EXACT DETERMINATION OF THE PHASE IN NEUTRON REFLECTOMETRY

We show that by using a known reference layer having three tunable values of scattering density, an exact determination of the complex amplitude R =ReR+iImR for neutron specular reflection can be made for any unknown real potential (i.e., no absorption). This straightforward yet remarkable general result is valid even in the dynamical regime (where the conventional Born approximation fails) and makes it feasible to consider direct inversion methods for obtaining the scattering-length-density profile normal to the reflecting surface.