Exotic Nucleus Helium-9 and its Excited States

The ground state and several excited states of ${}_{}{}^9{}_{}{}^{}\mathrm{He}$, the most neutron-rich nucleus to date, have been identified by means of the reaction ${}_{}{}^9{}_{}{}^{}\mathrm{Be}({\pi }^{-}, {\pi }^{+}){}_{}{}^9{}_{}{}^{}\mathrm{He}$. The mass excess of the ground state has been measured and it is found that the nucleus is unbound against single-neutron decay by 1.13±0.10 MeV only. It is found that the excited-state spectrum of this nucleus, which is very far from the valley of stability, is in good agreement with the predictions of "no-core" shell-model calculations whose parameters were optimized for the stable nuclei in the valley.