Neutron halo in the exotic first excited state of $^9$Be

Differential cross sections for elastic and inelastic $^9\mathrm{Be} + \alpha$ 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 $^{11}\mathrm{Be}$ nucleus. This result is the first observation of a halo in an unbound state lying above the neutronemission threshold.