First excited state of doubly-magic $^{24}$O

Neutron separation energy systematics indicate the formation of a new magic number $N=16$ close to the dripline. The energy of the first 2$^{+}$ state may indicate or invalidate the existence of a shell closure. The search for excited states in $^{23.24}$O using in beam $\gamma$ ray spectroscopy has yielded no results, which could indicate that the 2$^{+}$ state is neutron unbound. In order to unambiguously identify $^{24}$O as a doubly magic nucleus, we therefore have resorted to neutron decay spectroscopy. Experimentally, the two-proton-knockout reaction of a 86 MeV/u $^{26}$Ne beam on a Be target at the fast- fragmentation radioactive beam facility of the National Superconducting Cyclotron Laboratory was investigated and $\sim 500$ neutron-$^{23}$O coincidences were recorded using the Sweeper/MoNA setup. From these events, a decay-energy spectrum was reconstructed which combined with the neutron separation energy of $^{24}$O yields an excitation energy of the first excited state of $^{24}$O in the order of 3.6 MeV, in agreement with new shell-model calculations.