Ground-state neutron decay of $^{21}$C

The ground state of neutron-unbound $^{21}$C was measured for the first time in a neutron-fragment coincidence experiment at the National Superconducting Cyclotron Laboratory at Michigan State University. This is the heaviest neutron-unbound N = 15 nucleus and provides a measurement of the $\nu$(1s$_{1/2}$) - $\nu $(0d$_{5/2}$) shell gap in the presence of proton holes in the $p$ shell. $^{21}$C was produced via one-proton knockout from a $^{22}$N secondary beam at 69.7 MeV/u. The Modular Neutron Array (MoNA) was used to measure the time-of-flight and position of emitted neutrons, while $^{20}$C fragments were detected in a series of position and energy-sensitive detectors behind the MSU/FSU Sweeper magnet. The decay of $^{21}$C was then reconstructed event-by-event from the four-momentum vectors of the neutron and fragment. Preliminary results will be presented.