Unbound states in the lightest island of inversion: Neutron decay measurements of 11Li, 10Li, and 12Be

The light nuclei in the N ~ 8 region provide easily accessible systems for studying the structure of exotic nuclei. The nuclide 11Li is a two-neutron halo system, a Borromean nucleus, and located in the N ~ 8 island of inversion. The next lightest lithium isotope, 10Li is neutron unbound, located on the western edge of the island of inversion, and important for the understanding of the 11Li system. 12Be is the heaviest N = 8 isotone in the island of inversion. Neutron unbound states from all three systems have been measured and analyzed in this work.;The experiment was conducted at the National Superconducting Cyclotron Laboratory. Beams of exotic boron isotopes (12B and 13B) were provided by the Coupled Cyclotron Facility and delivered to the MoNA-LISA-Sweeper setup, each at 71 MeV/u. The neutronunbound isotopes were created in a secondary beryllium target where they immediately decayed into a charged daughter fragment and one or two neutrons. The charged daughter fragment was measured by a suite of charged particle detectors and the neutrons were detected by MoNA-LISA.;Measurement of the 11Li unbound system showed a state at 1210+/-80 keV excitation energy with a width greater than 1200 keV, consistent with previous measurements. Correlations between the three decay products were examined with use of Jacobi spectra and Dalitz plots, which revealed a correlation between the two neutrons stronger than that expected from the decay of two neutrons uncorrelated in the nucleus. This is the first correlation measurement attributable to a specific unbound state in 11Li. Unbound states in 10Li were populated with a two-proton knockout from a 12B beam, a reaction expected to populate primarily positive-parity states. The decay energy spectra of 10Li was fit with three peaks at 110+/-40 keV, 500+/-100 keV, and 1100+/-100 keV. The lowest two of these peaks are expected to be a 1+/2+ doublet. Calculation of the 0 p1/2 single particle energy in the 10Li system and comparison with Faddeev calculations suggests that both states of the 2-- /1-- doublet are below 50 keV, consistent with previous measurements of one ℓ = 0 state at 50 keV or below. In the 12Be system, a measured decay energy of 1243+/-20 keV corresponds to an excitation energy between 4400 and 4800 keV, depending on the combination of final states populated in 11Be. A lack of decays to 10Be placed an upper limit of 5% on the branching ratio to the ground state of 10Be via 2n emission. This low branching ratio, as well as spin and parity selectivity from the 1p-knockout production mechanism, suggests that the spin and parity of this state is 1-- or 2--.