Investigation of the Neutron-Rich Oxygen Isotopes at the Drip Line

To investigate the appearance of a new magic number at neutron number 16, resonance states around the oxygen (Z=8) drip line have been investigated. The neutron unbound states were populated by a nucleon removal reaction from a radioactive 26F ion beam and the invariant mass method was used to reconstruct the resonance decay spectrum. The secondary radioactive 26F beam was created by the fragmentation of a 48Ca beam, produced by the National Superconducting Cyclotron Laboratory's Coupled Cyclotron Facility, and was cleanly selected by the A1900 fragment mass analyzer. A complete kinematic measurement was made of the decay neutron and fragment using the Modular Neutron Array (MoNA) to detect neutrons, and the large gap dipole Sweeper magnet combined with charged particle detectors to select and measure the charged fragments. Resonance spectra were reconstructed for both 24O+n and 23O+n coincidence data to investigate the resonance states in 25O and 24O, respectively. To extract resonance energies and widths from the spectra, a Breit-Wigner line-shape function was input into a Monte Carlo simulation that reflected the experimental response. A fit which minimized chi-squared was completed for each spectrum and for the 24O+n coincidence data a single resonance at a decay energy of Edecay=770(30) keV and with a width of Gamma=172(30) keV was determined. Two resonances of energies Edecay=0.63(4) MeV and Edecay=1.24(7) MeV were found in the fit to the 23O+n coincidence data. The single state in the 24O+n coincidence data is the first mass measurement of the ground state of the lightest neutron unbound oxygen isotope, 25O. The two states in the 23O+n data have been inferred to be the first observation of the 2+ and 1+ neutron unbound excited states in 24O. The ground state mass of 25O was used to determine the location of the 0d3/2 orbital at the oxygen drip line, and hence, determine the size of the N=16 shell gap to be 4.86(13) MeV at this location. Also, the ground state mass measurement provided the one- and two-neutron separation energies for 25O. Each of these observables were compared to shell model calculations using various interactions. Curiously, the interaction which best reproduced the experimental data was the universal sd shell model which calculates a bound 26O ground state, contrary to experiment. The two excited states observed in 24O were also compared to a number of theoretical calculations. The best description of the data was found by a calculation which explicitly included a continuum d3/2 orbital into wave function. The N=16 shell gap was also determined from the two states to be 4.95(16) MeV, in excellent agreement with the 25O result (4.86(13) MeV). Finally, the energy of the first excited 2+ state observed in 24O was systematically compared with the same state in other nearby even-even nuclei. It was determined that 24O is in fact a doubly magic nucleus having a magic number of protons Z=8 and neutrons N=16, due to the relatively high energy of its 2+ excited state.