Sample alignment for neutron (and in some cases x-ray) reflectometry can be complicated due to a coupling between angle and position which occurs when slits are used to define the path of the beam. Misalignments in sample position or sample rotation angle give rise to systematic errors in the experiments. By measuring the reflectivity both from the back and from the front faces of the sample (twin reflectometry) these alignment errors can be detected and corrected for.

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Journal J. Neutron Res.
Bouwman, W. G, Vigild, M. E, Findeisen, E, Kjaer, K, Feidenhans'l, R, & Mol, E. A. L. (1997). Two-axis neutron and x-ray reflectivity: How to avoid alignment pitfalls and how to correct for them. J. Neutron Res., 5, 133–146.