First evidence for a virtual 18B ground state

The decay of the neutron unbound ground state of 18B was studied for the first time through a single-proton knockout reaction from a 62 MeV/u 19C beam. The decay energy spectrum was reconstructed from coincidence measurements between the emitted neutron and the 17B 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 18B provides an experimental verification that the ground state of 19C includes a large s-wave component. The presence of this s-wave component shows that s-d mixing is still present in 18B and that the s1/2 orbital has not moved significantly below the d5/2 orbital.