Abstract
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 $^{9}\mathrm{Be}({\ensuremath{\pi}}^{\ensuremath{-}}, {\ensuremath{\pi}}^{+})^{9}\mathrm{He}$. 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\ifmmode\pm\else\textpm\fi{}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.
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