research papers J. Appl. Cryst. (2018). 51 https://doi.org/10.1107/S1600576718001528 1 of 7 Received 16 October 2017 Accepted 23 January 2018 Edited by K. Nakajima, J-PARC, Japan Atomic Energy Agency, Japan 1 This article will form part of a virtual special issue on advanced neutron scattering instrumentation, marking the 50th anniversary of the journal. Keywords: neutron reflectometry; interface kinetics; neutron optics; neutron prisms. Supporting information: this article has supporting information at journals.iucr.org/j RAINBOWS: refractive analysis of the incoming neutron beam over the white spectrum. A new fast neutron reflectometry technique exploiting a focusing prism 1 Robert Cubitt, a * Jaime Segura Ruiz a and Werner Jark b a LSS, Institut Laue–Langevin, 71 Avenue de Martyrs, Grenoble, Isere 38042, France, and b Elettra – Sincrotrone Trieste S.c.p.A, S. S. 14 km 163.5 Area Science Park, Basovizza (TS), I-34149, Italy. *Correspondence e-mail: cubitt@ill.fr Neutron reflectivity is a powerful technique for characterizing interfaces in many areas of science. The traditional method of time of flight for measuring the wavelength of neutrons in a white beam is extremely wasteful, as the vast majority of neutrons must be absorbed in the choppers in order to produce a pulsed beam. A prism operates continuously, with a transmission up to two orders of magnitude higher than choppers. The wavelength-dependent deflection of the beam by the prism, coupled with a high spatial resolution detector, results in excellent wavelength resolution. The theory of how the resolution is considerably enhanced by curving the surface of the prism is described in detail for a real experimental arrangement. It is demonstrated how this can be used for faster neutron reflectometry, including the merging of different angles and subtraction of background. The technique shows considerable promise for neutron reflectivity, opening up new areas of science particularly in the realms of kinetics and small samples. 1. Introduction Neutron reflectometry is a powerful technique for character- izing nanoscale surface structures. It is complementary to X-rays in that it has a particular sensitivity to light elements, can differentiate between isotopes and can separate magnetic from nuclear structure. With the brilliance of neutron sources being many orders of magnitude below state-of-the-art X-ray sources, efforts have been made to reduce measurement times without paying too high a price in blurring the spatial reso- lution (Ott & Menelle, 2006, 2008; Ott & Vismes, 2007). Unlike scattering techniques such as small-angle neutron scattering (SANS), specular reflectometry preserves infor- mation on the incoming beam paths after interaction with the sample. The use of the detector to recover this information allows the incoming beam collimation to be relaxed without any loosening of the resolution, resulting in considerable flux gains (Stahn et al., 2012; Cubitt et al., 2015). Conventional neutron reflectometry exploits one of two traditional methods to measure the neutron wavelength. The first involves the use of a multilayer or crystal that provides a source of monochromatic neutrons to the collimation system. To vary the momentum transfer at the sample the reflection angle needs to be scanned. This makes this sequential point- by-point method particularly unsuitable for kinetic measure- ments with a timescale similar to the time taken to make the scan. The other method is time of flight (TOF). With a synchrotron the neutron beams are naturally pulsed, but with ISSN 1600-5767 # 2018 International Union of Crystallography