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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 2 p − 2 h 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 n p − n h configurations play a major role in the structure of 30 Mg. [ABSTRACT FROM AUTHOR]

The properties of nuclei with extreme neutron–to–proton ratios, far from those naturally occurring on Earth, are key to understand nuclear forces and how nucleons hold together to form nuclei. 7H, with six neutrons and a single proton, is the nuclear system with the most unbalanced neutron–to–proton ratio known so far. However, its sheer existence and properties are still a challenge for experimental efforts and theoretical models. Here we report experimental evidences on the formation of 7H as a resonance, detected with independent observables, and the first measurement of the structure of its ground state. The resonance is found at ∼0.7 MeV above the 3H+4n mass, with a narrow width of ∼0.2 MeV and a 1 / 2 + spin and parity. These data are consistent with a 7H as a 3H core surrounded by an extended four-neutron halo, with a unique four-neutron decay and a relatively long half-life thanks to neutron pairing; a prime example of new phenomena occurring in what would be the most pure-neutron nuclear matter we can access in the laboratory. [ABSTRACT FROM AUTHOR]

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 19B, 22C, and 29F as well as that of 34Na. In addition, the most precise determinations to date for 23N and 31Ne are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in 22C, with a dominant v2s21/2 configuration, and a single-neutron halo in 31Ne with the valence neutron occupying predominantly the 2p3/2 orbital. Despite a very low two-neutron separation energy the development of a halo in 19B is hindered by the Id25/2 character of the valence neutrons. [ABSTRACT FROM AUTHOR]