Unbound States of Neutron-Rich Oxygen Isotopes: Investigation into the N=16 Shell Gap*

The energy of the first excited state in $^{24}$O and the ground state decay energy of $^{25}$O have been measured for the first time. Due to the lack of observation of an excited state in $^{24}$O using $\gamma$-ray spectroscopy, as well as the known unbound nature of $^{25}$O, techniques involving neutron spectroscopy had to be used. $^{25}$O ($^{24}$O$^{*}$) was populated via proton (proton-neutron) removal from a $^{26}$F beam. Break-up neutrons were detected by the Modular Neutron Array (MoNA) located at 0$^{\circ}$ relative to the beam direction. Charged fragments of desired rigidity were selected by a large-gap dipole (Sweeper) magnet and their properties were measured in the focal plane. Complete 4-vector reconstruction for neutrons and fragments at their break-up point revealed resonant energies of 750(50) keV and 840(30) keV for n-$^{23}$O and n-$^{24}$O coincidences respectively. Using the known neutron separation energy (S$_{\rm n}$) of $^{24}$O, its first excited state is calculated to be 4.45(40) MeV. These results are to be compared with current shell model predictions to aid in understanding of the N=16 shell gap.