Very large neutron-nucleus scattering lengths

\nThe interaction of neutrons and nuclei at low energies may potentially lead to strongly resonant states, characterized by scattering lengths several orders of magnitude larger than the effective range of the interaction. In order to explore the scattering of neutrons off unstable nuclei, we describe a simple analytical formalism to characterize the scattering parameters of a neutron-nucleus system created following the fast removal of a few nucleons from a slightly heavier beam. The case of the n-17B system is considered, and the implications of a potentially very large scattering length on the structure of 18,19B discussed.\n