  The flux-across-surfaces theorem establishes a fundamental relation in
quantum scattering theory between the asymptotic outgoing state and a quantity
which is directly measured in experiments. We prove it for a hamiltonian with a
point interaction, using the explicit expression for the propagator. The proof
requires only assuptions on the initial state and it covers also the case of
zero-energy resonance. We also outline a different approach based on
generalized eigenfunctions, in view of a possible extension of the result.
