The low-Z shore of the Island of Inversion: Invariant-mass Spectroscopy of the heavy Fluorine Isotopes 29F and 30F at SAMURAI with NeuLAND

The island of inversion is the region in the chart of nuclides at Z∼11 and N∼20 where the N=20 shell gap is quenched and intruder configurations are already dominant in the ground-state wave function. First experimental evidence for shell-structure changes compared to the naive shell model was found already in the 1970’s. This thesis studies the neutron-rich fluorine isotopes 30F and 29F that are located just at the predicted lower proton-number boundary of the island of inversion. The experiment was carried out in inverse kinematics at the SAMURAI setup at the Radioactive Ion Beam Factory (RIKEN Nishina Center, Tokyo/Japan). The nuclei of interest are populated in quasi-free proton- and neutron-knockout reactions at ∼250 MeV/u on the 15cm LH2 target of the MINOS device. The scattered charged nucleons are tracked in the MINOS TPC, while the heavy charged fragment is analyzed by the large-acceptance dipole magnet SAMURAI. The NeuLAND demonstrator and the NEBULA neutron detectors measure coincidentally neutrons in forward direction. The DALI2 array detects γ-rays around the target region. The NeuLAND demonstrator, part of the R³B experiment at GSI/FAIR (Germany), was added to the SAMURAI setup for a two-year experimental program. The detector was commissioned in a particular experiment where the one-neutron detection efficiency is determined at 110MeV and 250MeV. Therefore, a quasi-monoenergetic neutron beam is produced in the p(7Li,7Be)n reaction. The one-neutron detection efficiency is determined to be 31.0(13)% and 27.4(10)% (for ∆E>5MeV) at 110MeV and 250MeV, respectively. The results agree with the simulations. The SAMURAI setup allows to perform the complete spectroscopy of neutron-unbound nuclei. The first spectroscopic information for 30F is here obtained in the 31Ne(p,2p)29F+n reaction. By applying the invariant-mass method, the relative energy is calculated from the momentum measurement of the fragment and decay neutron. The ground-state resonance is determined to be at 583(85)keV with a width of Γ=730(151)keV. In the single-particle limit for a Breit-Wigner resonance the value of the width indicates a significant contribution from a valence neutron in the 2p orbital. This is a signature as found for nuclei in the island of inversion. Bound and neutron-unbound states are studied for 29F. The excited states are populated in the 30Ne(p,2p)29F* reaction. A known bound excited state at 1063(7)keV is confirmed in this experiment and a new one with transition energy of 287keV is found. Above the separation threshold 29F decays into 27F+n+n, the relative energy of the three particles is reconstructed and analyzed. Five more excited states are identified. The correlation analysis of this three-body decay in Jacobi coordinates shows that 29F* decays dominantly in sequential-decay mode via resonances in 28F. 28F is also separately investigated in the 29F(p,pn) reaction. The Breit-Wigner line shapes for sequential decay are introduced to determine the three-body resonance energies. The level and decay schemes are obtained. Eventually, the comparison to a shell-model calculation with the SDPF-M interaction and a ab-initio Self-consistent Green’s function theory using a N2LO_sat+N3LO(lnl) interaction is shown. The shell-model calculation shows reasonable agreement, while both theories do not predict the bound-state spectrum correctly. It is concluded that the N=20 shell-gap quenching persists at Z=9 (N~20) and intruder configurations are crucial in the description. The studied neutron-rich fluorine isotopes show characteristics as found in the island of inversion.