Measuring oxygen isotopes beyond the neutron dripline: Two-neutron emission and radioactivity

The availability of rare isotope beams has made it possible to extend nuclear structure measurements to nuclei far away from stability. Drastic changes in the structure, properties, and available decay-modes of these exotic isotopes have been observed in comparison to their stable counterparts. The oxygen isotopic chain has been particularly interesting with observations of new shell closures at N=14 and N=16. The MoNA-LISA/Sweeper setup at the National Superconducting Cyclotron Laboratory at Michigan State University has allowed for studies of the oxygen isotopes to be extended beyond the neutron dripline. Recently, the $^{26}$O ground state was observed for the first time and shown to be unbound by less than 200~keV. The low energy ground state of the two-neutron unbound $^{26}$O opened the possibility for the discovery of two-neutron radioactivity. A new technique was developed to measure the lifetimes of neutron unbound nuclei in the picosecond range. This technique was applied to the $^{26}$O decay and a half-life of 4.5$_{~-1.5}^{+1.1}$~(stat.)~$\pm$3~(sys.)~ps was extracted. This corresponds to $^{26}$O having a finite lifetime at an 82\% confidence level and, thus, suggests the possibility of two-neutron radioactivity.