Stacy Kim

The decay of five neutron-heavy rhodium isotopes were studied at the Radioactive Isotope Beam Factory (RIBF) Facility at the RIKEN Nishina Center after relativistic fission of 238U beam on a thick beryllium target. Previously unknown associated gamma-ray decay energies are reported for each nuclide, and through evaluating the intensity of the 2+ → 0+ E2 transition in the even-even palladium daughter nuclei, 120,122,124Pd, from the beta-tagged gamma-ray spectra an upper or lower limit of beta-delayed neutron emission is deduced for each nuclei. A general, expected trend of increasing Pn is observed in the direction of the neutron drip line.

Detailed spectroscopy of the neutron-unbound nucleus ²⁸ F has been performed for the first time following proton/neutron removal from ²⁹ Ne / ²⁹ F beams at energies around 230 MeV / nucleon . The invariant-mass spectra were reconstructed for both the ²⁷ F ( * ) + n and ²⁶ F ( * ) + 2 n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the ²⁸ F ground state, with S _n ( ²⁸ F ) = − 199 ( 6 )     keV , while analysis of the 2 n decay channel allowed a considerably improved S _n ( 27 F ) = 1620 ( 60 )     keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of ²⁸ F . Importantly, in the case of the ground state, the reconstructed ²⁷ F + n momentum distribution following neutron removal from ²⁹ F indicates that it arises mainly from the 1 _{p 3 / 2} neutron intruder configuration. This demonstrates that the island of inversion around N = 20 includes ²⁸ F , and most probably ²⁹ F , and suggests that ²⁸ O is not doubly magic.

Background: Very neutron-rich isotopes, including Ne30-32, in the vicinity of N=20 are known to exhibit ground states dominated by fp-shell intruder configurations: the “island of inversion.” Systematics for the Ne-isotopic chain suggest that such configurations may be in strong competition with normal shell-model configurations in the ground state of Ne29. Purpose: A determination of the structure of Ne29 is thus important to delineate the extent of the island of inversion and better understand structural evolution in neutron-rich Ne isotopes. This is accomplished here through a combined investigation of nuclear and Coulomb-induced one-neutron removal reactions. Method: Cross sections for one-neutron removal on carbon and lead targets and the parallel momentum distribution of the Ne28 residues from the carbon target are measured at around 240 MeV/nucleon. The measurements are compared with reaction calculations combined with spectroscopic information from SDPF-M shell-model wave functions. Results: The deduced width of the inclusive parallel momentum distribution, 98(12) MeV/c (FWHM), suggests that the ground state of Ne29 has a spin parity of 3/2-. Detailed comparisons of the measured inclusive and partial cross sections of the two targets and the parallel momentum distribution of the carbon target with reaction calculations, combined with spectroscopic information from large-scale shell-model calculations, are all consistent with a 3/2- spin-parity assignment. Conclusions: The results indicate that Ne29 lies within the island of inversion and that the ground state of Ne29 is dominated by a Ne28(0+1)xp3/2 neutron configuration. Combined with recently measured interaction cross sections, it is concluded that Ne29 may exhibit a moderately developed halo-like distribution

Cross sections of 1n-removal reactions from the neutron-rich nucleus Mg37 on C and Pb targets and the parallel momentum distributions of the Mg37 residues from the C target have been measured at 240  MeV/nucleon. A combined analysis of these distinct nuclear- and Coulomb-dominated reaction data shows that the Mg37 ground state has a small 1n separation energy of 0.22+0.12−0.09  MeV and an appreciable p-wave neutron single-particle strength. These results confirm that Mg37 lies near the edge of the “island of inversion” and has a sizable p-wave neutron halo component, the heaviest such system identified to date.