Abstract
The unbound nuclei 9He and 10He have been produced in proton-knockout reactions from a 280 MeV/u 11Li beam impinging on a liquid hydrogen target at the ALADIN-LAND setup at GSI. Information on their nuclear structure has been obtained from the relative-energy spectra, He 8 + n and He 8 + 2 n , employing reaction models incorporating the structure of 11Li. The He 8 + n relative-energy spectrum is dominated by a strong peak-like structure at low energy, which may be interpreted within the effective-range approximation as the result of an s-wave interaction with a neutron scattering length of − 3.17 ( 66 ) fm . This spectrum also shows evidence for resonance states at 1.33 ( 8 ) MeV and at 2.4 MeV above the He 8 + n threshold. It is argued that the s-state might not be the 9He ground state. For He 8 + n + n , the analysis of the relative-energy spectrum gives two alternative possibilities for an interpretation: either a narrow resonance at 1.42 ( 10 ) MeV ( Γ = 1.11 ( 76 ) MeV ) superimposed on a correlated background, or two overlapping resonances, a ground state at 1.54(11) MeV ( Γ = 1.91 ( 41 ) MeV ) together with an excited state at 3.99(26) MeV ( Γ = 1.64 ( 89 ) MeV ). The two possible interpretations are discussed.
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