Evidence for an l=0 ground state in 9He

The unbound nuclear systems 10Li and 9He were produced in direct reactions of 28 MeV/u 11Be incident on a 9Be target. The distributions of the observed velocity differences between the neutron and the charged fragment show a strong influence of final-state interactions. Since the neutron originates in a dominant l=0 initial state, a selection-rule argument allows a firm l=0 assignment for the lowest odd-neutron state in 10Li. We report the results suggesting a very similar unbound state in 9He, characterized by an s-wave scattering length more negative than −10 fm corresponding to an energy of the virtual state of less than 0.2 MeV. Shell-model calculations cast light on the reasons for the disappearance of the magic shell gap near the drip line.