Studying the neutron-unbound $^{18}$B

The decay of the neutron-unbound ground state of $^{18}$B was studied for the first time through a single-proton knockout reaction from a 62 MeV/u $^{19}$C beam. The decay energy spectrum was reconstructed from coincidence measurements between the emitted neutron and $^{17}$B fragment using the MoNA/Sweeper setup. An $s$-wave line shape was used to describe the experimental spectrum resulting in an upper limit for the scattering length of $-50$ fm which corresponds to a decay energy $< 10$ keV. Observing an $s$-wave decay of $^{18}$B provides an experimental verification that the ground state of $^{19}$C includes a large $s$ component. In addition, our results show that the $s-d$ mixing proposed for $^{19}$C is also present in $^{18}$B, therefore no clear signs of an inversion between the $s_{1/2}$ and $d_{5/2}$ orbitals can be suggested.