Abstract
Differential cross sections for elastic and inelastic 9 Be + α scattering (the inelastic scattering leading to the 1.68- and 2.43-MeV states) were measured at the energies of 40 and 90 MeV. The radii of the excited states in question were determined on the basis of a modified diffraction model. The root-mean-square radius of the level at 1.68 MeV proved to be approximately 1 fm larger than the radii of both neighboring states. The reason is that, for this level, there is a neutron halo whose properties are similar to the properties of the known halo in the 11 Be nucleus. This result is the first observation of a halo in an unbound state lying above the neutronemission threshold.