First Evidence for a Virtual B-18 Ground State

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The decay of the neutron unbound ground state of B-18 was studied for the first time through a single-proton knockout reaction from a 62 MeV/u C-19 beam. The decay energy spectrum was reconstructed from coincidence measurements between the emitted neutron and the B-17 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 B-18 provides an experimental verification that the ground state of C-19 includes a large s-wave component. The presence of this s-wave component shows that s-d mixing is still present in B-18 and that the s(1/2) orbital has not moved significantly below the d(5/2) orbital.