The Origins of Neutron Reflectometry

The optical properties of slow neutrons, which includes refraction, reflection and interference, were recognized in the very early days of neutron physics. Fermi and co-workers [1] reported the total (or critical) reflection of slow neutrons. Goldberger and Seitz showed that the intensity of reflected and transmitted neutrons follow the same laws as electromagnetic radiation. The early applications of those neutron optical phenomena included the determination of neutron scattering lengths, and the production of neutron guides and neutron spin polarizers. The development of graded multi-layer mirrors, “supermirrors” [2] quickly followed. From such developments and the pioneering neutron guide installations at the Institute Laue Langevin, ILL, in Grenoble, sophisticated neutron optical devices are now commonplace on all neutron sources. The potential to use neutron reflectivity to probe surface structure arose out of those early applications. As such neutron reflectometry (NR) has evolved [3–6] over the past quarter century to become an established probe of the structure of thin films and periodic multilayers of both soft and hard condensed matter. From the beginning, specular neutron reflectivity measurements have been used to determine scattering length density depth profiles related to chemical composition and, with polarized beams, the vectorial magnetization depth profile in magnetic thin film systems. The success of NR can be attributed not only to instrumentation advances and the formulation of quantitatively accurate theoretical methods for the analysis of reflectivity data, but to the advances in various thin film deposition techniques required to prepare satisfactory sample systems for study. In this article, a partial history of neutron reflectometry development is briefly outlined in the form of complementary perspectives from the two authors. Length limitations, however, make it impossible to acknowledge here, let alone adequately describe, all of the important work that so many people have contributed to the implementation of neutron reflectometry in many fields of scientific investigation.