Low-lying resonance states in ${}^7\mathrm{He}{(}^6\mathrm{He}+n)$, formed after fragmentation reactions of a 227 MeV/nucleon ${}^8\mathrm{He}$ beam on a carbon target, have been studied. Coincidences between ${}^6\mathrm{He}$ nuclei and neutrons, corresponding to the one-neutron knockout channel in ${}^8\mathrm{He}$, were selected. The relative energy spectrum in the ${}^6\mathrm{He}+n$ system shows a structure, which is interpreted as the ${}^7\mathrm{He}$ ( $I^{\pi }{=3/2\mathrm{}}^{-}$) ground state, unbound with 0.43(2) MeV relative to the ${}^6\mathrm{He}+n$ system and a width of $\Gamma =0.15\mathrm{}\left(8\right)\mathrm{MeV}$ overlapping with an excited ( $I^{\pi }{=1/2\mathrm{}}^{-}$) state observed at 1.0(1) MeV with a width of $\Gamma =0.75\mathrm{}\left(8\right)\mathrm{MeV}$.

• Received 26 February 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.102501