Methods for separating isotopes are provided. An embodiment of such a method comprises directing a supersonic beam characterized by an average velocity v and velocity distribution Δv/v, the beam comprising a first isotope and a second isotope, at a single-crystalline surface at an angle of incidence θi such that the first isotope elastically scatters from the surface with a peak angle θfl and the second isotope elastically scatters from the surface with a peak angle θf2; and selectively collecting the scattered first isotope, the scattered second isotope, or both. Apparatus for carrying out the methods are also provided.