Isotopic superlattices are chemically not visible and therefore yield no x-ray contrast. However, due to the isotope dependence of the cross sections, neutrons are sensitive to different isotopes in the sample.
The unpolarised neutron reflectivity from a 56Fe/57Fe superlattice clearly shows the layering of the sample with sharp interfaces.
The inset reproduces the film thickness oscillations from the superlattice with a total thickness of 164 nm, demonstrating the high resolution of the ADAM reflectometer.
(Courtesy of B. Hjörvarsson)