 level in 30 Mg was found to have a strength much weaker than expected in the conventional picture of a predominantly 2p − 2h intruder configuration having a large overlap with the deformed 31 Mg ground state. In addition, negative parity levels were identified for the first time in 30 Mg, one of which is located at low excitation energy. The results are discussed in the light of shell-model calculations employing two newly developed approaches with markedly different descriptions of the structure of 30 Mg. It is concluded that the cross-shell effects in the region of the island of inversion at Z=12 are considerably more complex than previously thought and that np − nh configurations play a major role in the structure of 30 Mg.